rch 1951 - americanradiohistory.com€¦ · what's your problem? 24 change of address should...

INGE AREA ISSUE

FIELD STRENGTHMEASUREMENTS

FRINGE AREASERVICING

-Page 9

TV ANTENNA,111111111rPLIFIERS

Fringe Area reception is ofte rg D32 6I Z012improved with antenna rotators, VNN3d NO.T4NVI-11/IDN

shown here in construction at the Ra 3AV NOVCNINSVA SL ISIISDN3S0 A NI.VIVN

limppgr-

RCH 1951

the Du Mont type 20cp4For the set that follows the "seventeen" inrectangular pictures, Du Mont supplies the"twenty" all -glass rectangular Type 20CP4.Another in the series of Big -Picture Teletronsemploying the NEW Bent -Gun for edge -to -edge sharpness that makes the difference inpicture tubes of this size. The gray filter faceplate improves contrast.The first picture tube designed with theindustry "standard" neck length of 73(6".Our Commercial Engineering Departmentinvites all design inquiries.

ont Laboratories, Inc., CathodeRay Tube Division, Clifton, N. J.

Trademark

RADIO INDUSTRY NEWSLETTER

RCA RELEASES3 -COLOR TUBE

COLOR TELEVISIONDECISION IMMINENT

ENLARGE PLANS FOR'UTILITY' TV SETS

NBC TO WORK OUTSLAVE TV -FILM UNIT

FM STATIONS TO HITMFRS. FOR NEGLECT

NEW BILL TO CHANGEFCC SCOPE, POWERS

MFRS. START SERVICETRAINING PROGRAMS

Data on the manufacture of the RCA three -color television tube was just releasedby the company to other tube makers licensed to use RCA patents. Although therelease of the information will probably not speed the coming of color TV, dueto the war situation, it is the action which FCC asked the company to take monthsago. It is expected that more months will pass before the tubes are available,since even RCA is not producing them yet.

A decision from the Supreme Court on the color television issue, appealed to thehigher court after the inconclusive decision of a Chicago court, was forecast for thismonth by FCC chairman Wayne Coy. The commissioner also said that the wareconomy will dictate whether color TV makes a large or small beginning.

Further plans for "utility" TV receivers are being made by set manufacturers, withtransformerless power supplies, selenium rectifiers, electrostatic focusing anddeflection, and smaller speaker magnets scheduled to cut down on the consump-tion of vital materials. The so-called utility sets are said to be the last stepbefore what would be actually lower -quality units, or "austerity" sets. Utilitymodels will be advertised, probably, as using substitute materials without anyloss in quality.

A new slave camera for TV broadcast is on the schedule of experiments at NBC.The device, which will be a movie camera attached to the side of the TV camera,will permit higher quality in kinescope recordings, since the films will be madedirect, controlled by the same switches that control the TV pickups, instead ofbeing filmed from a monitor. The development is expected to improve the qual-ity of re -broadcast programs considerably.

FM broadcasters, it is reported, will soon attack RTMA members for laxity inpromoting the suffering medium of audio broadcasting. With FM stations drop-ping fast, the broadcasters will request cooperation from the manufacturinggroup's members. FM stations have been successful in only a few localities, andthe nationwide picture is not encouraging to the system's backers. The output ofFM receivers has, according to the station operators, been "disproportionatelysmall" in relation to the production of AM and TV sets.

A change in the operation of the FCC is foretold by the passage of a bill in theSenate. The bill provides the commission with power to issue "cease and desist"orders to stations for violations of minor rules. At present, the commission's onlypositive weapon against infractions was the power to revoke operating licenses.Heretofore, the group has allowed small offenses to go unpunished rather thanto put the violators out of business. FCC will retain its position of granting andrenewing licenses.

With the shortage of radio and television servicemen growing more acute, sev-eral manufacturers are initiating plans to increase technical education oppor-tunities. Sylvania Electric has begun a series of meetings on TV service, to con-tinue throughout the year. RCA Institutes, the manufacturing company's service -training department, has begun a "home -study" course, also for workingtechnicians.

RADIO AND TELEVISION MAINTENANCE MARCH, 1951 3

Don't bevague. . .

ask for

SPRAGUECAPACITORS

SPRAGUE"CERA-MITE"

DISCCERAMICS

SPRAGUETVA "ATOM"DRY ELEC-TROLYTICS

Sprague's Puiedeed 7044BIGGEST LITTLETROUBLE -SAVERSIN TV SERVICING.. Avoid costly callbacks

WRITE FOR CATALOG

SPRAGUEPRODUCTS COMMDISTRIBUTORS- DIVISION OF THE SPRAGUE ELECTRIC COMPANY

91 MARSHALL ST., NORTH ADAMS, MASS.

ATOM, TWIST-LOK, TELECAP endCERA-MITE are SPRAGUE Trademarks

VOLUME VII NUMBER 3

RADIO AND TELEVISIONMAINTENANCE

is published monthly byInternational Publishing

Corporation at16 Union St.,

Somerville, N. J.,U. S. A.

EXECUTIVE OFFICESAmbassador HotelAtlantic City, N. J.

Phone: 4-3474

Entered as second classmatter October 16, 1950,at Post Office, Somer-ville, New Jersey, underAct of March 3, 1879.

PRESIDENTPhilip Harrison

PUBLISHER

ADVERTISING DIRECTORCharles R. Tighe

BUSINESS DIRECTORPaul Lightman

EXECUTIVE EDITORRobert Letwin

EDITORMaurice de Angell

CONTRIBUTING EDITORSMorton G. ScheragaNorman L. Chalfin

CIRCULATIONMANAGERLee Rand

ART DIRECTORJohn Cantwell

ADVERTISING OFFICES

New YorkRoom 70540 E. 49:h St.MUrray Hill 8-7820

Representatives:

ClevelandHarold L. Stuart852 Leader Bldg.Main 1-9357

ChicagoPrendergast and

Minahan168 No. Michigan Ave.Financial 6-0993

West CoastKeith H. Evans3757 Wilshire Blvd.Los Angeles 5, Cal.DUnkirk 8-2981

Subscription rates: U. S.and U. S. possessions,$2.00 per year by mail.Canada, $3.00 per yearby mail. All other coun-tries, $4.00 per year bymail.

RADIO andTELEVISIONmathienance

MARCH, 1951

FEATURES

Field Strength Measurements 5-By Rufus P. Turner

Getting the Most Out of the Antenna 7-By David T. Armstrong

Fringe Area Servicing 9- By Rudolf F. Graf

"Box Office" Television 10-By Richard L. Browne

Pushing Back the Fringe 12

Build This Simple Home ApplianceTester 13

-By Harry F. Leeper

Television Preamplifiers 14- By Norman L. Chalfin

DEPARTMENTS

Newsletter 3

Products for the R Trade 16

Fix on the Facts 17- By John T. Frye

Trade Literature 18

The Maintenance Mill 21

What's Your Problem? 24

Change of address should be sent to RADIO AND TELEVISIONMAINTENANCE, BOX 867, Atlantic City, N. J. Change requiresfour weeks' notice. When notifying of change of address, pleasefurnish stencil impression from a recent issue.

DEVOTED TO SERVICE OF RADIO -AUDIO -VIDEO

4 RADIO AND TELEVISION MAINTENANCE MARCH, 1951

In Fringe AreasYou Should BeFamiliar With

ANTENNAI N PUT

TERMINALS

FIG. I

1 N56

RF TYPE FIELD STRENGTH METER

0-50 DCMI CROAM METER

-RTM-

Field Strength Measurements

AFTER the first fires of the DX crazeburned low, and sensitive super-

heterodyne receivers and high-powered,locally -situated broadcast stations as-sumed dominance in radio, service tech-nicians stopped thinking about fieldstrength. This is quite understandable.Good, strong broadcast signals havebeen available in most communities formany years. The outside antenna al-most completely disappeared, becausesatisfactory signals could be picked upwithout it. Only engineers concernedwith the performance of a transmittingstation needed to determine how stronga signal was at a given receiving loca-don. Many radio service technicianswho have come up during the last tenor fifteen years have never given a

thought to field strength.

TV Range LimitedTelevision, however, with its limited -

range transmitters and variations in re-ceiving conditions, has brought thepractical technician again face to facewith field strength problems. The TVserviceman uses field strength measure-ments in one form or another to evalu-ate the merits of antennas, guide himin situating and orienting antennas forstrongest signal pickup, and to surveyareas for appraisal of reception condi-tions to be expected by potential TVcustomers. Practical field strength testseliminate much guess work and specu-lation.

Moderately -priced instruments havebeen made available for TV fieldstrength measurements by service tech -

By RUFUS P. TURNER

nicians. This equipment, plus the in-evitable makeshift gimmicks and meth-ods which have put in their appearance,have provided the TV technician withnew tools for doing a better job.

Correct procedure for tests and a clearinterpretation of the results obtained infield strength operations depends largelyupon a good understanding of the prob-lem and of the instruments and methodsemployed.

Signal Field Strength

The radio wave emitted by a trans-mitter consists of an electric and a mag-netic component. These componentsoperate at right angles to each other.Both induction and radiation fields areset up by the transmitting antenna. Theinduction field decreases rapidly in in-tensity as the distance from the trans-mitting antenna is increased. Thestrength of the induction field is neg-ligible a short distance from the trans-mitter. But the radiation field decreasesless rapidly in intensity as the distanceincreases. If signal strength is meas-ured quite close to the transmitter loca-tion, chances are that the induction fieldis the predominant one checked. At adistance from the transmitting antennaequal to the transmitter wavelength di-vided by 2 7r, the radiation and induc-tion field intensities are approximatelyequal.

The induction field is of little inter-est to the practical TV technician, sincemeasurements are made at such a greatdistance from the transmitter that in-

ductive effects are not present. In orderto encounter a strong induction field, itwould be necessary to test within a veryfew feet of a transmitting antenna oper-ated in any of the twelve present-dayTV channels.

It is the radiation field which bringspicture and sound information to thereceiving point, and it is the strengthof this field that is of interest in receiver -location field -strength measurements.

Minimum Intensity

Attenuation conditions, reflection phe-nomena, polarization, and absorptioneffects all are quite different on TVfrequencies from those in the broadcastband and on the lower short waves. Allof this adds up to the fact that an im-practically weak TV signal is more aptto appear at a relatively nearby receiv-ing point than is the case with lowerfrequencies. The boundary at which thesignal strength has fallen to the mini-mum value which will give satisfactoryTV reception with a conventional in-stallation is the beginning of the fringearea. Actual depth of the fringe area,before the point is reached where thereceived signal drops to a level nolonger useful, will depend upon condi-tions in the neighborhood of the receiv-ing antenna. Authorities have specifiedthe fringe area signal strength as 500microvolts per meter. This means thata TV receiver must be capable of fullperformance at this signal intensity, inorder to operate satisfactorily within thefringe area.

Because of their design, some TV re -

RADIO AND TELEVISION MAINTENANCE MARCH, 1951

ceivers inherently are more capable thanothers of doing a good job with weaksignal input. This increased sensitivityordinarily is obtained through the me-dium of higher gain in the IF video,and sync amplifiers. Similarly, some an-tenna arrays deliver stronger signal volt-ages to the receiver input terminals thanthose of different design. Antennaheight and placement likewise are fac-tors often in the utilization of low fieldintensities.

Many factors act singly or in combi-nation to cause low field strength in agiven locality. Among these are dis-tance from the transmitter; topographyof the region lying between transmitterand receiver; channel frequency; natureand type of buildings and other promi-nent objects in the vicinity of the receiv-ing antenna; and objects obstructingthe transmitter -receiver path.

Field Strength Meters

A TV field strength meter consistsessentially of a receiver which may betuned to each of the TV channels andwhich actuates an indicating meter. Ininstruments designed for accurate, quan-titative measurements, the meter scaleis calibrated to read in microvolts. In-struments intended for less demandingindications have arbitrary meter scalessatisfactory for relative indications offield strength. While the latter type ofscale does not indicate microvolts di-rectly, it will show the magnitude of onesignal (or signal condition) with re-spect to another.

Several portable field strength meterslately have been placed on the marketfor the technician. As a rule, the morecomplex of these instruments use a su-perhet front-end tuner. This is followedwith an IF amplifier and second detec-tor. This last stage drives a simplifiedDC vacuum tube voltmeter stage which

ANTENNAINPUTTERMINALS

FIG 2

IN2I

in turn actuates the indicating meter.At the receiving location of interest,

the meter is connected to an exploringantenna or to the antenna which is tobe used later with the receiver. Thetuner is switched successively to thechannels on which broadcasts arrive, theantenna is rotated for maximum meterdeflection, and the reading recorded.

Commercial MetersCommercial meters include the fol-

lowing, which are listed alphabeticallyrather than as to specific merit: AP-PROVED ELECTRONICS CORP., Mod.A-460; 30-50 microvolts and 100-20,000microvolts. A phone jack is providedfor aural monitoring of the receivedsignal. NATIONAL COMPANY, INC.;a complete 7 -inch TV receiver with amicrovoltmeter added. The actual tele-vised picture, as well as the meter indi-cation, may be observed with this in-strument. SIMPSON Mod. 488; 0-50,0-500, 0-5000, and 0-50,000 microvoltscales. A phone jack is provided foraural monitoring of the received signal.This instrument is exceptionally light inweight. SIMPSON Mod. 351 TV An-tenna Compass; consists of a sensitiveDC microammeter, germanium diodesignal rectifier, and connecting cables.Gives relative field strength readingswhen connected to the video electrodeof the picture tube in the TV set.

TRANVISION Mod. FSM-1; Superhettype with indicating microvoltmeter.

MakeshiftsFigures 1 and 2 show makeshift FS

meter arrangements which have beenused by TV technicians. The first cir-cuit employs a conventional TV boosteras an RF amplifier ahead of a rectifierand indicating DC microammeter. Themeter rectifier is a bridge circuit com-posed of four 1N56 high -conductiongermanium diodes. This rectifier setup

vGLTAGE GAIN1000 MIN

UNMODULATEDSIGNAL GENERATOR50-230 Mc.

HETERODYNE - TYPE METER

1 N34

0-100DC MICROAMMETER

-RTM-

gives maximum meter deflection onweak signals. A calibration chart orcurve, in which microammeter readingsare recorded against RF microvolts in-put, may be prepared by means of astandard signal generator having a mi-crovolt -calibrated output attenuator.

The signal generator output is con-nected to the antenna input terminalsof the meter. The generator outputimpedance must be matched to thebooster input impedance. Generally,the generator impedance will be some-what lower than that of the booster, andsufficient additional resistance (in theform of carbon or non -inductive wire -wound resistors) must be added in serieswith the booster input to make up theimpedance.

The generator output impedancemay be 15 ohms, while the boosterinput impedance is 300 ohms. Theimpedance difference in this case is

285 ohms. In order to match the twoinstruments, one 142.5 -ohm resistormust be connected in each lead to thebooster.

The sensitivity of the setup shownin Fig. 1 is governed entirely by thegain of the booster. Furthermore, thesensitivity usually will decrease as thechannel frequency is increased. Thisnecessitates making a separate calibra-tion for each channel. A complete cali-bration is very necessary, since the mi-croammeter reading will not be exactlyproportional to signal strength. This isbecause the 1N56 response curve is notlinear at the voltage level about whichthis circuit operates. A meter -readingratio of 2 to 1 thus does not necessarilymean that one signal is twice as strongas another.

Heterodyne Detector

A somewhat different arrangement isshown in Fig. 2. This is a silicon crys-tal type heterodyne detector followed bya high -gain audio amplifier, germaniummeter rectifier, and indicating DC mi-croammeter. The heterodyning signalis furnished by an unmodulated signalgenerator tuneable from 50 to 230 Mc.The generator is tuned to beat with adesired TV signal, the audio section am-plifies the beat note, and the 1N34 dioderectifies the audio output voltage andactuates the meter.

In order for this setup to be reliable,the AF amplifier must have constantgain, and the signal generator (or testoscillator) must have constant output

to page 25

6 RADIO AND TELEVISION MAINTENANCE a. MARCH, 1951

GETTING THE

PROBABLYno single piece of tele-

vision receiver equipment is moredisregarded than the antenna. Yet innearly all fringe areas, the antenna is achief cause for complaint about thequality of reception. Where an ordi-nary antenna works well, and the instal-lation is near the stations to be received,a haphazard setup may suffice; but inthe fringe areas, particularly wherethere are all kinds of man-made inter-ference, a top-notch antenna installa-tion is often the difference betweenprofit and loss on a job.

It is assumed here that the reader isfamiliar with the variety of antennatypes, and perhaps owns a good bookon antennas. We will indicate heresome items to look for in connectionwith an antenna installation that willprevent service problems involving theantenna.

Basic Requirements

The fundamental requirements of agood antenna system for TV include:

1. It should be installed so that thereare no reflections in the antenna systemwhich would cause smearing of the pic-ture or distortion of the sound.

2. It should be so directed that therewill be no reflected signals from nearbybuildings or metal structures that mightcause multiple images (or ghosts) toappear on the screen, and distortion inthe sound.

3. It should be so placed that thepickup of interference from man-madesources of TVI is minimal.

4. The system should be sufficientlywell designed that it is capable of pro-viding good signal pickup on the lowTV band and the high TV band. Thisis one of the most difficult requirementsfor an antenna to meet.

It Makes the DifferenceBetween Profit and Loss

MOST OUT OF THE ANTENNA

By DAVID T. ARMSTRONG

Bear in mind that nearly 90 percentof reception difficulties in the first 30days are traceable to losses, distortions,and interferences which affect the in-coming signal before it reaches the firstrube in the RF or converter stage.

The television antenna is like the sec-ondary of a transformer, of which thetransmitting tower is the primary. Takea typical transformer and measure theamount of voltage induced in it as yourotate the secondary through 360 de-grees. Obviously, position of the sec-ondary of a transformer is important inconnection with the induction of a de-sired voltage; likewise the position ofthe antenna is most important sincenothing can be done to reduce the dis-tance between the primary and second.ary. It then becomes exceedingly im-portant to position the receiving an-tenna so that the maximum signal isobtained.

Tall Mast ImportantIt is for this reason that a steel struc-

ture between an antenna and the trans-mitting tower may become the second-ary of the transformer and absorb muchof the signal that should go to the an-tenna. There are ways of getting aroundthis in some cases, but none is better

300 OHM TRANSMISSION LINE

SHORTED PARTOF LINE

TTANTENNA TERMINALS

1/4 WAVE AT CHANNEL 4 FREQUENCY FOR LOWBAND, WITH 5µµt CAPACITORS.

1/4 WAVE AT CHANNEL 10 FREQUENCY FOR HIGHBAND, WITH 2µµt CAPACITORS

FIG.1 -RTM-

than erecting an antenna high enoughto get the signal over the interveningstructure. Where this is impossiblesome less satisfactory method will haveto be tried.

Resonant Antennas

In modern television, resonant an-tennas are the type generally used be-cause for a given field strength themaximal voltage will be developed inan antenna that is resonant to the fre-quency of the desired signal. Thismeans, of course, that the best receptionwould be attained with, say, seven sep-arate antennas for seven separatechannels; but this is a ridiculous an-swer to the problem.

An antenna has a certain amount ofinductance, capacitance, and resistancewhich help it to act like a resonant cir-cuit. The inductance and capacitanceare not lumped as they would be in asingle coil or condenser, but rather dis-tributed along the length of the an-tenna. The ungrounded antenna thenwill be resonant whenever its totallength approximates a multiple of thehalf wave length of the frequency towhich the receiver is tuned.

A resonant circuit with a low Q willhave a broad response. The Q of theantenna should be sufficiently low sothat the frequency response is broadenough to cover at least seven channels,separated as far as channel 2 and 13.The Q of the resonant circuit for theantenna depends upon the diameter ofthe conductors in relation to theirlength. When the diameter is large,the inductance per unit length decreaseswhile the capacity per unit length in-creases. This means that a decreasedL -C ratio causes the Q of the equiva-lent resonant circuit to become low.

RADIO AND TELEVISION MAINTENANCE * MARCH, 1951 7

Conversely, when the diameter of theconductors is small, the L -C ratio is in-creased, with a resultant increase in theQ of the resonant circuit.

A half wave dipole then, made withquarter wave elements using two-inchdiameter tubing, will operate over a

much wider frequency range than a di-pole made with /8 -inch tubing. Toomany installation men use the cheaperproduct without regard for the effectthis may have on the range of fre-quency that must be covered. Whenyou need range of coverage from Chan-nel 2 to 13, an antenna with 1/2 -inch or3/4 -inch tubing will be more effectivethan one with thinner tubing.

Antenna Selection

The particular type of antenna usedand the transmission line connecting itto the receiver will vary from installa-tion to installation, depending upon thelocal conditions. Some of the basicfactors which influence the antenna se-lected and its installation are:

1. The exact location and height ofthe array. Use a sensitive fieldstrength meter to measure signalstrength, if it is at all possible, andfind the best spot for the tower.

2. The man-made noises originatingnear the antenna which would be likelyto affect the transmission line.

3. The possibility of multiple signalpaths that may be picked up by theantenna, the transmission line, or thereceiver itself.

4. Whether the high and low bandchannels are located in the same generaldirection from the antenna, or are inwidely separated positions.

General Recommendations

The following suggestions grow outof much long and hard-earned experi-ence with transmission lines:

1. If unshielded transmission line isused, it should be of the outdoor typeto prevent excessive losses as a resultof weather conditions.

2. Avoid running the line inside anytype of conduit as this will change thecharacteristic impedance.

3. Avoid splices in the line; run asingle continuous piece of transmissionline if it is possible.

4. Caution the customer not to paintthe line, particularly with a lead -base oraluminum paint.

5. Run the transmission line through

a hole in the window casing or througha hole in the wall of the house.

6. If there is a run of more than fivefeet inside the house, particularly a rununder a rug, use the heavy outdoor typeline. This type may be run up to 20feet under the rug without adverselyaffecting impedance.

7. In running the indoor line alongmoldings, use standoff insulators. Besure to keep the indoor transmissionline away from radiators, pipes, andmetal objects.

8. Use a lightning arrestor with ev-ery installation. Be sure to use thespecial type for TV which does notchange the characteristic impedance ofthe line and which protects both sidesof the line.

There are other suggestions thatcould be made, but these are basic.Following them is likely to help inmaking a satisfactory installation.

Servicing HintsHere is a quick check on the proper

orientation of a TV antenna. Discon-nect the transmission line from thereceiver and touch one antenna termi-nal and then the other. If there is

much change in the picture received itmay be due to improper positioning ofthe antenna. It helps to make thisrough test with the contrast control setwhere the picture is fairly light.

It is urged that you make it a prac-tice to use two separate transmissionlines for good high -low TV receptionfrom the low frequency antenna andthe high frequency antenna. Both linesshould be twisted one turn per foot inthe conventional manner, should bekept at least 12 inches apart and shouldbe held firmly in place with standoffinsulators spaced about four feet apart.

There may be a point between thereceiver and the antenna terminalswhere a piece of tinfoil wrapped aroundthe transmission line will improve thepicture brightness. Find this spot foreach separate transmission line, whenthere are two, and fix the metallic foilfirmly in place.

Standing WavesStanding waves in the transmission

line, or a mismatch in impedance(caused sometimes by proximity to for-eign objects) may be eliminated some-times by use of a matching stub madefrom about 40 inches of transmissionline. Remove the insulation from theends, baring the wire. Connect two of

the bare wires together, thereby short-ing one side of the transmission line.Connect the remaining two to the an-tenna terminals where the antenna leadsare. If there is improvement in picturequality, shorten the stub an inch at atime until there is no further improve-ment in picture quality. The standingwaves in the line will then be dissi-pated. A simple test for the existenceof these standing waves in connectionwith this matching stub is to squeezethe transmission line between the fin-gers at six-inch intervals from the re-ceiver terminals to about five feet fromthe receiver. When there is no im-provement in the picture upon squeez-ing the transmission line the standingwaves have been balanced out.

Some types of common interferencecan be minimized considerably by usinga quarter wave stub of 300 ohm trans-mission line with one end shorted.From the other two ends connect a lowvalue capacitor, something like 2 to 5mmf, in series with the transmissionline where it connects to the antennaterminals. This is a type of absorptionwave trap and will have the effect ofpreventing any change in the R -F re-sponse curve, which might result inimpairment of picture detail. (SeeFigure 1.)

Inevitable Losses

A survey of the antenna installationsin any given neighborhood will revealto the practiced eye of a careful work-man that there are many sloppy andinadequate installations which are rob-bing much of the signal strength pres-ent and preventing it from getting tothe receiver where it will do somegood.

Consider the losses that are inevitablein a good installation. Perhaps thesefigures will help the serviceman takehis work a little more seriously. In anarea where the field strength is of theorder of 5000 microvolts there will belosses in a good installation of suchmagnitude that only about 1000 micro-volts will be delivered to the antennainput terminals. These losses are pri-marily the result of the connection ofthe antenna to the transmission lineand the connection of the transmissionline to the antenna terminals. If it werepossible to connect the antenna directlyto the antenna terminals the loss wouldstill be about 50 percent. For good and

± to page 28

RADIO AND TELEVISION MAINTENANCE MARCH, 195T

StrikeTech'. e

INGt AREA SERVICINGProblems May Often Tax Your Ingenuity

But They Can Be SolvedBy RUDOLF F. GRAF

ALTHOUGH television receivers,upon leaving the factory, are prop-

erly adjusted to give satisfactory recep-tion on all channels, it is still the ser-viceman who makes it possible, by hisinstallation, to get pleasing and depend-able reception at the final location ofthe receiver. The installation of setsin areas more than about 40 miles fromthe transmitter, in the fringe areas, pre-sents a number of problems. These dif-ficulties, though they may at times taxthe ingenuity of the television techni-cian, can be solved.

The factors which are involved in theproper operation of the receiver are thestability of the sync circuits, signal-to-noise ratio, proper definition, interfer-ence and sound. The elements involvedin obtaining peak performance in fringearea installations are: the antenna, thetransmission line, the booster amplifier,and the receiver itself.

Signal StrengthLet's see, first of all, determine what

we can expect in the way of signalstrength in a fringe area location. Thepresent allocation plan as set up by theFCC specifies a field strength of 5,000microvolts per meter for a primary serv-ice area, and 500 microvolts per meterfor a secondary service area. The pri-mary service area is generally foundwithin a radius of ten miles from thetransmitter, and the secondary servicearea between that limit and the 30 -mile mark. Within these confines mostcommercial receivers will yield satis-factory pictures with the usual installa-tion.

We shall consider the problemsencountered when the field strengthdrops to values of about 50 microvolts.

RUDOLF F. GRAF

Field strength is the intensitynal at a certain distance from the trans-mitting antenna. This intensity is a

measure of the number of microvolts ofthe signal, from any one particular sta-tion, which would be intercepted by anantenna exactly one meter (3.281 ft.)long. Hence the term 'microvolts -per -meter." Usable pictures have been pro-duced on the screens of television re-ceivers in areas where the signal strengthwas as low as 3 microvolts per meter.

The AntennaThe first and most important link of

the fringe area installation is the an-tenna. It must have as high a gain aspossible and, if interference from otherstations is a problem, be sufficiently di-rective to pick out only the desired sig-nal. There are many good antennas onthe market, which may be stacked forhighest gain. We shall not go into allthe available types, but restrict our dis-cussion to three antennas which are par-ticularly suitable for fringe area work.These antennas are most effective foronly one channel. To use them, a sep-arate antenna is generally required for

of a sig-

each channel. These may be employedin addition to an antenna which is usedonly for other nearby stations.

Yagi AntennaThe Yagi is a very good fringe an-

tenna. Because of its flatness and sim-plicity, it can be easily stacked forhigher gain. Its high front -to -backratio makes it ideal for rejection of co -channel interference. Several Yagis canbe mounted one on top of the other onthe same mast, each cut and orientedfor a particular channel. For best fringearea results, the antenna should be cutat or near the picture carrier. If thesound comes in too weak, the front di-rector may be shortened by about fivepercent to improve it. As many as fourYagis may be stacked for higher gain.

Corner ReflectorThe corner reflector has an excellent

front -to -back ratio (100:1), a narrowbeam angle, and a gain of approximately10 Db over a simple dipole. The drivenelement is mounted a half wavelength,or slightly less, from the vertex of thereflector and the reflector elements arespaced one -tenth wavelength apart. Thetwo groups of reflector elements aremounted at an angle of 90 degrees andmay be connected together electrically,although that is not necessary. Thetransmission line should be run out atthe rear of the reflector, in order to keepthe whole system as symmetrical as pos-sible. The antenna should be used witha 300 ohm line and is generally satis-factory for reception on only onechannel.

Rhombic AntennaIf there is sufficient space available

for its erection, the rhombic antenna is--->- to page 21


`New

Developments`,

Techniques Used hyZenith In Its TestSystem for

The Phonevision decoder unit, which is attached to the chassis of the subscriber's TVset. This supplementary unit is shown as part of the receiver block diagram below.

"BOX OFFICrTUIVISIONBy RICHARD L. BROWNE

IF TELEVISION broadcasting, as weknow it, cannot provide the pro-

grams the American public wants tosee, some form of box-office TV mustbe found to supplement the presentcommercial sponsorship of commercialshows."

Those are the words of Wayne Coy,chairman of the Federal Communica-tions Commission, the group that willdecide, in a few months, whether the"Phonevision" system is sufficientlyworthwhile for commercial licensing.

At the moment, of course, Phonevis-

R.F.AMP.

SOUNDIF.

I STDETECTOR

MIXER

LIMITER

ion, which is the form of subscriber -television backed and developed by theZenith Radio Corp., is under test in 300homes in and around Chicago. Thistest, which is claimed to be bringinggood results all around, will last untilMarch 31, for a 90 -day experimentamong a sample of the population.

If the first four weeks' results are anycriterion of the eventual acceptance ofthe system, it is fairly plain that it willbe a success. For the families concerned"bought" movies over television sets onan average of just less than twice aweek for each installation during thisperiod; and each time, it cost a dollar

OSCILLATOR

VIDEO I.F.

DISCRI-MINATOR

A GC.

VIDEODETECTOR

AUDIOAMP.

PULSECLIPPER

SYNCSEPARATOR

HSYNC.

VIDEOAMP.

PVDECODER

< SPEAKER

<PICTURETUBE

H

SWEEP

I -CODED DRIVE

V

SWEEP

KEY INPUT

to see the program. The movies, tomake the results more interesting, wereall at least two years old, and were com-peting with the latest releases beingshown in the city's theatres. In addi-tion, about 20 percent of the audiencehad already seen the pictures.

Decoding UnitsPhonevision installations, at the re-

ceiving end, consist of decoding unitswhich straighten out the scrambled pic-tures sent from the TV transmitter.A decoder is a small supplementarychassis installed within the set. Thesignal, which keys the decoding process,comes over the user's telephone wirefrom the exchange station, on a carrierwhich is at a higher frequency than theaudio band of the telephone, so thattelephone conversations are not inter-fered with. A cable from the telephoneinstallation to the television receiverbrings the decoding signal into the de-coder unit, while the main section ofthe broadcast signal comes throughfrom the transmitting antenna in theusual way.

The code signal, which originates atthe transmitter as the transmitted signalis being regulated, is distributed throughthe telephone exchange just at the timespecified by the subscriber. As out-lined by Zenith, the subscriber woulddial a certain number in advance of theprogram he wished to see. The figures


Part of the Phonevision coder at the trans-mitter. The "heart" of the system, the random -

noise tube, is circled.

which he dialed would identify him forlater billing purposes, and at the sametime would set the exchange mechan-ism to send through the decoding sig-nal throughout the duration of the pro-gram he wished to see. This procedure,however, is only the basic one. Addi-tional details will be decided on whenand if the system is prepared for full-scale operation.

Traffic ProblemsCertain problems of traffic -handling

would arise, as might be expected, withthe prospect of a very popular programwhich would be requested at a largenumber of receivers throughout the lis-tening area. These difficulties, it is ex-pected, would almost solve themselves,if the customers were willing to requestprograms more than a few minutes be-fore they started.

The code signal itself, which clearsup the scrambled picture at the receiverwhile it is being jumbled at the trans-mitter, shifts the video information inthe signal with respect to the horizon-tal synchronizing pulses, so that part ofthe time, the video seems normallyphased with respect to horizontal pulses,and the rest of the time, there is a

noticeable phase shift. These two con-ditions, or modes, are interchanged ata random sub -field rate. Thus, the pic-ture may be normal for several fields,then out of phase for several more. Thechangeover from one mode to the otheris determined by a noise source, so thatit will be entirely random, and thus sec-ret. If the coded signal is tuned in ona receiver which has no coder, the pic-ture moves back and forth horizontallyon the screen, at an irregular rate.With a decoder in operation, the keysignal causes the circuit to compensatefor the phase shift.

To allow for small variations in thenormal delays occurring in the trans-mission of the key signal over the tele-phone line, the key is sent from thetransmitter about one field before it isused at the receiver. Synchronizationof the keying signal with the properfield in the receiver is accomplished bycombining the key with a pulse fromthe vertical sweep. Thus, the mode ofa particular field is determined by thepresence or absence of the keying signalwhen the vertical pulse appears.

120 -Cycle BandwidthThe bandwidth, then, which is needed

for the transmission of the key is about

Antenna of Zenith's experimental Channel 2TV station in Chicago, center. At left is the

company's FM station antenna.

120 cps. The highest possible keyingrate, or change from one mode to theother, is 30 per second.

Extra equipment needed at the trans-mitter for sending Phonevision, that is,the coder, consists of a unit inserted be-tween the convention sync generator'shorizontal section and the horizontal

sweep and shading sections. In addi-tion to shifting the video phase, thecoder also adds "jittered blanks" to thesignal, for the purpose of eliminatinglow -frequency modulation of the signalcaused by the changing phase relation-ships of the video and the steady blanks.While the coder is producing thesevariations in the transmitted signal, itis also sending out the keying signalfor receivers.

Noise GeneratorThe first section of the coder unit,

the random key generator, is activatedby the normal transmitter vertical drive.The vertical signal is mixed up with therandom noise pulses in a blocking oscil-lator circuit, after the noise signal is

fed through a limiter for regulation.The blocking oscillator can be set tooperate between 60 and 20 times persecond. In connection with a multi -vibrator, alternate signals from theblocking oscillator give square -wavecontrol signals. The square wave con-trols an oscillator which operates at thecarrier frequency of the key signal. Theoutput from this stage is fed to thelink which connects to the receiver.

The oscillator output also is fed tothe part of the coder which generatesthe horizontal drive for the camera,monitors, and shading generators at thetransmitter. A gating circuit, actuatedby the oscillator signal plus the verticalpulses, controls a single -trip multivi-brator. Application of the pulses,which are positive in the presence ofthe key signal and negative in their ab-sence, to the multivibrator, causes achange in the mode of operation. Thechange occurs at the time of the first

± to page 28

The scrambled TV picture as it is received without the decoding signal is shown at left. Videoinformation and horizontal sweep are out of phase port of the time, producing a picture which

shifts back and forth horizontally. Right, normal picture when the decoder is operating.


New

elopmenls, MOUNT ATTOE Of TOWN

MOUNT AlYSI OF TOWN

10 S "1-4- CNANN. RECD., AT ANTENNAS

14. 1-0 1-10. ANITENNAS,- 0011. STACKED YGIS

MINIMUM ANTENNA SIGNAL WOUIRIO 100 MICRO -VOLTS

WMC-I, MASTER CHASSISA WINA. PITEAMPUTIER STRIP

11114AMPLIFIER HanC= WW -C. 13 W. EWAN/ CONVERTER SRN

humps CHASMSA -WCAd.AMNMME SRN

= WC. 11041. CONWETEEAMPLINEE SNPC= WCON OA, CONVNITERAMPLINEE SRN

0-.S

101,1,40001 MM.

m1=1=1F1VI90

Block diagram of the Jer- 111.1.1. POE WMC-I# ®, (;), 0 ETC. WMC-I, MASTER CHASSIS

rold Antenna system instal-lation for a town. At pres-ent, the system delivers amaximum of three channels,with an individual amplifierstrip for each channel ineach repeater -amplifier unit.

A - WCAA 1.111.111611 SETOWU ANFUEIER

C WCA- AMPUNER SNP

wfirmrtE .11.101 E.EE- . NEEL NumEnAL

EQUIPMENT REQUIRED TO BRING SIGNALS INTO TOWN

RG41.-10001 RG-11.404104E.-30CY ADO ROSY,: -SW

NW MA

MOW MIS44,59. ACC/ MATE TO W 14014

F_._'-WAD0.3 WA0040

EG-30,.-900' WA.. 70 W 111014

SW 1401-T

o- -0 -0-- -1014

Z_ MODEL W ism -rsocAno 1111012400661161 HA. Of W 1401 WITS

MOOS W 1401-1-TEEMMIE°!MAHON UNIT

omy opopygo op.moo. oo.

04.6..= %CO 14011.1.

ALL EU SW. REFER TO ZNIOLIFYR 1000 14101-VOLIS

AU. CAM LOSSES CASCOULTE0 ATCHANNEL 6, ES AICS

EQUIPMENT REQUIRED TO MAKE CONNECTION IN TOWN TO HOMES

Pushing Back the FringeAGOOD many television -less com-

munities are likely to have recep-tion in the comparatively near future,whether the new -station freeze is liftedor not, thanks to a development broughtabout by the Jerrold Electronics Corp.of Philadelphia.

The new system, which operates onthe principle of the usual multiple -receiver installation, like those now usedin many apartment buildings, hotels,and dealers' stores, but expanded to sup-ply as many as 25,000 receiver outletswith usable TV signals, is already inoperation in several locations, whichare between 100 and 125 miles awayfrom the nearest transmitter.

Jerrold Electronics, which manufac-tures the "Mul-TV" multiple receiversystem described in the July, 1950 issueof RADIO AND TELEVISION MAINTE-NANCE, is now prepared to install 150of its large "booster antenna stations,"as they might be called, at costs vary-ing with the size of the installations.

Operation of System

The operation of the installations isanalogous to that of conventional mul-tiple systems. A large tower, with in-dividual antenna arrays cut to the properlengths for the channels to be received,is set up on high ground near the townto be serviced. The signal -boostingequipment is installed at the antennatower, and the signals are then pipedthrough lines to the receiver outlets,the owners of which subscribe, by amonthly charge, to the service.

At the present time, the system isoperating in Mahonoy City, Pa., Astoria,Ore., Bellingham, Wash., and Lansford,Pa.

The Lansford installation, which usesan 85 -foot tower, erected on the peakof a nearby mountain, brings televisionwithin reach of the residents of PantherValley, about 70 miles away from thethree stations in Philadelphia, a locationwhich is completely hidden by sur-rounding mountains from line -of -sightreception.

Business Possibilities

A boon to advertisers, to local deal-ers in TV sets, and to station operators,the installation opens up a new vistaof business possibilities which werepreviously out of the question. AtLansford, the sponsoring company ofwhat is really a community project in-vested $30,000 in the system. Eachreceiver connection to receive one sta-tions costs $100, plus $25 for each ad-ditional channel to be received. Aservice charge of $3 monthly for thefirst channel, plus $1.50 for every addi-tional one, is also made. The grouporiginating the utility, the Panther Val-ley Television Company, expects to getits investment back within six months;from that date, only maintenance ofthe equipment will be required, and therest of all collections will be profitable.

One possible flaw in the system, thefact that the spot commercials used byPhiladelphia advertisers will have littleuse for local listeners, is being worked

out. In the future, it will be possiblefor local advertisers to break into thesignal during the Philadelphia com-mercial.

Double Reception AreasDealers in the town, who had almost

given up hope of ever being able toprofit from the huge public interest intelevision, can now sell nearly all thereceivers they can supply. Since thefirst news of the system was revealed,groups of dealers throughout the coun-try have shown great interest in thesystem. Many station operators feelthat the arrangement could double theireffective listening areas, if enough ofthe installations are put up. Accord-ing to Jerrold President Milton J.Shapp, it is possible to set up thebooster stations as much as 250 milesfrom the transmitters, thus bringingsignals into thousands of unexploitedhomes. For practical purposes, the in-stallations could be thought of as en-tirely new TV stations, operating onnetwork bases, which neither requirethe heavy expenditures necessary for set-ting up conventional broadcasting sta-tions, nor violate the FCC rule againstnew construction.

Supplementary BoostersTo maintain the high signal level

reached after the received signals areput through the initial amplifier, whichprovides gain of about 500, the cableson the route from the antenna tower

-->- to page 28


BUILD THIS SIMPLEHOME APPLIANCE TESTER.

By HARRY F. LEEPER

SINCE most radio and television tech-nicians are often called upon to do

some work on home appliances otherthan receivers, it is a good idea to havea test tool that will do the job withouttying up the conventional shop equip-ment. Here is an appliance tester thatwill do much of the work coming intothe shop, and that will do some thingsthat could not be done with radio andTV instruments, without a number ofspecial adapters.

This piece of equipment, which canbe easily and cheaply assembled fromparts found in the shop, consists of anAC voltmeter, a standard two -outletfuse block, a dummy female receptacle,and a pair of iron prongs, like thosefound on the older flatirons or otherdevices.

Mounted on a sheet of press board,and fitted into a woden case, the socketsof the fuse block are connected in series,while the voltmeter is hooked up across

The complete home appliance tester in its case. ACvoltmeter is optional, may be replaced with a pilotlight to show when the tester is activated. Dummyreceptacle in the lid of the case holds power plug when

it is not in use.

the line voltage input. The dummyreceptacle, left open, is mounted on thebox cover, and rile

Prongsprovide

pl.( >figs, connected

connected together behind the panela test for appliance cords suspected.

of open circuits.

together in the rear, are mounted onthe press -board panel.

For use, a screw -in receptacle is in-serted in one outlet of the fuse block,

Appliance cord is tested for short circuits, below, left. Twisting andfolding the cord will reveal an intermittent short if it is present. Whenthe short contacts, the lamp will light. In the center, a heater unit,which draws about 10 times as much current as the lamp, is inserted

and a 40 or 60 -watt lamp is screwedinto the other socket. The line cord isplugged into a power outlet, and themeter reads the line voltage, serving asan indication that the unit is energized.

To check an appliance cord, the cordis plugged into the receptacle. Twist-ing and folding the cord will reveal in-termittents, since a short circuit willlight the lamp. To find an open in thecord, the female end is pushed ontothe shorted prongs mounted in thepanel, and the male plug is inserted inthe fuse -block receptacle. If the line isopen, then the lamp will not go on. Ifthe lamp lights up, however, the cordis proved to be good.

For particularly stubborn cases of in-termittent operation, a heater unit canbe substituted for the lamp. The heaterunit, drawing about 660 watts, puts agreater load on the cord, of course, andis more likely to reveal the break.

± to page 29

in the socket to put a greater load on the suspected cord. At right, aflatiron is tested and found to be good, since the lamp lights up. Inthis test, care must be taken to avoid shock at the bare prods of the

test leads, which are in series with the power circuit.

RADIO AND TELEVISION MAINTENANCE MARCH, 195113

Design and Service Data on

TELEVISION PHAMPLIFIIRS

Parts arrangement in a typical all -channel TVbooster. Note selenium rectifier and use ofa single tube, a duo -triode used for two -stage

amplification.

THE fact that best reception of tele-vision stations can be had over a

"line -of -sight" path has challenged radioengineers to find means of extendingthis range over the horizon.

One approach to increasing the num-ber of elusive microvolts which getdown the lead-in and into the first stageof the TV receiver has been to erectcomplex multi -element antenna arrays.A second approach has been throughthe development of antenna preampli-fiers.

In addition to the signal increase, anantenna booster for TV receivers willadd to the selectivity of the set, so thatbetter image rejection can be effected.This will result in reduction of inter-ference from stations in the 88-108 McFM band and from amateur and otherRF signals which may, through hetero-dyning with the TV set's local oscillator,cause interference with the picture. Afurther advantage of the added selec-tivity is the reduction of interferencefrom local oscillators of nearby tele-vision receivers.

By NORMAN L. CHALFIN

Different Circuits

Many different circuits have been em-ployed in TV preamplifier application.One of these, shown in Fig. 1, employsa 6AK5 or 6AG5 pentode RF ampli-fier with a standard L -C circuit as thetuned element. This tuned elementmay be either on the grid, as shown, onthe plate, or on the cathode. The oper-ation of the tuned -grid and tuned -platecircuits are familiar to most technicians.

One feature which is often used inTV boosters is a resistive -load balancedinput as in Fig. 3. This arrangementreduces the likelihood of standingwaves, which mar the clarity of theimage.

Narrow BandIt will be obvious by this time that

while the circuit shown is a good am-plifier with considerable gain, the use oflumped constant L -C arrangements withvariable capacitor does not allow a widerange of tuning. To cover the rangesfrom 50 to 88 Mc and 174 to 216 Mcfully, one must switch for two tunedcircuits, each covering one of theseranges. A ratio of two to one for tun-ing is the most practical obtainablewith L -C combinations employing avariable capacitor. A further featureof L -C circuits is their higher Q with

Under chassis view of the booster. Twelve -position selector switch can be seen, with coilsattached. Short connecting leads are particu-larly important in replacing any parts of TVboosters, since losses and noise are easily

introduced.

the consequent narrow band pass char-acteristics.

This narrow band pass can, however,be used to advantage for those sets thathave appreciable dips in their IF band-pass characteristics. The booster canmake possible selection of a frequencyband to fill in the dip; this may notnecessarily provide much gain althoughpicture fidelity may improve. A bandswitch, however, can cause considerablecircuit losses, reducing overall efficiency.Thus a lumped -constant booster circuitwith a variable condenser and 2 -bandswitch will have less gain than a boosterof the same type for each band. Somecommercial boosters have a cut outsystem wired to the on -off switch, asshown in Fig. 2. This is a five -pole,three -position switch. The third posi-tion is for switching high to low band.

High -Low SwitchThe high -low band switch can be a

second wafer on the same shaft withthe three -position, five -pole switch. The


D pole

To Receiver

Iloy

FIG.2

5 Pole3 Position

-I- H, Bond-2- Lo Bond

o -3- Of f

Input

Output

To PowerFor Booster

-RTM-

second wafer has two poles and threepositions. All the wiring necessary forthese switching operations is likely toresult in more losses. In spite of this,however, the amplifiers will still haveuseful gain.

Fig. 3 shows one way out of the prob-lem of switching in a unit employing a6J4 tube. If the 6J4 is not used, a6J6 with the elements of the two tri-odes tied in parallel operates in thesame circuit. A pentode could be used,too, but this requires more parts. Thissame arrangement is sometimes used ingrounded grid application for coaxialinput. In that case, position 1 is tied toground at the input and the coax goesbetween position 2 and ground. Theresistor R2 should be equal to the coaximpedance for the coax application.

Wiring PrecautionsWhen servicing boosters like that

shown in Fig. 3, the usual wiring pre-cautions should be observed. Theseapply as well as to all the other circuitsdescribed here. All grounds should bewired to a common point. This can bevery practically accomplished by bring-ing a piece of bus bar up from the cen-ter shield on the bottom of a seven pinminiature socket. All resistors and con-densers that go to ground should thenbe brought from the socket terminal tothis ground.

In working on the coils to the selectorswitch, it is important that all con-

nections of the coils come as close aspossible to the switch terminals. Thecoils which provide selection of chan-nels from 13 down to 7 (6 turns) canbe a single coil stretched so that eachturn touches one switch position. Thebalance of the coils are wired as closely

as possible to the switch. With someswitches just a single strap of wireforming the 7-13 channel tuning coilcan be arranged, as in the early RCATuner.

Each individual coil should be per-fectly concentric for best results. Sometuning adjustment may be required.This is accomplished by spreading orsqueezing the turns together. Squeezeto lower the resonant frequency. Spreadto raise the resonant frequency.

Wiring should be point to point andall leads should be as short as possible.Where a greater length is necessary, itshould be allowed only at the groundor end, the shortest possible lead goingto the hot side. This applies, of course,

as a general rule for wiring all high fre-quency apparatus.

Tuned Grid -Tuned Plate

The tuned circuit of Fig. 3 can, as wementioned before, be employed in thegrid circuit as well as in the plate cir-

cuit. One commercial booster uses atuned grid, tuned plate RF amplifieremploying additive inductance tuning.The tube is a 6AK5. For such a circuit(Fig. 4), it is important that the gridand plate tuning elements be isolatedfrom one another. This can be done bysetting a shield between the switchwafers and wiring. (This is employedin some Jerrold boosters.)

Many other arrangements are possiblefor boosters involving more than onestage. Since impedance matching withinductances over the wide range re-quired for television receivers is rathera complex matter a resistive match isdesirable. Photographs are shown ofthe mechanical arrangement employedin such a booster.

The unit in the photographs wasmade with a selenium rectifier powersupply derived from the line. Thissupply is shown diagrammatically inFig. 5.

The tuning coil assembly for this in-strument is the same as that for the 6J4booster in Fig. 3. This unit, however,uses a 12AT7. It is just as well, though,to use two 6J4 tubes or two 6C4's, withRF chokes (Z-144 Ohmites) in each legof each filament.

The two tube arrangement can oper-ate slightly better, because there is

some loss due to the degenerative actionof feedback coupled through the com-mon filament of each section of the12AT7.


Some of the inter-esting new items be-ing made availablecurrently in the Radioand TV service fieldare presented in thiscolumn. For furtherinformation, write to:Products Editor, RA-DIO AND TELEVISIONMAINTENANCE, P.O.Box 867, Atlantic City,N. J.

WIDE -BAND BOOSTERS

Two new television antenna boostershave been developed by Industrial Tele-vision, Inc. Called the "Autobooster,"the first of the units is an automatic,broad -band unit for a single receiver,while the second device, the "Multi -booster," is intended for use in multipleinstallations.

The Autobooster, which covers boththe TV bands and the FM band, with-out tuning or switching, turns on and

off the receiver through a relay. Withtwo 6BC6's, it is said to give 9x gainon the low TV band, and 5x gain onthe high band. It has provision foreither single or double input, permit-ting separate high and low -frequencyantennas and leads to be used withoutswitching.

The Multibooster, which uses similarcircuits to those of the Autobooster, isalso automatic in operation, and con-tains separate high and low -band gaincontrols. Where excessive local oscil-lator radiation is a problem, the makerrecommends use of this unit in conjunc-tion with the Autobooster.

-- T M-TWO-STAGE TV PREAMP

The "Super Skychief," a new, two -stage television preamplifier is now be-ing produced by the Mark SimpsonMfg. Co.

The two -knob control, which is re-ported to prevent overloading of thereceiver, provides exact gain control.When the booster is turned off, the setwill operate through a direct connectionto the antenna.

Containing eight tuned ciruits, which

the manufacturer claims assure "bal-anced" picture gradation and sound am-plification, the unit is said to give uni-form gain on all channels, with full 4.5Mc bandwidth.

A pilot light and switch are includedin the booster, as is a receptacle for theAC plug of the television receiver. Theset may also be turned on and off bymeans of the booster switch.

-ATM -----

MANUAL ANTENNA ROTATOR

A new television antenna rotator,which is operated manually, is theSelect -A -Beam, developed by Neo Prod-ucts. The mechanism, which is driven

by a worm gear, can be operated byeither an outdoor or an indoor controlbox.

Weighing three pounds, the deviceis equipped with guy wire anchoringlugs, and is claimed to fit "any standard

antenna, whether for home receiver orfor amateur transmitter and receiver."

The rotator turns the antenna through360 degrees. A mechanical meter toindicate orientation is included with thecontrol box. - T M -LIGHTNING ARRESTER

A new lightning arrester with twosawtooth contacts, for standard TV trans-mission line, has just been put on themarket by LaPointe-Plascomold.

Part of the Vee-DX line of antennasand accessories, the new device is sim-ilar to the company's RW-204 arrester,which handles four -wire or two -wireline, but takes only two -conductor line,and is sold at a lower price.

- T -REDESIGNED BOOSTER

In order to cope with allocation dif-ficulties, the Regency TV booster hasbeen redesigned. The new model,called the "DB 410" by the manufac-turer, is similar in appearance to earliermodels.

The unit containsfilar coils with push-pull triode connec-tions for balanced output. Internalimpedance match accommodates either300 -ohm twinlead or 72 -ohm coaxialline. Bandwidth, according to themaker, I. D. E. A., Inc., is sufficientlywide for satisfactory operation on 12channels.

The revised booster, like the com-pany's other models, measures 41/2 by

to page 19

contra -wound bi-


In Fringe Areas

Promise Less Than

They're Likely to Get

D0 YOU remember wicked Tanta-lus of Greek mythology whose

punishment in the lower world was tobe forever dying of hunger? Deliciousfruit dangled just above his head, butwhenever he reached forth his hand totake it, the fruit drew back so as toremain always just beyond the tips ofhis fingers.

Many TV fans in the fringe areas re-mind us of this wretched father of ourword tantalize. They seem to feel thatif they can just put another section orso on their TV tower, can just get thatantenna up in the air a few more feet,they will suddenly get perfect recep-tion. My experience, and that of otherfringe -area TV installers does not bearout this fond belief.

Miniature Eiffel TowersIn a true fringe area where it is pos-

sible to secure line -of -sight reception bygoing up fifty feet or so, every extra tenfeet of antenna height may well show amarked increase in signal strength; butin an area such as the one where Ilive-seventy miles from the nearesttelevision transmitter-it would be nec-essary to put an antenna one thousandfeet in the air to approach line -of -sight.While some of the miniature Eiffel

JOHN T. FRYE

towers sprouting from rooftops thesedays make one hesitate to say that sucha dizzy height will never be achievedby some TV fan with a bulging pocket-book, certainly true line -of -sight recep-tion will remain out of the question formost people living at great distancesfrom TV transmitters.

These people have to depend uponthe TV signals coming down to themthrough refraction or reflection of thewave. Sometimes this occurs; at othertimes it does not; but an impressiveamount of evidence indicates that whenthe signal is really coming in, it will bepicked up just about as well on an an-tenna that is fifty feet in the air as onone that is twice that high; and, by thesame token, when the signal is not beingbent or reflected into the receiving lo-cation, neither the fifty -footer nor thehundred -footer will be able to pull it in.

Diminishing ReturnsI'm certainly not arguing that no

height at all is necessary. I contendthat after you have the antenna severalwave -lengths above ground, the law ofdiminishing returns sets in very rapidlyand makes shoving the signal -sniffer onup into the stratosphere scarcely worththe added cost, trouble, and danger. Ofcourse, when raising the tower -height afew feet enables the antenna to clear thetop of a nearby building or power linedirectly in the path of reception, that isa different question.

It must be remembered that raisingthe antenna necessarily increases thelength of the lead-in and the accom-panying lead-in losses. There must,then, be a really substantial increase insignal -interception at the antenna toinsure a noticeable increase in signaldelivered to the set. That is why ex-periments with a telescoping mast arelikely to be quite misleading unless thelead-in is shortened each time the an-

tenna is lowered. It is unfair to saddlethe lower antenna positions with theunnecessary lead-in losses of the greaterelevation.

We all have experienced cases whereraising or lowering the antenna a fewfeet has resulted in marked improve-ment in reception, but I look with con-siderable doubt on the theory that thisproves TV signals exist 'in the form of"space loops," "ether swirls," or "strengthlayers." It seems much more likely thatthe change in antenna height results ina change in its impedance, producing amore favorable match at either the an-tenna or receiver. Possibly a standingwave on the lead-in moves up or downso as to deliver more power to the set.

Critical HeightsThen, too, hams have long known

that there are critical heights above vir-tual ground (not necessarily the earth -level) that greatly favor reception ofsignals arriving at certain vertical an-gles. It is quite possible to shift theelevation of the antenna a few feet,especially when it is not too high tobegin with, so as to throw out the welcome -mat for a signal coming in on aparticular "glide path." Moreover, ifthe signals consistently arrive at or nearthat angle, the shift in elevation caneasily produce a permanent improve-ment.

"Standard" InstallationA local TV installer, with nearly a

hundred fringe -area installations to hiscredit, has done considerable experi-menting on his own. He told me thathe has just about "standardized" on aninstallation system for this weak -signalarea. The antenna is put up about 50feet above the ground, the tower usu-ally mounted on the roof. The lead-inpasses directly through a small hole

± to page 30


TV SERVICE

"T ELEVISION SERVICING," a newhandbook covering complete pro-

cedures in the maintenance of receiv-ers, has just been put out by McGraw-Hill. In simple language, the book,which was written by Solomon Hellerand Irving Shulman, shows exactly howto locate and correct the commontroubles encountered in TV work, withtest pattern analysis, installation, effectsof component breakdown, and othersections devoted to specific aspects oftelevision.

Priced at $6.00, the volume contains434 pages.

Troubles outside the receiver itselfare also covered, with explanations ofthe causes and cures of many types ofinterference, noise, weak signals, exces-sive signals, and other difficulties. Inthe parts of the book which deal withinternal faults, the book gives directionson isolating the trouble to sections ofthe set, then on narrowing the possi-bilities to the exact part which isdefective.

-R T M -SERVICE BUSINESSHOWARD W. SAMS & Co. is the

publisher of "Making Money inTelevision Servicing," a new work de-scribing, as the title suggests, methodsof finding out the necessary economicfacts that are required for the success-ful operation of a service shop.

The technical requirements, the stateof the industry, the nature of businessprospects in particular areas, and manyother factors are of the highest impor-tance in maintaining a business. Thisbook shows the reader just how to findout for himself what procedures heshould follow for profitable operation.

Among the information included ismaterial on contractual relationshipsset up between dealers and service orinstallation agencies, as well as thosemade between consumer and servicecompany.

Pitfalls to avoid in business, as wellas sound procedures to follow, are out-lined. As the author, Eugene Ecklund,presents it, the material may well pre-vent some of its readers from beingforced out of business.

With 136 pages, the book sells forS1.25. -N. L. C

VTVM BOOKLET

DIFFERENT types of vacuum volt-meters, their adjustment, operation,

and use are covered in a new book re-cently prepared for Sylvania Electric byRufus P. Turner.

The 48 -page booklet will be availableto dealers buying Sylvania TV picture

tierticioo Radio

and Trinisiona Iacono' Tithe

lolloieter

tubes through distributors, from April1 to May 31. After that date, it will besold at $1.00 per copy.

Divided into five chapters, the bookgives radio receiver, audio amplifier, andtelevision receiver tests and measure-ments. Full title of the book is "Servic-ing Radio and Television with a VacuumTube Voltmeter."

-R T 81-TV ACCESSORIES CATALOG

LAPOINTE-PLASCOMOLDCorp. has

1--' just issued its new 1951 catalog oftelevision antennas and accessories.

In addition to detailed descriptions oflightning arresters, a three-way switchbox, an antenna impedance matchingdevice, and the company's line of an-tenna towers, a section of general infor-mation for the servicing trade is in-cluded. Installation instructions, tech-nical specifications, and other materialis included, relevant to the antennaspresented, along with information onordering. -R T -GE REPLACEMENTS

REPLACEMENT parts for sets pro-duced between October, 1945, and

December, 1950, by General ElectricCo. are listed in the company's new

"Radio and TV Replacement Parts Cata-log," a 144 -page, cloth -bound volume.

A feature of the new compilation is across reference between drawings andpart numbers. Also included are twochange -of -address cards, to facilitatemailing of supplementary loose-leafsheets.

Among other features of the catalogare alphabetical listing by part num-ber, complete part descriptions, setmodels in which the various parts areused, list prices, and a revision service.

-ATM-AUDIO EQUIPMENT

TWO NEW editions of semi -tech-nical booklets by G. A. Briggs, on

audio equipment have just come out.The third edition of "Loudspeakers-

the How and Why of Good Reproduc-tion," thoroughly covers the subject ofloudspeaker operation, with chapters ofimpedance, sound levels, frequency re-sponse, power, resonance and vibration,and other subjects of equal interest toaudio fans. Illustrated with many dia-grams, the book also gives details onbaffles and speaker housings.

"Sound Reproduction," in its secondedition, is Mr. Briggs' explanation ofthe sound field to music lovers who arenot technically educated. The handlingof the subject is complete enough thateven technicians will be able to learnmuch -from it.

Both booklets are available from theBritish Industries Corp., New York.

-A. G. C.--RTM--

COMPONENTS CATALOGcTACKPOLE CARBON COMPANY," Electronic Components Division, re-cently issued a 42 -page catalog includ-ing descriptions of both new and al-ready -established components for thetrade.

In addition to standard lines of re-sistors, fixed and variable, the itemsmentioned in the catalog include lineand slide switches, iron cores, chokeforms and capacitors. Among newlines shown are various controls,switches, non-metallic "U" and "E"cores, width control and segmented de -

to page 28


Products forthe Trade--)"- from page 16

51/2 by 33/4 inches, and utilizes a singletuning knob. One exterior change isthe addition of a new gold -coloredpanel for the dial face. There has beenno change in price.

-R T M -ROTARY SWITCH

Solid silver contacts are included in anew rotary switch developed by Leedsand Northrup Co. Contact resistance inthe unit, which is designed for precisionapparatus, is about .001 ohm.

Electrical strays are also low in theswitch, the manufacturer states.

Available in combinations from sin-gle pole, 12 position, to 6 -pole, 12 posi-

jtion, the switch, which is a shortingtype, has an adjustable detent, with ascrew for setting torque to the exactpressure desired.

Current handling capacity is one am-pere at 110 volts, 60 cycles. The unitis tested for 100,000 revolutions.

R T M -

OSCILLATION ELIMINATOR

4Tnacani

Barkhausen oscillations in horizontaloutput tubes can be eliminated with anew device put out recently by Perfec-tion Electric Co., the "B. 0. Eliminator."

The eliminator, a magnet -clip device,

which is slipped over the offending tube,is reported by the maker to remove theeffect, in most cases, of Barkhausen os-cillation, which is radiated from thescreen grid of the horizontal outputtube to the television set's front end,and which causes vertical black bars toappear on the picture. The effect ismost noticeable in fringe areas.

For installation, the unit is put onthe tube, and moved up or down androtated until the bars disappear fromthe picture. -R T M -SINGLE -STAGE PREAMPLIFIER

Continuous, one -knob tuning is afeature of the new booster just devel-oped by Standard Coil Products Co.High gain on all channels is claimedfor the unit, with the single stage oper-

ating at a low noise factor. Printed cir-cuits are used in the booster.

The device is made in a dark -brownplastic cabinet. It is called the "B-51Standard Booster."

R T M

17 -INCH RECTANGULAR

The Tube Department ofjust announced production ofmetal, rectangular picture

RCA hasa 17 -inch,tube-the

first metal rectangular to be put onthe market.

Labeled the 17CP4, the new tube hasa high voltage rating of 16 Kv. De-flection angle is 70 degrees on the di-agonal, and 66 degrees horizontally.The ion trap requires only a single -fieldexternal magnet. Picture area is 145/8inches by 11 inches.

Use of metal for the shell, the com-pany says, permits use of higher qual-ity glass for the face plate than is pos-sible in all -glass kinescopes. The faceplate is made of frosted Filterglass.

The tube's overall length is about 18inches. -R T -SHOP LADDER

For stock roomsnew ladder, madeCompany, has been

or service shops, aby the Ballymore

announced.

Mounted on casters which retractwhen a person steps on the ladder, thedevice is made of tubular welded steel,

with expanded -steel steps for non -sliptraction.

The ladder is 30 inches high, 48inches long, and 171/2 inches wide.The top step has a 20 -inch tread, whilethe other steps have 7 -inch treads. Theladder is finished in aluminum paint.

-RTM---NEW WRENCH

Rack-and-pinion is utilized in a newwrench recently introduced. Desig-

nated the "Power -Grip," the tool has anangle head, to afford heavy pressure inordinarily inaccessible spots. Thicknessof the head is Vs inch, while thewrench's overall length is 51/4 inches.The jaw opening ranges from zero tomore than 1/2 inch.

Made of alloy steel and guaranteed tobe unbreakable, the wrench is finishedin a dull black, with polished straps.

to following page


Products forthe Trade

from preceding page

PRECISION POTENTIOMETER

Linearity within plus or minus 11/2percent is claimed for a new precision -built potentiometer now being pro-duced by Clarostat.

Developed to meet growing demandfor precision components, the new con-trols are being manufactured in limitedquantities. With mechanical tolerancesheld as close as plus or minus 0.00025inch, the unit is reported to operate de-pendably over extreme ranges of tem-perature, humidity, altitude and baro-metric pressure, and is treated to with-stand fungus growths and corrosive con-ditions without loss of efficiency. A sil-ver contact is used to ride on the wind-ing and contact rail.

g T m

FIVE -INCH 'SCOPE

The Model 640, a new five -inch os-cilloscope, has recently been put outby Hickok.

With a wide -band amplifier, the fre-quency response of which is zero to 4.5Mc, the instrument has 10 mv-per-inchsensitivity on the vertical DC and AC

amplifier, with sensitivity switch inhigh position, and 25 mv-per-inch char-acteristic with switch in low position.

Input impedance is two megohms,while direct connections to both hori-zontal and vertical deflection plates areprovided for.

Linear time base from two cps to30,000 cps is available, with either re-

current or driven sweep.Modulation on the "Z" axis so that

15 volts capacitatively coupled to thegrid of the CR tube will blank the tracefully at normal intensity is possible.

R T M -

RESONANT CAPACITORSReduced cost and improved IF filter-

ing are features claimed for new res-onant capacitors put out by Aerovox.The units, which act as series resonantcircuits, are designed primarily for by-pass use.

The resonance, which occurs in the

425-485 Kc range, is achieved by wind-ing sections of the foil so that induct-ance is increased. With a certain place-ment of the tabs, the resonance is con-trolled to fall in the IF band.

The condensers are available in threestandard values -0.05, 0.1 and 0.2 mfd,400 WVDC. -RTM-

MINIATURE TUBE CAP

For use with 1X2 or similar Imma-ture tubes, a new high voltage, low -losstube cap, with the wire molded intothe plastic cover as an integral unit,has been developed.

Described by the manufacturer, Al-den Products Co., as the 90 ISTL, thecap has a long skirt designed to pre-vent danger of flashover from cap tochassis when operated at high voltages.Minimum strain on leads is said toresult from the insulating molding,which is made of flexible polyethylene,while the design of the contact itself,with two smooth curved surfaces, is

claimed to reduce the effect of corona.-R T M-

DEVELOPMENTS IN THE NEWSNew Tube for Color TV Shown

Los ANGELES-The newest inventionfor use with the CBS color televisionsystem, a tube using a rotating colorphosphor disk within the envelope, wasshown here recently by its inventors,Eugene Singer and Dr. Irving Rehman.

At the demonstration, which utilizedthe first working model of the tube, theRehman-Singer system included projec-tion of the tube image on a screen.Singer, a television set manufacturer,said that the greater light transmissionof the tube permits projection on ascreen of any size. He said that theprevious arrangement of a disk rotat-ing in front of the tube reduces thelight output by 80 percent.

The tube can also be used for blackand white pictures, the inventors saidin a public announcement. They alsosaid that, due to the war emergency, itwould probably be some time beforecommercial exploitation of color TV ispossible, and that they are working onan adaptation of the unit for militaryuse.

Bureau of Standards DevelopsAccurate, Stable RF Generator

WASHINGTON-Extremely simple de-vices which produce voltages at a verylow impedance and at a wide range offrequencies have recently been inventedand developed by M. C. Selby of theNational Bureau of Standards here.Known as "RF micropotentiometers,"they provide accurate voltages from oneto 100,000 microvolts without the useof attenuators at frequencies up to 300megacycles and above. Thus, conveni-ent standards of low voltages are madeavailable which should greatly reduceequipment and shielding problems en-countered in calibration of present-daycommercial voltage generators, attenu-ators, voltmeters, and other radio -fre-quency equipment.

The micropotentiometers shouldprove especially useful in measurementsof radio receiver sensitivity. Here thelarge disagreement between variousstandard voltage generators at high fre-quencies and low voltage levels hasbeen due to three major causes. First,


generator output impedance and re-ceiver input impedance are not ordi-narily known as functions of changingfrequencies. Second, extreme care isnecessary in using precision voltage -dropping attenuators. Finally, thelong-time calibration stability of vac-uum tube voltmeters is uncertain. Forthese reasons, manufacturers of voltagegenerators have not been able to guar-antee the accuracy of their equipmentat all frequencies. Development of themicropotentiometers now appears tohave removed most of the obstacles tostandardization of receiver sensitivity.

The new instruments consist essen-tially of appropriately housed andmounted current -carrying elements to-gether with means for monitoring thecurrents they carry. Their electricalconstants are simply determined by us-ing known DC voltages and currents.The current -carrying elements are an-nular membranes, either metallic ornonmetallic, of various radii, thick-nesses, and electrical resistivities. Moni-toring may be accomplished by meansof thermocouples, thermoelements, bol-ometers, stable vacuum tube voltmeters,or other devices whose indications areindependent of frequency. Thermoele-ments have been used in measurementsof one to 100,000 microvolts at fre-quencies from zero to 300 Mc and alsofor 100,000 microvolt measurements inthe region of 1000 Mc.

These micropotentiometers are thefirst low -impedance (of the order ofmilliohms) devices which provide r -fvoltages in the microvolt range andwhich make these low voltages avail-able without the use of attenuators.They thus provide useful tools for manyproblems where constant voltage andlow voltage sources are required. Thedevices are inherently frequency insen-sitive up to and above 300 Mc.

Extremely low and essentially non -reactive output impedance facilitatestheir use for checks and references withstandard voltage generators. They maybe used for direct calibration of per-centage -modulation indicators. Bymeans of known voltage ratios, the mi-cropotentiometers may be used to ex-tend the range for checking attenuatorsup to 120 db or higher.

In comparing the micropotentiom-eters with other sources, such as a volt-age -measuring thermistor bridge, abso-lute reproducibility and agreement havebeen limited only by the relative com-plexity of the standards of comparison.Verification of the exact frequency andvoltage ranges of the micropoteniom-eters in terms of other independentstandards is still in progress at the Bu-reau, along with other phases of designand application. Probably the greatest

single difficulty encountered in thiswork has been the lack of stable sensi-tive receivers which can indicate onemicrovolt (or lower voltages) at 100Mc and higher frequencies with accu-racies of 10 percent or better.

- T M -FRINGE AREA SERVICING

from page 9ideally suited for fringe and ghostareas. This unit is characterized by ahigh gain over a relatively broad fre-quency range and can be made sharplydirectional by terminating the closedend of an 800 ohm non -inductive re-sistor. If front -to -back ratio is impor-tant, different values can be tried forbest results. It can also be made bidi-rectional, by omitting the terminatingresistance. The characteristic imped-ance of the rhombic antenna is 800ohms, but for short distances a 300 ohmline is satisfactory. For longer leads,an open line with the wires spaced ap-proximately 2 inches apart with a quar-ter wave matching section at the an-tenna is used. If the transmission linemust be run through an area of highambient noise, and it is necessary to usea 75 ohm coaxial line, a balanced -to -unbalanced transformer must be used.

Optimum Position

Not only is it necessary to use a highgain antenna for good fringe area re-sults, but it is also important that theantenna be located in a so-called "spaceloop." Such a spot can be located byusing a test antenna in this manner:construct a simple test antenna, such asa folded dipole, cut to the frequency ofthe desired station. Mount the test an-tenna on a light mast, about the sameheight as that to be used for the finalantenna used. Locate the test antennaat the desired site and orient it towardthe station.

Now move the antenna toward oraway from the station until you find aspace loop. This can be observed by amarked increase in picture contrast ora rise in AGC voltage. If it is desiredto get the best location for more thanone station, repeat the same procedure,and, if possible, find a spot which isgood for both stations. If the simpledipole does not pick up a sufficient sig-nal, a more elaborate antenna is indi-cated for the test.

A simple check to make sure that theantenna is properly located can be made

± to following page

the maintenance mill by Ghyssels

"You have to expect SOME snow in a fringe area."


FRINGE AREA SERVICING± from preceding page

if a twin lead transmission line is used.Reverse the leads at the receiver or atthe antenna and observe the picture. Ifthere is no change, the antenna is ori-ented properly. If, however, there is achange in picture quality, the antennashould be reoriented. This test cannotbe employed, of course, if an unbalancedcoaxial line is used.

Transmission LineIt would be senseless to erect an

elaborate antenna, then use a low qual-ity transmission line, and lose the care-fully -obtained signal on the way to thereceiver. If maximum signal is to bedelivered, it is important that the linematches the antenna as well as the re-ceiver. The line may be matched byusing either a Q section or a quarter-

wave matching stub.

In fringe -area work, it is essential tochoose a line which has a low capacity,low loss and a good quality dielectric.This is particularly important if the linemust run over a substantial distance. If

the location is one where the line is sub-ject to noise pickup, a shielded coaxialline or a shielded parallel line is neces-sary.

Booster AmplifierFor proper set operation in a fringe

area, a well -designed booster or pream-plifier is usually a necessity. The boostermust not only amplify, but it must alsohave very little inherent noise. It is

actually possible that with some boost-ers, the signal-to-noise ratio is worsethan when no booster at all was used.It is generally advisable to use a boosterwhich uses triode tubes rather thanpentodes, since the level of the tubenoise is much lower (because it hasfewer electrodes), thus giving an im-provement in the signal-to-noise ratio.It may sometimes be advisable to usetwo sharply tuned boosters in series,where one is tuned for best picture andthe other for best sound. Although thisis certainly not the most economical wayto use boosters, it is sometimes the onlyway left. If the transmission line hasto run for any appreciable distance, itoften helps if the booster is mounted asnear as possible to the antenna, so as to

sAftDoxiifu-Jection

TYPE PRSELECTROLYTICS

IIIIII

III

I

I

Aerovox "Dandees", ofcourse! Tubulars encased inaluminum containers. Especiallysuitable for tight spots. Singleor multiple sections. Insulatedstranded wire leads and safetysleeves. Common -negative wirelead. Vented for excessive gaspressure.

Normally supplied with etchedfoil for utmost compactness, butplain foil also available. High -

10092(capacitors

IM =EN EU MN NI 1111 IN NI

purity foil aluminum minimizescorrosion possibilities. Vented forexcessive gas pressures. Insu-lating jacket. Mounting strap.Widest selection of voltages andcapacitances. Single-, dual-,triple-, and quad -section units.

Ask your jobber for theseAerovox "Dandees." See list-ings for any voltage and capa-citance combination.

CAPACITORS VIBRATORS TEST

II

IIIII

IIII

III

INSTRUMENTS

4.9oh-ede ARDOR CORPORATION, NEW BEDFORD, MASS., U. S. A. Sales Offices In All P,inclA,l CHOI

Egad: 41E. 42W SE. Newra1 IA, N. V. Cabla. ilEALICAP. N. Y. In Canada, ALROYOX CANADA LTD., Hamilton. ORE

amplify the already weak signal beforeit picks up any additional noise alongthe line.

Correct Matching

As mentioned above, one of the im-portant factors in the proper operationof the complete installation is necessityfor correct impedance matching betweenthe booster, the antenna and the receiver.If standing waves appear either betweenthe booster and the set, or on the trans-mission line coming from the antenna,an improvement can be made in eitherone of two ways. If a coaxial line isused, a small variable condenser of about15mmfd maximum capacity can be con-nected across the line, at either the setor the booster, and adjusted for the bestpicture. For twin lead, the best thingto use is a small piece of aluminum foilwrapped around the line. The alumi-num is moved along the line for bestresults and held in place with tape. Anunshielded transmission line can bechecked for standing waves by holdingthe line between two fingers and mov-ing the hand along the line. If there isany marked improvement in the pictureat any one point along the line, there arestanding waves present, which should betuned out by the foil method or somesimilar technique

The ReceiverNow let us turn our attention to the

receiver itself. If it is possible for theserviceman to recommend the purchaseof any one particular receiver, it is ad-vantageous to suggest a set which hasfour high -gain IF stages, a good frontend, and a full bandwidth of four mega-cycles. There are a number of simplemodifications, however, that may bemade on even such a receiver, which willmaterially improve its operation infringe areas.

A simple modification in the RF stagewould be to remove the RF amplifiergrid return from the AVC line andground it. This will prevent the AVCvoltage from cutting down the necessarygain of the RF stage. This changeshould not be made, though, if strongstations are also received, since theywould then overload the receiver. If a6AG5, a 6AU6, a 6BC5 or a 6CB6 isused, a slight increase in B voltage, tono more than 200 volts, will increase thegain of this stage.

Now let us see what we can do withthe video IF stages. One very simpleway of attaining a marked improvement

22 RADIO AND TELEVISION MAINTENANCE MARCH, 1951'

in gain is to replace the tubes in thisstage with high -limit tubes of the sametype or with some of the "hotter" newtubes. Since television receivers aremanufactured for reception in normalsignal strength areas, the tubes for theIF stages are not especially selected, butgood average tubes are used. A tubevery frequently employed in video IFstages is the 6AG5. This tube may bereplaced with the new and hotter 6BC5.The 6BC5 is an exact replacement, butwill give an appreciable increase in gain.(Transconductance of the 6AG5 is5,000 micromhos, while that of the6BC5 is 5,700 micromhos.)

Another good substitution for the6AG5 is a 6CB6. The transconduct-ance of this tube is 6,200 micromhos,more than 20% higher than the valuefor the 6AG5. If three or four tubesare substituted in the IF stages, the in-crease in overall gain is quite substan-tial. Pins 2 and 7 must be tied to-gether if the tube is used in place of a6AG5, since the 6CB6 has separate pinsfor the cathode and the suppressor. Theset may also have to be realigned, sincethe interelectrode capacitance of the6CB6 is slightly different.

Select Best Tubes

Another thing that may be done is toselect from your present stock thosetubes which are most used in video IFstages. Pick out the ones which givethe highest reading on a transconduct-ance tube tester and save them for justsuch an occasion. You may find thattubes of one particular manufacturerhave a higher transconductance thanthose of the same type made by a dif-ferent manufacturer. Make sure thatyou have a few of those hot tubes putaside from the rest. The tubes whichare replaced may be used in a differentreceiver, or perhaps in the sound sectionof the same set. If you have no sparetubes on hand, it may sometimes payto substitute tubes from the sound IFsection into the video IF stages, provid-ing that they are of the same type. Aconvenient way of observing whetherthere is any improvement is to measurethe AGC voltage while trying differenttubes. Be sure that a constant -strengthsignal is being received during thismeasurement.

Proper realignment will also generallyimprove the operation of a set. Sincetelevision receivers are aligned on a pro-

± to page 25

PULARBECAUSE THEY ARE

A TWO -BAY3 ELEMENT

FUL

JCONE -BAP5 ELEMENT

"J" Series YagisThe VEE-D-X "J" series were the first preassembled, low costYagis to gain national recognition. They were developed tomeet the demand of TV owners in areas where maximumsignal pick-up from only a few channels was required. Each ofthese famous Yagis is cut for a specific channel and will provideextremely high forward gain, excellent front -to -back ratio, andnearly complete immunity to man-made noise, ghosts, etc. Theycan be used singly or stacked and are engineered to matchstandard 300 ohm line. For further information write theLaPointe-Plascomold Corporation., Windsor Locks, Connecticut.

VEE-DoXTHE WORLD'S MOST POWERFUL ANTENNAS


PHOTOFACT Users

Write Our Best ADS!

Hundreds of unsolicited letters tell

what the world's finest Radio & TV

Data means to Service Technicians

ART RHINE, Rhine Television, New York.President, Television Contractors

Association of New York, Inc.

"In business, time is money. In our profession,PHOTOFACT is as indispensable as trainingand experience-saving one-third of the timerequired for re -calls. We consider PHOTO -FACT an investment for profit."

CHARLES W. ANDERSONRadio Service, Inc., St. Louis, Missouri

"It is a 'must' policy in our shop that we neverstart a service job on a radio or televisionreceiver without first getting out our files ofPHOTOFACT Folders. This wonderful datasystem is the finest thing that has ever beendone for the service man."

NOW! GET THE PROOF FOR YOURSELF!

FREEWe'll Send You a FREE Photo -fact Folder on any postwar set

listed in the PF Index

Learn for yourself -at our expense-howPHOTOFACT pays for itself by earning biggerrepair profits for you! Ask for a FREE Foldercovering any postwar AM, FM or TV receiverlisted in the PHOTOFACT Cumulative Index.Examine it. Put it to work at your bench-thenjudge for yourself!

WRITE FOR FREE FOLDER TODAY!

HOWARD W. SAMS & CO., INC.2201 East 46th Street Indianapolis 5, Indiana

This department of RADIO AND TELEVISION MAINTENANCE is devoted to help-ing to solve the difficult service problems of our readers. Tough ones of generalinterest will be printed, and readers will send in answers. The best solutions will beprinted in later issues. If only one answer to a problem appears here, its originatorwill receive $5.00 in cash. If two or more different ways of beating the poser are ofnearly equal merit in the opinion of RTM editors, the second best will be worth $3.00to the man who submits it, and the third best will bring home $2.00. Send yourquestion or solution to: Problem Editor, RADIO AND TELEVISION MAINTENANCE,P. 0. Box 867. Atlantic City, N. J.

HEAVY INTERFERENCE

In the January issue, Charles Karafa,Detroit, explained a very tough case hehad with a TV installation opposite anautomobile factory. His pictures werefoggy, full of ghosts, and had horizon-tal black bars. Reader Maurice E.Reller gets $5 for his suggestions as toa possible cure for Mr. Karafa's diffi-cuty, which follow:

GENTLEMEN:

In reference to Mr. Charles Karafa'sproblem, my suggestion is to make an-other survey of the situation. Deter-mine whether the source of the black -bar interference is located in a posi-tion between the installation and thetransmitters, or direction of best signal.Il this is so, then I would say that theproblem is hopeless. You have to ap-preciate the fact that hopeless cases doexist.

Assuming this does not apply to anyor all of the channels, do a completejob of searching out all possibilities oflocating the antenna, running the linein a direction opposite and as far aspossible from the source of the inter-ference. Make certain that the direc-tion is such as to insure the least pickupon the transmission line. If it is pos-sible to back a thousand feet away fromthe source of interference, do it by allmeans, and worry about line losseslater.

If you are still with me at this point,I suggest trying a five -element yagi.Bring the receiver as close as possibleto the new antenna location you haveselected and probe the area thoroughlyfor best elimination of ghosts and otherinterference. If you succeed in obtain-ing a clean signal, then mount the an-

tenna and complete the orientation job.For a leadin, I would select 300 ohm

shielded line, or, second best, a line con-sisting of twin 72 ohm coaxial cable.Whichever is used, install good groundsat regular intervals on the horizontalrun. If it is possible to run the trans-mission line along or close to theground, I would recommend doing so,even if it means ducking around a build-ing or over an alley. After the trans-mission line has been installed and con-nected to the receiver, try stubbing atthe terminals to improve the signal. Ifintereference is still troublesome, de-termine whether there is any directpickup at the tuner. If so, install acopper screen, grounded, inside thecabinet, completely covering top, bot-tom and sides.

When all this has been completed,you may want to touch up the job bytrying any or all of the following: in-stallation of an isolation transformerwith additional power line filtering inthe primary; commercial interferenceeliminator and booster at antenna ter-minals of receiver; grounded copperwindow screen as an addition to an-tenna reflector, installed to the rear orsides of the antenna, wherever best re-sults are obtained.

Good luck!MAURICE E. RELLER,

St. Louis Park, Minn.

-RT M -

HUM DIFFICULTY

This department presented the case ofRoy T. Fischel, a TV technician ofWashington, D. C., last month, in whichMr. Fischel was having trouble with a

± to page 30


± from page 23

duction line basis, they are made to meetcertain minimum specifications set upby the manufacturer. It is generallypossible to "squeeze" out a little moregain with a slight touching up. Be sure,however, that the alignment is carriedout under the same conditions underwhich the set will operate, that is, witha weak signal. If the receiver is checkedin the shop with a signal from the shopantenna, insert an attenuator in the linewhich will give the same signal strengthas that available at the final location ofthe receiver.

If the IF amplifier is stagger -tuned,it is possible to increase the gain ma-terially by aligning the individual stagesto give a grated overall gain with anaccompanying decrease in bandwidth.This modification will increase the con-trast but reduce the detail in the picture.A bandwidth reduction to 3 or 3.5 mega-cycles will not materially reduce thequality of the picture. This alignmentshould be carried out with a very lowsignal, so as to develop an AGC volt-age corresponding to the amount devel-oped under actual operating conditions.Remember that the shape of the responsecurve is affected by the AGC voltage.

Other Circuit ChangesIf all these techniques do not produce

satisfactory results, there are still somecircuit changes which can yield a usable,if not perfect, picture. These stepsshould be taken only as a last resort.

One of the things that can be doneis to increase the size of the loadingresistor across the IF stage which istuned to the high frequency end ofthe response curve. This will increasethe gain at the end of the curve wherethe picture carrier is located. Try va-rious sizes for best results, but be sureto stop at a value low enough to keepthe IF's from oscillating. Now realignthe set for best picture. The definitionwill be lowered, but the marked in-crease in signal-to-noise ratio may meanthe difference between a usable pictureor none at all. Another modificationwhich will increase the gain at the lowend of the video frequencies is to in-crease the value of the video detectorload resistor, or increase the load re-sistor in the video amplifier. Theseresistors are usually from 3,000 to 5,000ohms and should not be increased tomore than 10,000 ohms.

FRINGE AREA SERVICING FIELD STRENGTHMEASUREMENTS± from page 6over the specified tuning range. Non -inductive resistors R1 and R2 in seriesmatch the antenna impedance. Thevalues shown in Fig. 2 are for a 300 -ohm antenna. R1 and R2 each wouldbe 36 ohms if the antenna impedancewere 72 ohms, and would be 75 ohmseach for a 150 -ohm antenna.

This arrangement, like the one givenin Fig. 1, must be calibrated individuallyfor each TV channel of interest with

the aid of a standard signal generator.The same calibration procedure recom-mended in the preceding case should befollowed here.

Since this FS meter utilizes a locally -generated signal in its input stage, itwill radiate a small amount of energy(comparable to, but somewhat less than,that radiated by some TV receivers).For this reason, it should not be usedover protracted periods of time, butmeasurements should be completed asquickly as possible and the signal gen-erator promptly switched off.

± to following page

CAN'T BREAKWON'T 540

WRITEFOR BULLETINNO. 60M

DOUBLE VEE ANTENNA

A great, new engineering idea! Walsco

Double-Vee Antenna with "TWIN -TUBE" element

construction eliminates sag ... Guaranteed not to

break. Elements stay firm in perfect alignment for

lasting high gain performance. Highly directive ...

extra high gain on all channels. No mechanical

failures even under severe weather. New molded

insulators guaranteed unbreakable. Outstanding and

lasting dielectric properties. Completely

assembled ... only 4 wing nuts to tighten.

WALTER L. SCHOTT CO.Los Angeles 18, Calif. Chicago 6, Ill.


FIELD STRENGTHMEASUREMENTS

from preceding page

Amateur Circuits UselessExperimenters will be disappointed

to learn that simple crystal -diode cir-cuits, such as those used to check fieldstrength around an amateur transmit-ting station, cannot be used for TV fieldstrength measurements. These arrange-ments have much to recommend them-simplicity, small size, light weight, free-dom from tubes and power supplies.They offer a continual fascination to thetechnician confronted with the task ofhauling test equipment to the roof top.But they do not possess the requiredsensitivity. Even in setups using veryhigh -Q coils in pushpull circuits withparallel crystals in each leg and a sen-sitive microammeter, the minimumreadable RF signal rarely is lower than0.01 volt. This is 10,000 microvolts,20 times the accepted fringe area sig-nal level.

Battery -operated superregenerative re-ceivers also have been suggested as TVfield strength meters. The superregen-r

*VOLOMETERSMODEL 102

(1000 ohms per voltmeter)

* 3" Square Meter*3 AC Current Ranges

(0-30/150/600 ma.)

I *Same zero adjustmentfor both resistanceranges (0-1000 ohms,

I0-1 megohms)

5 DC & 5 AC VoltageI Ranges to 3,000 Volts. Also 4 DC Cur -$1 A90

rent Ranges.

MODEL 104(20,000 ohms per volt

meter)41/2" Square Meter'(50 micro -amperes-alnico magnet)Includes carrying'strap 5 DC VoltageRanges at 20,000ohms/volt to 3,000 V.:5 AC Voltage Rangesto 3,000 V. 3 Resist-ance Ranges to 201meas. Also 3 AC (SrDC CurrentRanges & 5 $1)6951DB Ranges. L

IAll oz toe arzove have round cornered, bake Ilite, molded cases. ("Reg. Trade Mark for Volt-Ohm-Milliammeter)

Prices subject to change without notice.Export Dept., 303 W. 42nd St., N. Y. C.

Write Dept. D-3 for Free Catalog

I

Gives More Measurement Value Per Dollar

ELECTRONIC

MEASUREMENTS

CORPORATION

280 Lafayette St., New York 12, N.Y.

erative detector is extremely sensitive,but has two outstanding drawbacks: itradiates a compartively strong interfer-ing signal, in some cases even whenwell -shielded and preceded by an RFamplifier; and it is endowed with aninherent AVC action which almost com-pletely unsuits it to the job of distin-guishing between signal strengths. Infact, one superregenerative outfit wetried had such an unexpected degree ofautomatic volume control that it showedalmost no change in signal strength asthe exploring antenna was movedthrough regions of strong and weaksignals.

Meter CalibrationIn a field strength meter, the factor of

chief interest is the indication of signalintensity. The output meter readingtherefore is of greatest concern. Whereaccurate measurements are to be made,the meter readings must be in micro-volts. But when qualitative tests aregood enough, relative readings on anarbitrary meter scale are satisfactory.

It has been pointed out already thata standard signal generator covering onfundamentals the TV channel frequen-cies and provided with a micro -volt -calibrated output attenuator, is requiredfor accurate calibration of the fieldstrength meter. Even factory -made FSmeters should be checked occasionallyagainst a reliable generator to insureaccuracy of indications. It should bepointed out, however, that the absoluteaccuracy of a microvolt calibration is

somewhat limited. This is due to thepercentage accuracy of the attenuatorin the signal generator. An accuracyof calibration of better than 10 to 15percent seldom is bettered in the cali-bration process, due to the generatorerror plus the accumulated errors of theprocess. Nonetheless, such a calibra-tion will be sufficiently useful for FSmeasurements at TV receiving locations.

BARGAINCATALOG

RADIOPARTS

'BUYERS.0SYNDICATE.=VW=

BUYERS SYNDICATE30. TAYLOR ST., SPRINGFIELD 3, MASS.

Noise Generators

In some commercial field strengthmeters of the professional type, a self-contained noise generating circuit is in-cluded for calibration, in absence of astandard signal generator. The noisecircuit makes use of shot noise gener-ated in a special tube. Advantages ofshot noise as a calibration sourceare: this noise signal is evenly spreadthroughout the radio spectrum, therebyrequiring no tuning of the calibrationgenerator circuit; the noise amplitudelevel is high; and since the noise am-plitude is proportional to the DC platecurrent in the noise -generating diodetube, the circuit may be standardizedeasily and simply by adjusting the platecurrent to a predetermined calibrationvalue. Pure shot noise is obtained ina vacuum tube when space charge iseliminated. This elimination is accom-plished in the shot noise calibrator byreducing the filament temperature to thepoint where the plate attracts all of theelectrons emitted by the cathode. Spe-cial diodes having plain tungsten fila-ments frequently are employed as noisegenerators. However, triodes occasion-ally are used with their grids and platestied together for diode action, or withthe grid employed as the diode plate andthe regular plate left floating. The noisesignal is applied to the first tuned cir-cuit in the field strength meter. Thecalibrating voltage obtained is propor-tional to the impedance of this tunedcircuit. In calibrating the FS meteragainst shot noise, the noise circuit firstis standardized by adjusting the diodeplate current to the level recommendedby the instrument manufacturer. Thisis accomplished by switching the outputmilliammeter (or microammeter) tem-porarily to the diode circuit. With thenoise signal then applied to the FSmeter circuit, the gain of the instru-ment is adjusted by means of a calibra-tion control potentiometer to give a pre-scribed calibration level deflection whenthe meter is switched to the output cir-cuit of the instrument.

SPECIFY BLACO(formerly BLACKBURN)

A-1 Ground ClampsFit 3/s" to l'/4" Pipe

ASK YOUR JOBBERBLACO MANUFACTURING CO.

Formerly Mad/burn Spettalty C. .l

6525 EUCLID AVE., CLEVELAND 3, OHIO


Making Tests

The principal uses the TV technicianwill find for the field strength meter are:(1) checking antennas for best place-ment and orientation, (2) making sur-veys to determine the average fieldstrength in a particular locality, ( 3 )

checking antenna arrays for best typeto install permanently, (4) checking thestrength of local oscillator radiationfrom TV receivers, and ( 5 ) tracingnoise and signal interference on TVchannels. While operation of the in-struments is entirely straightforward,the technician should keep in mind thefollowing points regarding the use offield strength measuring equipment:

1. Use a battery -type instrumentwhenever possible. Extended powerlines tend to increase the effectiveheight of the receiving antenna andalso to introduce interference patterns,especially when the lines are near theactive elements of the antenna.

2. Connect the antenna to the meterthrough the entire length of the actualfeeder which later is to be used withthe TV receiver installed on the prem-ises. Make all connections solid.

3. Take a separate field strength read-ing on each channel used locally fortelecasts. In fringe areas, take separatereadings at various times of the day.

4. When testing for best antenna site,make separate field strength readingswith the antenna at various locations.At each location, orient the antenna formaximum meter deflection. Repeat theentire procedure with the antenna atvarious heights, to determine optimumelevation.

5. Keep out of the field of the an-tenna when making readings. Orientthe antenna and then step clear of theregion. Place the meter at the farthestdistance consistent with ability to seemaximum swing of the meter.

6. In localities where there are re-peated complaints of poor reception,try to interest the residents in a fieldstrength survey. (It might be one ofthose localities where signal strengthis so poor that no set will work right.)Payment of your fee can be divided be-tween a number of residents, all ofwhom can benefit from the project.

7. When making tests of local oscil-lator radiation from TV receivers, use avertical rod antenna (an auto antennawill do ). Support the antenna in a

stationary position, and keep out of itsfield.

EVERYTHING INTELEVISION!Now at yourfingertips, forquick, easyreference

THE

VIDEOHANDBOOKTHE COMPLETE TELEVISION MANUALby MORTON G. SCHERAGA, Tele-

vision Research Consultant,Allan B. Du Mont Labs.

and JOSEPH J. ROCHE

NOW in one volume-all the essentialknow-how of television! All the up-to-the-minute information on television isarranged for quick reference-in easy -to -understand language.

The VIDEO HANDBOOK gives youpractical answers to all your questions ontelevision. This best-selling TV encyclo-pedia tells you . . .

How Television WorksHow to Plan and Engi-

neer TelevisionHow to Troubleshoot and

Repair TelevisionHow to Select and Install a

Television AntennaHow to Produce a Television

Show. Technical AspectsHow to Build an Operating

Television ReceiverHow to Select a Television

Receiver

CONTENTS:Television, Past, Present and Fu-ture Fundamentals of ElectronicTelevision The Television Sta-tion, Pick-up-Control-Trans-mission The Television Re-ceiver Television Antenna Sys-tems l Creating a TelevisionShow, Programming and Produc-tion Description of ModernTelevision Receivers, Circuit Var-iations - Design - MechanicalFeatures Installing TelevisionReceivers Servicing TelevisionReceivers Troubleshooting

Interpreting Test Patterns-Alignment-Repair TelevisionTest Equipment; How To UseIt-How To Buy It Buildinga Television Receiver DataSection Television TermsBibliography.

The VIDEO HANDBOOK contains thousands of vital facts-cov-ering everything you need to knowfor working in Television. Everyimportant point discussed is visual-ized in diagrams or photographslover 860 illustrations).

More than 900 pages . . .

handsomely bound in durableFabrikoid. Only $5.95.

You must see this book toappreciate it! 20,000 copiesnow working for practicalTelevision men! Mail couponfor your copy TODAY.

-911111111111iiiimummimmimimilimmiumimumumminummum111111111111111minim

77.RADIO -TELEVISION PUBLICATIONSP. 0. Box 867, Atlantic City, N. J.

Please rush me my copy of The VIDEO HANDBOOK postpaid. I am enclosing $5.95

= NAME

= ADDRESS

= CITY ZONE STATE


TRADE LITERATURE± from page 18

flection yoke types for television.Copies will be sent on request to the

company, at St. Marys, Penna.

- T MNEW HANDBOOK EDITION

10C OVERAGE of nearly all phases of

radio and television is provided inthe second edition of the "Radio andElectronics Handbook," by William F.Boyce and Joseph J. Roche, an 890 -page revision of the earlier compilation.

Selling for $5.95, the handbookshould serve a definite need for thetechnician who wants a reference workthat can be used quickly, to get briefinformation on almost any subject thatis likely to come up in radio or tele-vision work. -N. L. C.

-R T M

GETTING THE MOSTOUT OF THE ANTENNA± from page 8

satisfactory operation 500 microvolts isdesirable at the front end, with a min-imum ranging from 150 to 250 micro-volts depending upon the sensitivity ofthe receiver. You have to be carefulabout a fringe area installation to getthat kind of signal into the set's frontend.

In conclusion, it is suggested that ina new installation that gives trouble,immediately suspect the antenna. Thereceivers that are coming off the as -

FREE

CATALOG

MAILING LISTSWrite for your free copy of the latest Polk mailinglist catalog, giving counts and prices on guaranteedlists of all business classifications-wholesalers-retailers-manufacturers-catalogued by type and bystate. Also, hundreds of selections of specialized lists-professional people, auto owners, income lists, etc.

World's Largest City Directory Publisher

R. L. POLK & COMPANY431 Howard Street, Detroit 31, Michigan

Branches in Principal Cities

sembly line today are well aligned, andin most instances reception should beimmediately good when the receiver isplugged in. The bumping around areceiver gets in transportation has lesseffect than is often thought. Make asgood an antenna installation as youknow how, based on the suggestionsgiven here, in the manufacturer's litera-ture, and from the basic texts on thesubject. Most servicing problems willbe forestalled, and many fretful callsfrom the customer will be eliminated.

--RTM-PUSHING BACKTHE FRINGE

from page 12

into the city are interrupted with sup-plementary boosters.

The manufacturer, which marketsthe systems direct and also throughPhilco distributors, says that the onlyreal stumbling block which is foresee-able is that the shortage of materials,which is constantly becoming more se-vere, may cut off production, or seri-ously reduce it. For six months, atleast, the company expects to be ableto meet the demand for the installa-tions, however. After that, it is thoughtthat development of substitute mate-rials will have become sufficiently ad-vanced so that some production maycontinue. A lowering of the originalcost for the equipment is also in view,the company said.

With this system, enterprising tech-nicians and dealers in many sections ofthe country not now served by regularTV transmitters will be able to boosttheir own business and aid the commu-nity, while at the same time they are

mosumimmomimmilowSPEAKER PARTS SPECIALS!II

4" CONES.9/16" I.D., 5/8"deep $46/M; 100/$7.2551/4' CONES....9/16" I.D..

1 13/16" deep $50/M;100/$7.95

E 6" CONES.3/4" I.D 1.1/8"E deep $65/M: 100/59.95

4" to 12" ASSORTMENTI (incl. free -edge) ..12/$1.98

SPEAKER REPAIR KIT....E Liberal assortment of Spi- ders, Rings, Shims, V.C.

forms, Cement, Felt, Cham-ois Leather & Instructions

$2.49SPECIAL: $1.98 CONE 6 REPAIR KITS $3.951

(All Cones Less Voice Coils)E "FACTORY SPEAKER REPAIRS SINCE 1927"I Min. Order $3.00 20% deposit on all C.O.D.'s 0

Full Remittance with foreign orders.Please add sufficient postage-

excess refunded.

RADIO CO. ILEO OE67-A Dey Street

I

New York 7, N. Y.- IM

extending the limits of successful TVpropagation, which has become an assetto every area in which it is found.

R T M -

"BOX OFFICE"TELEVISION* from page 11

vertical pulse with the key, or at thefirst pulse without the key.

Normal horizontal drive pulses aresent into a phasing circuit which shiftstheir phase with respect to normal andfeeds them into a blocking oscillator,which they trigger. The pulses are thenfed to a long line with a specified delaytime, and an electronic switch chooseseither the pulses from the input orthose from the output of the delay line,as controlled by the single -trip multi -vibrator. In this way, the coded hori-zontal drive is produced for the rest ofthe station equipment.

Horizontal BlanksSince the phase shift time in the

video signal occupies some of the timenormally assigned to horizontal blank-ing, the new horizontal blank signalmust be narrower. This signal is gen-erated in another section of the coderby feeding the horizontal pulses fromthe output of the delay line into a sin-gle -trip multivibrator, after amplifica-tion and isolation. The multivibratoroutput is then mixed with normal ver-tical blanking signals, and the resultingsignal is amplified, clipped, and dis-tributed to the system.

Coded blanks are inserted by the finalsection of the coding unit. The blankscome from a single -trip multivibrator,which has been triggered by the codedhorizontal drive, and are maintained atthe average video level, both polaritiesof the wave form having been combinedin two tubes with a common load im-pedance, and the resultant signal mixedwith the video from the camera.

Decoder Operation

Only the relatively minor change ofconnecting the decoder is needed at thereceiver, to convert a conventional set so

you'll find it in

PF INDEX

FREEGET YOURCOPY FROM

YOUR PARTS JOBBER


that it will be able to receive Phone -vision.

In the set's sweep section, the verticalpulse is separated normally, and fed tothe vertical deflection circuit. The hor-izontal pulse, however, is sent into thePV decoder unit, which will be eitherbuilt into future receivers, or used as aseparate adapter for present-day TVsets.

Operated by the key signal from thetelephone line, the decoder applies acorrection to the horizontal pulse. Withthe key coming in at a level of about 50millivolts, the first stage of the decoderprovides gain. A gating circuit thencombines the key information with thereceiver's vertical pulse, and uses thecombined signal to trigger a squarewave generator. Between the gatingcircuit and the square wave generator,the first vertical pulse when the key ispresent, and the first pulse when thekey is absent, can be detected. In thisway, the generator changes the modesaccording to the changes in the key.

The square wave controls the phaseof the pulse from the normal circuits ofthe receiver, and consequently that ofthe horizontal drive on the picture tube.The amount of compensating phaseshift, then, agrees with that shift in-serted at the transmitter and includedin the key. A blanking circuit in thedecoder, controlled by the horizontalpulses, restores the abridged blankinginterval to its normal width.

Applications

The usefulness of such a system ofsubscriber television, whether it is theZenith arrangement or some other, isnot limited to the motion pictures nowbeing shown in the Zenith tests, ofcourse. Many special broadcasts, ofmore -than -average attraction to the con-sumer, could be presented through thismeans, and could thus raise the incomesof the broadcasters, according to analy-ses of the subject. Movies would prob-ably be the chief substance of the pro-grams, however, and with the incomederived from subscriptions, it is pre-dicted that first -run pictures could com-monly be shown, instead of the olderpictures now available for television.

Judging from the reaction of the "testfamilies," who now may view Phone -vision, the system, if approved, couldgrow rapidly throughout any commu-nity where installations were possible.

This would probably mean an overallincrease in the number of potentialcustomers for television sets, since thepossibilities of the new system wouldbe added sales points for dealers, andthe additional equipment would addonly about $10 to $15 to the price ofreceivers.

Final decision from the FCC on sub-scriber television may not mean a greatdeal, however, as far as immediate com-mercial exploitation is concerned. Asin the case of color TV, progress de-pends on the current state of the wareffort, and even FCC approval, coupledwith the willingness of manufacturersto produce such civilian goods, wouldnot likely be sufficient to divert menand materials into what would no doubtbe considered non -essential production.It probably will be a fairly long time,then, before this new development isput into common use.

-R T M -

Build This SimpleHome -Appliance Tester± from page 13

to the contacts on the appliance. Witha little practice, the operator will be ableto make an estimate as to the conditionof the unit under test by judging thebrightness of the lamp. If the lampdoes not light at all, an open circuit inthe device is indicated.

The AC voltmeter can also be usedseparately, if desired, with the probesconnected in the same way. To avoidthe possibility of voltage feedback, theplug on the tester's line cord is insertedinto the dummy receptacle. The lampis replaced with a plug fuse, and volt-age across the terminals of an appli-ance is read directly, up to the capacityof the meter.

The usual precautions against shockmust be taken, particularly when thetest probes, with their bare terminals,are used.

If voltage measurement is not con-sidered necessary the voltmeter may beomitted, and a low -current pilot lightused instead. The meter shown, ob-tained from the surplus market, wasnot expensive, however, and the likeli-hood is that any serviceman, if he doesnot have one in the shop, can buy oneat low cost.

LIGH

te

4 -WIRERW-204 150

"6, r LIST

The first and only arresterthat will accommodate 4 -wire rotator line as well asregular 2 -wire transmissionline.

E.D.X.Prae.otedeING ARRESTERS

oevede Pu'eed

7leat2 -WIRERW-200

o4125LIST

The new VEE-D-XModel RW-200 is thepopular low-priced ar-rester. Similar in designand construction to theRW-204. Two saw toothcontact points assurepositive protection forany 2 -wire installation.

The Original 2 -WireRW-300

For use with 2 -wirestandard transmissionline. An air gap plusresistor provide doubleprotection. RW-300 ismanufactured of mois-ture resistant Mica -fillBakelite. RW-300A -highest quality thermo-setting plastic. ally 200LA POINTE-PLASCOMOLD CORP., WINDSOR LOCKS, CONN.

ALLIED1951 CATALOG

FREE!212 -PAGEBUYING GUIDE

COMPLETEUP-TO-DATE

Allied Radio Send for it!

EVERYTHING IN RADIO,TV and ELECTRONICS

Service Technicians and Engineers:ALLIED'S latest Catalog brings you allthe newest equipment and money -savingvalues-from the world's largest stocksof test instruments, amplifiers, P.A. sys-tems and equipment, tubes, parts, tools,books and accessories. Send today foryour FREE 212 -page ALLIED Catalog.

ALLIED RADIOALLIED RADIO CORP., Dept. 27 -B -Itl33 W. Jackson Blvd.. Chicago 7, IIL

Send FREE Catalog No. 126.

Name

Address

City State

RADIO AND TELEVISION MAINTENANCE MARCH, 1951 29'

ESSENTIALS ofELECTRICITYfor RADIO and

TELEVISIONNew Second Edition develops the

I principles of electricity needed tounderstand the various phases of radioand TV operation. Analyzes electric andelectronic circuit components and action in

terms of electron flow, treat-ing filter circuits, coupledcircuits, band-pass circuits,etc. By Morris Slurzburgand William Osterbeld, In-structors of Electricity andRadio, Wm. L. DickinsonHigh School, Jersey City.

528 pages, 391 illus., $5.50.

ELEMENTS OF

RADIO SERVICINGI Shows you how to handle 95% of allL radio service problems. Covers thesuperheterodyne A.M. receiver, as wellas phonograph -combination service andauto -radio jobs. Gives special attentionto the modern, most commonly used test-ing equipment-the multimeter, signalgenerator, etc. By William Marcus, co-author of Elements of Radio, and AlexLevy. Instructor of Radio Mechanics,Manhattan Trade Center for Veterans. 471pages. 461 illus., $4.75.

BASIC TELEVISIONPRINCIPLES and

SERVICING3Practical, on-the-job guidance on to-day's most efficient television servic-

ing practices. Tells how the video andaudio signals originate . . . how they aretransmitted . . . how they are received.Gives a balanced, detailed picture ofboth AM and FM circuits-their operation and main-tenance. By Bernhard Grob,Instructor, RCA Institutes,Inc. 592 pages, over 400tables, charts, and illus.,$6.50.

TELEVISION TELSERVICING SEI

4Brings you the informa-tion you need to know to

service television receivers.Covers the operation and servicing ofevery section of the TV receiver, describ-ing typical circuits and answering alarge number of questions on antennainstallations. Shows how to locate anyexternal or internal trouble. By SolomonHeller, Instructor, American Radio Inst.,and Irving Shulman, Chief Engineer, Fed-eral Television Corp., 434 pages, 266illus., $6.00.

RADIO -TELEVISION

PUBLICATIONS

P. 0. Box 867

Atlantic City, N. J.

%Vliat's YourProblem?

from page 19

large hum bar on two local channels,on a Motorola set, while two other chan-nels came through all right.

He had already checked all the elec-trolytic condensers in the set, and foundthem to be good, and he also made surethat the resistance and voltage valueswere correct according to the manufac-turer's specifications.

RTM's $5 award for the best answergoes to Frank Florio, a New York TVstation engineer.GENTLEMEN:

In regard to Mr. Roy T. Fischel's prob-lem appearing in the February issue ofRTM, I'd like to explain how I havefixed a similar trouble in various makesof receivers.

The hum is neither radiated nor un-filtered B supply ripe ripple. It is ofthe heater -cathode leakage variety, andtakes place in the RF oscillator.

Due to the frequencies involved andthe value of distributed capacitances, itwill usually show up on one or maybetwo channels, while the other channelswill be clear.

Changing the oscillator tube is thesolution.

FRANK FLORIO,

North Bergen, N. J.-RTM-

FIX ON THE FACTSfrom page 17

bored in the roof, which is then care-fully sealed with roof cement. Insidethe attic, this lead-in goes directly toan automatic broad -band booster fas-tened to the rafters. Sometimes thisbooster is turned off and on by meansof a switch -and -pilot -light combinationmounted near the set; at other locationsthe power is fed to the TV set throughthe booster by means of a Romex lineand outlet near the set. In this lattercase, of course, the booster is turned onautomatically when the set is switchedon. The lead-in connecting the boosterand the set is twisted, as is the lead-in from the antenna to the booster.

Advantages

The installer claims the following ad-vantages for such an installation: lead-in length and losses are held to a mini-mum; ignition noise pick-up by the

lead-in is cut down; signal-to-noise ratiois much better than when the booster isplaced at the set; by bringing the an-tenna into the attic, the receiver can beplaced wherever wished, even against aninside wall, without materially increas-ing the length of the lead-in; since thebooster is where the customer cannotreadily get his hands on it, there is less"operator trouble."

In installations where the owner insistson being able to adjust things, a two -stage tuned booster is installed at theset; but the records show that this typeof installation results in considerablemore service calls than do the attic -booster jobs. This is not the fault ofthe equipment, of course. In the handsof an expert, a tuned -type booster canusually be made to produce slightly bet-ter results than are had with the broad-band type. Whenever you have any-thing to adjust by hand, though, youare certain to find many customers whowill display an absolute genius in mis-adjusting it!

This same dealer is one of the smart-est TV salesmen in the area, and thesecret of his success is that he deliber-ately and consistently "under -sells"fringe -area TV reception. To hear himtalk you would think he was trying totalk the customer out of buying a set.He tells him flatly that there will bemany nights when reception will be toopoor to enjoy. He advises that if thecustomer is not the sort of person whocan enjoy TV reception when it is goodand simply turn the set off and forgetit when it is not, he should not buy areceiver. As he puts it, "If you're thekind of guy who insists on fightingthis thing, it will drive you nuts."

Good Reputation

Human nature being the contrarything it is, people still buy sets fromthis man, and invariably the results theyobtain are better than he has led themto expect. But when an occasional badnight does come along, and when thetelephones of most dealers are beingrung off the wall, this fellow's customerssimply remember what he told themand let it go at that. At the same time,he has built up a reputation for truthand honesty that contrasts very stronglyand favorably with the reputations ofcompetitors who have promised morein the way of reception than ultra -fringearea conditions can deliver.


\i/62. itel-LreIn the research laboratories of hospitals,clinics, and medical schools throughoutour country, the lights burn late . . . asscientists constantly strive to halthumanity's greatest enemy-CANCER.

As the lights continue to burn, the hopefor a cure grows brighter ... here's why:Cancer Research Is Paying OffThrough research-which you havehelped to support by donating to theAmerican Cancer Society-medicalscience now has new weapons to combatthis disease more effectively than ever:Drugs-there is evidence that a chemicaltreatment for cancer may be perfected.Certain drugs will prolong the lives ofcancer victims ... other promising com-pounds are being tested.Hormones-treatment with hormones,such as ACTH and Cortisone, hasbrought about dramatic, although tem-porary, effects in some types of cancer.Other hormones have helped control ad-vanced cancer of certain organs.X-rays-the development of more power-ful machines promises to make this formof treatment more effective.

Isotopes-radioactive chemicals are be-coming increasingly useful in treatingcertain rare forms of the disease.

In addition, surgical technics havebeen improved so much that once hazard-ous operations can now be performedsafely. And progress is being made inthe development of tests to detect cancerin its earliest stages when the chancesfor cure are best. Research has made

Help Science Help You

CAWC E% zetk?, 004-t If=4-e-

...your g ift will reach your

American Cancer Society Division

these life-saving advances possible. Butas long as cancer continues to kill some210,001) men, women, and children in ourcountry each year, we must keep thelights burning in the laboratories! Muchmore research needs to be done beforecancer can be dealt the final blow!

Your life-the life of everyone youknow-is at stake. Give generously tothe 1951 Cancer Crusade.

. Give To Conquer Cancer

011-'-'_ - - - - _CANCER,

ca -c of Your Local Post Office

Here is my contribution of $to fight Cancer.

Na me

Address

City lone

State

"Sure we can fill many Shod

See how versatile SYLVANIA tubes

help you solve

shortage problemsYes, radio tubes are still in short supply. But, Sylvania is doing itsutmost to serve all its loyal customers.

Production facilities are being increased, and all Sylvania Distrib-utors are being taken care of on the fairest possible allocation basis.

But, there's no shortage in ingenuity at Sylvania! Now this com-pany offers you service dealers a great new tube substitution manual.

Here's a complete classified listing to assist service technicians andengineers in making substitutions for tube types not immediatelyavailable. This booklet includes circuit modifications and substitutiondirections for battery types,150 ma. and 300 ma. types, as well as forTransformer and Auto Tube types . . . Television Tubes and PictureTubes, too.

40 pages of valuable, up-to-the-minute information . .. FREE fromSylvania. Get your copy from your Sylvania Distributor NOW, ormail the coupon below.

This book is being given away FREE by Sylvaniaas a service to its good friends, the country's

radio -television service dealers.

SYLVA NIAELECTRIC

Sylvania Electric Products, Inc.Dept. R.2003, Emporum, Pa.

Please send me new booklet "Sylvania Tube Sub-stitution Manual."

Name

Street

City 7one_____State

RADIO TUBES; TELEVISION PICTURE TUBES; ELECTRONIC PRODUCTS; ELECTRONIC TEST EQUIPMENT; FLUORESCENT TUBES, FIXTURES, SIGN TUBING, WIRING DEVICES; LIGHT BULBS; PHOTOLAMPS; TELEVISION SETS

rch 1951 - americanradiohistory.com€¦ · what's your problem? 24 change of address should...

Documents