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SYLVANIA ECG®SEMICONDUCTOR GUIDE

HELPS SOLVE GEAND RCAF PROBLEMS.

No one knows better thanSylvania that the show mustgo on. So when a faulty semi-conductor in any brand ofhome entertainment productturns into a showstopper, it'sSylvania to the rescue. OurECG semiconductor guideis the most used, and useful,guide in the industry. It makesit easy to replace any ailingperformer with an ECG semi-conductor that will get theshow back on the road.

I

" Ait"*. "ft.%,_ . .*11.

When it comes to replace-ment parts the right place tocome is Sylvania. Your Sylvaniadistributor carries a completeline of ECG semiconductors,picture tubes, receiving tubes,and test equipment designedto make any brand work likeit's brand new.

Sylvania. The easy way toget even the hardest part.

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No. %

ECG is a registered trademark of GTE Sylvania Circle (1) on Reply Card

Electronic ServicingEditorial, advertising and circulation corre-spondence should be addressed to P.O.Box 12901, Overland Park, KS 66212 (a

suburb of Kansas City, MO); (913) 888-4664.

EDITORIALBill Rhodes, Editorial DirectorCarl Babcoke, EditorCindy Nelson, Managing EditorJoan Jackson, Editorial AssistantJoAnn Vella, Editorial Assistant

ARTDudley Rose, Art DirectorLinda Franzblau. Graphic Designer

CIRCULATIONJohn C. Arnst, DirectorEvelyn Rogers, Manager

ADMINISTRATIONGeorge H. Seferovich, PresidentGeorge Laughead, Publisher

ADVERTISINGCameron Bishop, Marketing DirectorJim Reilly, National Sales ManagerLauri Gash, Production

Regional advertising sales offices listednear Advertiser's Index.

Member,ABPAmerican Business Press

Member,Audit Bureauof Circulation

ELECTRONIC SERVICING (USPS 462-050)(with which is combined PF Reporter) ispublished monthly by Intertec PublishingCorp., 9221 Ouivira Road, Overland Park,KS 66212. Controlled Circulation Postagepaid at Shawnee Mission, KS 66201. SendForm 3579 to P.O. Box 12901, OverlandPark, KS 66212.

ELECTRONIC SERVICING is edited fortechnicians who repair home -entertainmentelectronic equipment (such as TV, radio,tape, stereo and record players) and forindustrial technicians who repair defectiveproduction -line merchandise, test equip-ment, or industrial controls in factories.

Subscription prices to qualified sub-scribers: 1 year-$10, 2 years-$16, 3

years-$20, In the, USA and its posses-sions. All other foreign countries: 1

year-$13, 2 years-$22. Subscriptionprices to all others: 1 year-$25, 2

years-$50, in the USA and Its posses-sions. All other foreign countries: 1

year-$34, 2 years-$68. Single copy price$2.25; back copies $3.00. Adjustmentnecessitated by subscription termination tosingle copy rate. Allow 6 to 8 weeksdelivery for change of address. Allow 6 to 8weeks for new subscriptions.

INTERTEC PUBLISHING CORP.

ONE parts bookfor all brandsNOW...The Sperry Tech Pricing Bookplaces over 8,000 fast-moving partsright at your finger tips!...

Over 3,000 direct factory replacementparts, including 17 major MFR's bothdomestic and off shore',Over 1,100 popular receiving tube prices0Complete universal parts listings...capacitors,circuit breakers, circuit boards, crystals,interlocks, rectifiers, resistors, sockets,tuners (rebuilt), batteries, fuses, pilot bulbs,tube brightners, etc.

750 picture tube prices (including a complete interchangeability guide)Manufacturers directory with names, addresses, and telephone numbers

When you appear on the job with a professional parts pricing book you make the bestimpression on customers...cultivates repeat business. At the same time you can adjustprices in a manner that will compensate for obsolete parts in inventory you can't resell orreturn, so you can start realizing a profit from your parts sales.

When price changes occur you automatically receive an up -dated section reflecting newprices...this way you know your parts prices are always current.But that's not all...as part of our subscription service, parts usage is ranked by popularityand is up -dated each year...a vital factor in controlling obsolescence.

CALL TOLL FREE/800-228-4338.(Between 8:00 AM and 5:00 PM CST Monday thru Friday)

-and we'll send you a complete parts book, postage prepaid with the mark-uppercentage you feel is correct for your shop for only $24.50. You'll have the opportunity toexamine the contents before deciding on our automatic up -dating service.

sperry tech inc. P.O. Box 5234 Lincoln, Nebraska 68505

Circle (4) on Reply Card

In September

Electronic servicing

Forest Belt opens up the newindustrial servicing department;Wayne Lemons reports on testingindustrial semiconductors; a specialtest lab by Gill Grieshaber; movieprojector repairs; and more onmicroprocessors.

August 1979 Electronic Servicing 3

August, 1979 P,

Volume 29, No. 8 Electronic servicing13 Reports from the Test Lab

14 Eliminating RF interferenceWayne LemonsRF carriers can be demodulated by audio circuits, thuscausing noise or undesired sounds to be heard. Here isa new and effective method of removing suchinterferences.

22 MRO Industrial electronic servicingGeorge LaugheadMaintenance electronics, whether industrial, biomedicalor in other areas, is the growth area for electronicservicing. Starting this month, each issue of ElectronicServicing will feature articles aimed at the maintenancetechnician.

24 Typical repairs of medical equipmentJoseph J. CarrElectronic medical equipment fails in predictable andrecurrent ways. Several common problems are explained.

29 A second look at waveforms, Part 3Gill GrieshaberScope waveforms illustrate many facts about fast -recov-ery diodes, varistor rectification, horizontal drive and apassive circuit with gain.

35 Instructing a microprocessorJack WebsterThe binary code of microprocessors should be changedto a faster condensed code.

38 Sam Wilson's Technical NotebookJ. A. "Sam" WilsonA charged capacitor can be constructed without themoving of any electrons. Described also is anexperiment with innovative results.

Departments 5 Electronic Scanner6 Symcure7 Readers' exchange

10 People in the News40 Test Equipment

42 New Products43 Catalogs & Literature44 The Marketplace44 Advertisers' index

About the cover Graphic design by Linda Franzblau©Copyright, 1979, by intertec Publishing Corporation. All rights reserved. Material may notbe reproduced or photocopied In any form without written permission of publisher.

4 Electronic Servicing August 1979

dImEnThtscannernews of the industry

RCA is reported to be planning large-scale production of SelectsVision videodisc playersbeginning this fall in Indianapolis. Also being prepared is a catalog of about 250 videodiscs whichwill include feature films. TV replays, music by popular artists, opera and ballet. Initially, thevideodiscs are to be sold by the same channels as the Selectavision players. However, it isexpected the discs later will be distributed through normal phonograph -record channels.

Color TV sales to dealers totalled 4,789,500 for the first six months of 1979, an increase of 1.6%over the same period of 1978.

Magnavox expects to have high -volume production of Magnavision videodisc players by the thirdquarto'. of 1980. Test sales in two cities have been very successful. However, most of thevideodisc player components now are manufactured in Europe by Philips. By next year, 90% ofthe components should be supplied by American manufacturers.

Zenith Radio Corporation has offered to buy the Heath division from Schlumberger Limited. Heathis famous for sales of Heathkit products, and recently has begun to sell small computers. printersand video terminals.

Electrohome of Toronto, Canada has agreed to produce four 19 -inch color TV models for theVictor Company of Japan. Electrohome is a licensee of Victor, and the TVs will be sold in theUnited States.

The National Electronic Service Dealers Association [NESDA] has changed their address to 2708Berry St., Fort Worth, TX 76109. The new phone number is (817) 921-9061. Additionally there is anew phone number at the same address for The Internationa. Society of Certified ElectronicTechnicians (ISCET). That number is (817) 921-9101.


IT7curelSymptoms and curescompiled from field reportsof recurring troubles

Chassis-RCA CTC87PHOTOFACT-1778-2

C93

1/2F

R92

27

R96

506

C94TO

+1__ AUDIO

100µFTRANS

CR8

SHORTED

Symptom-No sound or weak soundCure-Check diode CR8, and replace it if shorted


Q16

VERT OSC CR102

DEFECTIVE

TO Q17

R81A

HEIGHT

Symptom-Vertical rolls intermittentlyCure-Replace diode CR102 as a test for erraticoperation


LEAKY

POSITIVE CR13 / FB5FLYBACK

PULSES

C75

001

32V

R98

-31V

10 2W

Symptom-Intermittent loss of height followed byshutdownCure-Check diode CR13, and replace it if leaky

I


VERTICAL

Q18 PREDRIVER

CR104 CR103

co

Q21

DRIVER

SHORTED

+173VR117

220K

Symptom-Compression at the top, perhaps retracelinesCure-Check diodes CR103 and CR104, and replaceif shorted or leaky


VERTQ18 PREDRIVER

COCR104 CR103

+173VR117

/220KOPEN

Q21

DRIVER

Symptom-Height shrinks severely during first halfminute, and 020 overheatsCure-Check resistor R117, and replace it if open


C623

0022 SCR600

REGULATOR

FLYBACK

OPEN

GATE

+74V

R607

680

7W

REG

OSC

+115V

SUPPLY

Symptom-Shuts down occasionallyCure-Replace regulator SCR600 as a test for anopen gate


nralexchangeThere is no charge for a listing in Reader's Exchange, butwe reserve the right to edit all copy. If you can help with

a request, write directly to the reader, not to ElectronicServicing.

Editor's note: Beginning with the September '79issue, all ads will be edited to show no more thanfive items. Requests already submitted will be brokendown into as many ads as necessary. Since space islimited, this will accommodate more readers.

Needed: E. H. Scott radio parts and a ScottPhilharmonic radio. Robert Teska, 334 Willard,Toledo, OH 43605.

For Sale: More than 200 new Clarostat radio/TVcontrols, with shafts and switches, $75 plus shipping.Hughes Electronics, Route 2 Box 280, Kings Mountain,NC 28086.

For Sale: B&K-Precision equipment: model 415sweep/marker with probes and manual, $350; model1243 color/bar generator, $50; model 801 capacitorAnalyst, $75. Shipping paid when money orderreceived. Needed: stereo power monitor meters anddummy loads. Bob Mitchell, Apt. 208, Turek Building,Tavernier, FL 33070.

Needed: Horizontal -output transformer (VZ12017) fora model 6911C Broadmoor color TV. Don Gossage,4721 Ipswitch Street, Boulder, CO 80301.

For Sale: Bell & Howell home -entertainment electron-ics course, $70; Bell & Howell school model -24 5 -inchscope with manual, like new, $95. Prices includefreight. Also, have some out -of -print Photofacts totrade for ones I need. Gordon Handy, 300 ViennaDrive #214, Palm Springs, FL 33461.

Needed: Schematic for a Devry Technical Institutescope. Bailey's Radio & TV, 709 Madison Road,Williamsburg, VA 23185.

For Sale: B&K-Precision model 466 CRT tester andrejuvenator. Like new. Send $95 cashier's check forpost-paid delivery. John S. Messier, 2945 Novus St.,Sarasota, FL 33577.

Needed: Old RCA signs, displays or figures of "HisMaster's Voice" featuring the dog Nipper. C. E.Garrison, Box 604 VHFS, Warrenton, VA 22186.

For Sale: Heath IM -5228 VTVM, new, with HV probe,under warranty, $75; Heath IM -28 VTVM, $50; HeathIT -3120 FET/transistor tester, $60; Heath IT -27transistor tester, $10; Heath IT -5230 CRT tester/rejuv-enator with 6 sockets, $90; Heath 0-11 scope, -5 dB at5 MHz, $60; EICO 625 tube tester, $20. All in goodshape and operation, with manuals, cables andprobes. Shipped prepaid for the first check. Richard

Muller, Rich's TV, 3731 West 55th Place, Chicago, IL60629.

Needed: Original or copy schematic of model RCB-15Roberts mobile CB radio. R. T. Blinkhorn, WilcoxBrothers, 5157 Liberty Avenue, Pittsburgh, PA 15224.

For Sale: Telematic color test rig with adapters;Heathkit mini color -alignment generator with manual;Bell & Howell scope with manual and accessories.Like new for half price. Clarence Gillow, 608 BlackDrive, Prescott, AZ 86301.

For Sale: Sylvania CK3000 test jig with 57 adaptersand manual; Sencore YF-33 Ringer, new with trainingtape; Heath IT -5230 CRT tester; Heath IG-57Asweep/marker generator; Heath IG-28 color/dot gene-rator; Heath IG-102 scope with special probes; HeathIG-37 stereo generator; Heath IG-102 RF generator;Polaris HV probe; model 213 EMC tube tester; andTelematic substitute tuner, battery model. Makeoffers. Mike Murphy, 40512 Regency Drive, SterlingHeights, MI 48078.

Needed: Zenith radios with big round dials; cathedral -type radios; type 53 and 3KP4 tubes; and manual orcopy of instruction manual for a model E200CPrecision signal generator. Don Patterson, 636Cambridge Road, Augusta, GA 30909.

Needed: Any technical information about Microswitchkeyboard, 82SW??-3, customer part numberA59731000 -017P. Will buy or copy and return. PascalLarmet, 1429 Elva Drive SW, Atlanta, GA 30331.

For Sale: The following Heath equipment: IM -48 audioanalyzer, $45; IG-37 FM generator, $50; IG-28color/bar generator, $50; IG-72 audio generator, $15;IT -3120 transistor tester, new, $45; IG-102 RFgenerator, new, $25; educational kits EF-1, EF-2 andEF-3, $30; PKW-101 probe, $12. Also 3 years ofElectronic Servicing, $30. D. J. Mace, RD4 Box 84,Bellefonte, PA 16823.

Needed: Model 680 Conar color generator, need notwork. J. A. Quarato, 802 Holland, Vandergrift, PA15690.

For Sale: Model 970 B&K-Precision transistor Analyst,$250; VIZ WR-50C RF generator, never used, $100.Gary Castellini, 3567 Lincoln Avenue, Vineland, NJ08360.

Needed: TLY306 (S) deflection yoke, for 114° 22mmneck size, and it has 5 leads; or a suitable substitute.Michael Helgerson, Box 103, Electric City, WA 99123.

For Sale: Lectotech V7 combination color/bar genera -


Readers' exchange

tor and vector scope; model TR-15A Sencoretransistor checker. Both in excellent condition inoriginal cartons and with manuals. Max Goodstein,25-11 Union Street, Flushing, NY 11354.

Needed: Schematic and parts list for model 500CFisher receiver. Bill Agard, 19255 S.E. 269th, Kent,WA 98031.

For Sale or trade: Large quantities of 6AU6, 6AG5,6AL5 and 6CB6 tubes, and 100 TV tuners. Makeoffers. Troch's, 290 Main Street, Spotswood, NJ 08884.

Needed: These parts for a model 770 Concertonestereo -tape recorder: flywheel idler wheel, tapetransport motor, flywheel -idler tension spring, and apressure roller. Or a source of Concertone parts. J.Baud, 1008 5th Avenue N.W., Austin, MN 55912.

For Sale: Radiation -sensor head containing 6 1B85geiger tubes, salvaged from Warwick. Tubes sell for$36 each, but condition is not known. Send $35 checkfor shipment by UPS. Radio Control Central, P.O. Box56122, Chicago, IL 60656.

Needed: Service or operating manual for model 535Tektronix scope. Will buy, or copy and return. JohnMaxin, 217 West Larkspur, Munhall, PA 15120.

For Sale: Heathkit model 10-12 5 -inch scope with 2probes, manual and schematic, $85; B&K-Precisionmodel 415 sweep/marker generator with cables,manual and schematic, $300; Castle VHF tunersubstituter with cables and manual, $25; model A-460Approved Electronic VHF field -strength meter, $40;180 Photofacts below folder 400, $90; Sencore YF-33HV probes, new, $25; Bendix AN5851-1 sextant withmanual, $75. Long's TV Service, 720 Goshen, SaltLake City, UT 84104.

Needed: Original or replacement interstage audiotransformer for model 55C Atwater Kent antiqueradio. Has been replaced with a Thordarson T -33A91,although originally the interstage and output trans-formers were in the same can. Frodge TV & Radio, 41East Main, Mt. Sterling, KY 40353.

Needed: 12FR8 Tube. The Lectronic Shoppe, 102 W.Main St., Everson, WA 98247.

For Sale: Heathkit IG-57A sweep and markergenerator, assembled and calibrated with probes,manuals, attenuator, cables, $150. L. Hynar, 6408 E.Ellis, Mesa, AZ 85205.

For Sale: Zenith color TV test rig model 800-880;11/2 -years old, in like -new condition, with 21 -inch

diagonal color picture -tube and all cables forconnection to tube or solid-state TVs, $280. FredBlair, 2114 Cowlin Ave., Commerce, CA 90040.

For Sale or Trade: Portable record players withautomatic changers; 500 paperback books for radioand TV test equipment. Troch's, 290 Main Street,Spotswood, NJ 08884.

For Sale: Rem cathode recovery unit and CRT checkerwith adapters, $190; B&K-Precision DVM model 280with direct 100k probe, $65, needs resistor; SencoreHybrider, $200. Raymond Duffy, 1821 NE 65 St., Ft.Lauderdale, FL 33308.

Needed: Scopes, Heath IT -12 signal tracers, wow andflutter meters, distortion analyzers, frequency count-ers and other test equipment. J. C. Clark, 1702Converse, Cheyenne, WY 82001.

Needed: Hitachi digital MOS IC, Hitachi type HD3107.Robert W. Miller, Rt. 1, Anadarko, OK 73005.

For Sale: Sencore Super Mack color pix tubetester -rejuvenator, like new, $250 or best offer. DonSteadman, 8822 Juniper Court, Orland Park, IL 60462.60462.

For Sale: EICO 10MHz scope with probes, solid-state,

triggered sweep, $300; RCA RF generator, 170kHz-50MHz, $60. Heath electronic switch, ID -101, factorycalibrated, $40; Heath VC -2 voltage calibrator, $15;Heath IP-27 low -voltage power supply, 1/2-50V at 1.5a,factory calibrated, $150; Heath IP-17 high -voltagepower supply Ov to 400Vdc factory calibrated, $150.All units are complete and have manuals. John A.Alvarez, P.O. Box 522, Jackson Heights, NY 11372.

Needed: Craig FM stereo, 8 -track floor -mount tapeplayer, model 3142. Charles Wilson, 1406 StephensAve., Anniston, AL 36201.

For Sale or Trade: 75 trunk and gutter mounts andcable for 3/8 whip, four 8 -ft high -output fluorescentfixtures with tubes. Need CRT tester and dual -tracescope. A. D. Electronics, 108 Carey St., Deerfield, MI49238.

Needed: Operating manual for Superior TV50 Genome-ter. For sale: RCA 113599 flyback. Pauls Radio andCB, 1108 Normdave Dr., Dayton, OH 45418.

Needed: Schematic or manual for model CB Solarcapacitance/resistance checker. Will purchase, orcopy and return. For sale or trade: 100 loctal tubes,all types, some in cartons, all checked for quality andshorts, $50 postpaid. Howard Adams, 209 W.Shadywood Dr., Midwest City, OK 73110.

new kid on the block.Aa0

We're PTS and our reputation as a full service company has grownlike blockbusters. In fact, more and more servicing dealer/tech-

nicians are using PTS services as building blocks for a profit-able foundation for tuner and module repair. You see, we do it

all ... tuner repair, module rebuilding and exchange, and wecarry a full line of equipment, tuner test analysts and instru-ments, original or superior tuner parts, replacement baluns,

replacement semiconductors and alignment tools andaccessories. We're PTS. The Only Name You Need To Know.

PTS products are available from any PTS stocking distributorand PTS tuner/module servicenters located nationwide.


PTS ELECTRONICS, INC.The Only Name You Need To Know

P.O. BOX 272, BLOOMINGTON, IN 47402

,;*' 6 \,6,6*66,.",,V66.',6-6 '`- ':'41t0f,AV 6

O 0 in Me news

Hank Hermes, US JVC Corpora-tion's vice president of service andengineering has been presented thefirst annual Distinguished VideoIndustry Service Award. Hermeswas cited for his work in expandingand coordinating the work of ser-vice personnel with that of dealersand distributors.

Wahl Clipper has announced thepromotion of Ruth Heflebower toassistant to the sales manager forWahl's line of ISO -TIP solderingirons, guns and accessories.

David C. Carlson has been ap-pointed manager, advertising com-munications for Quasar. Carlsonmost recently held the position ofpublic relations manager for Wal-green.

Donald M. Cook has been ap-pointed division vice-president andgeneral manager, RCA Distributorand Special Products Division. Cookwas previously division vice presi-dent, government services marketingfor the RCA Service Company.

Hitachi has announced the ap-pointment of Clyde W. Smith asvice-president of research and de-velopment. Smith was previouslydirector of a/v engineering at Thom-son-CSF and was responsible for themicrocam program.

William P. Feely, III, formerlydirector of transportation, Quasar,was named one of 15 winners ofFleet Owner magazine's 1979 awardfor outstanding vehicle color anddesign. The award was presentedMay 24 at the culmination of the17th annual National Fleet OwnerConference at the Drake Hotel inChicago.

Shure Brothers has announcedthe promotion of Joseph J. Kaleba tothe position of vice-president ofmanufacturing. Kaleba was previ-ously assistant vice president ofmanufacturing.

William F. Quinn has been ap-pointed personnel director of Chan-nel Master. Prior to coming to thecompany he was personnel managerof Transkit.

Burton Bard has been appointedregional sales manager for AlphaWire. Before joining Alpha, Bardwas with C.C. Electronics serving assales manager.

Dr. Charles M. Herzfeld has beenappointed director of research forITT. Herzfeld had been technicaldirector of ITT's Telecommunica-tions and Electronics Group, NorthAmerica. William W. Grossman hasbeen elected a vice president ofITT. Crossman is group generalmanager of ITT's Illumination andElectrical Products group world-wide.

James L. von Harz has beenelected a vice president of ITT, vonHarz is group general manager ofthe company's North American Com-ponents group and ITT Cannonworldwide.

Joseph V. Cherry, director -qualityassurance for ITT Cannon Electrichas been elected a vice president ofthe North American division of ITT.Previously, Cherry served as qualityassurance director for Trivex. Alex-ander R. Brishka, manager of RFconnector operations for ITT Can-non Electric, has been promoted to

manager engineering for ITT Can-non's plant at Phoenix.

George J. Mitchell has beenpromoted to director of MAXARProduct Operations, Communica-tions, Products Division, Motorola.Previously, he was product managerof the MAXAR product line.

Continental Specialties has addedSid Cottin to its executive staff.Cottin will serve as consultingadvisor to CSC's sales and market-ing departments.

Charles Levine has been namedmerchandise manager for RadioShack consumer electronics storechain. Levine will be responsible forworking with the company's staff ofbuyers in the areas of productselection and advertising.

Becca Bowen and Bernie Hoch-man have been appointed districtmanagers for Gusdorf Corporation.Bowen is responsible for sales andmerchandising in southern Texas.Hochman is responsible for salesand merchandising Gusdorf Elec-tronics Furniture in Missouri, Iowa,and southern Illinois.

PTS President Roland Nobis (right) presents the "Man of the Month" award toBill Terrell, PTS regional manager. Terrell is responsible for the company'sbranches in Detroit, Grand Rapids and Columbus, OH.

10 Electronic Servicing August 1979 Circle (14) on Reply Card --o-

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EN N

-9/o o11 MT

ig_A test labEach report about an item ofelectronic test equipment isbased on examination andoperation of the device in theELECTRONIC SERVICINGlaboratory. Personalobservations about theperformance, and details ofnew and useful features arespotlighted along with tipsabout using the equipment forbest results.

By Carl Babcoke

Hickok model LX -303 digital multi -meter is pocket sized, but it hasmany performance and automaticfeatures of larger instruments.These features include a 31/2 -digitliquid -crystal display (LCD) that iseasier to read in bright lighting,automatic polarity indication, auto-matic zeroing, automatic overrangeindication and stable, accuratereadings.

DC voltsAccuracy of the five dc voltage

ranges (that provide readings from0.1mV to 1000V) is ±0.5% reading

0.50/o of full scale (FS). Inputimpedance is 10MQ for all ranges.An X10 slip -over probe extends themaximum readings (and the protec-tion) to 10kV.

AC voltsTwo ac -voltage ranges (0-100y

and 0-1000V) provide readings from0.1V to 600V with ±10/0 of reading±0.50/o FS accuracy. Frequencyresponse of the 100V range checkedflat to 400Hz, -6dB at 3.5kHz, and-12dB at about 6kHz. Input impe-dance is rated at 4.3MQ .

Resistance readingsSix ranges measure resistances

between 0.10 and 19.99M0 . Ac-curacy is ±0.50/o of reading .±.0.50/1)FS for all except the 20M0 rangewhich has 1.50/o accuracy.

Voltage across the test leads at afull 1999 readout is about 300mV(0.3V). Therefore, silicon transistorsand diodes do not conduct, andaccurate resistance readings can be

Figure 1 Advanced features of theHickok model LX -303 small digitalmultimeter include a range -switchknob that moves sideways, and anunblinking number 1 and decimaloverrange indicator.

obtained in -circuit. If a diode ortransistor resistance must be tested,the XIM g or X10M0 ranges shouldbe used because of the smallcurrent.

DC currentCurrent between 0.01nA and

199.9mA is measured in six dc -cur-rent ranges with ±0.50/o of reading±0.5% FS accuracy, except the100mA range which has ±2.5°/o.Voltage drop across the meter doesnot exceed 200mV.

Selecting functionsOhms or volts/milliamperes func-

tions are selected by the on/offswitch (Figure 1). Desired rangesare obtained by sliding the rangeswitch sideways. Multipliers forac -voltage and dc -voltage rangesare above the range -switch knob,and those for resistance and cur-rent are below. Four banana jacks(recessed to prevent electricalshocks) marked common, ohms/mil-liamperes, dc volts and ac volts areprovided for the test leads.

Maximum count of the LCD dis-play is 1999. For example, the100Vdc range can show a readoutup to 199.9V before overrangeoccurs. Overrange for any functionis indicated by a steady display ofthe left-hand number 1 and thedecimal point without other num-bers (Figure 2).

Minimum life of the 9V battery issaid to be 200 hours, but typicallyabout 300 hours can be expectedfrom an alkaline battery. A slidingdoor above the LCD display coversthe battery.

Voltage ranges are protected to1000Vdc or peak ac, and resistanceranges are protected to 120V.


Figure 2 At the meter's right is theX10 dc probe, while the padded caseand protective lid are above. The caseis optional, but the test leads and lidare provided with the meter.

General featuresHickok model LX -303 is small

enough to fit in a hip or coat pocket.Dimensions are 1 -3/8"x3 -3/8"x5-7/8" and the weight is 12 ouncesincluding the battery. A snap -oncover serves as storage for the twotest leads and protects the meterwhen it is transported in tool box ortube caddy. Readings on the sharpblack -on -white display are updatedthree times per second so tests canbe finished rapidly.

Optional accessories for LX -303include a padded carrying case, anadapter for 120Vdc operation, a10Adc current shunt, an X10 slip-over probe for dc voltage, and a40kVdc probe for checking TV highvoltages.

CommentsComparison with several digital

meters of 0.10/0 ratings showed theHickok LX -303 had good accuracy.Also, it successfully passed thedifficult test of providing accuratereadings with half -wave unfiltereddc voltage. (Some digital meters givelow or erratic readings during thistest.)

The slip-on X10 probe allowedsafe readings of dc voltage at thehorizontal -output tube plate and theCRT focus voltage. Also, it gave farless detuning or loading than otherprobes when measurements weremade in IF and oscillator stages.

The sample meter performed verywell on all functions, and noproblems were experienced. Espec-ially appreciated was the assurancethat no meter damage will occur ifthe meter (with test leads andcover) is inserted into the carryingcase and then tossed into a toolbox.


Case studies:

Eliminating RF interference

By Wayne Lemons, CET

Side effects of radio frequencypollution often take many differentforms. Symptoms range from inter-mittent interference lines on certainTV channels to the audio of CBtransmitters that is heard overpublic-address systems.

One reason for the variety ofsymptoms is that the interferencescan enter the equipment eitherthrough the RF/IF section orthrough the audio stages. Thetechniques for eliminating these twobasic kinds of interference are verydifferent.

Therefore, the unusual testingsystem described here applies onlyto accidentally detected audio thattravels through audio amplifiers.

Demodulated audioWhen modulated RF of sufficient

amplitude enters an audio amplifi-er, the high level can drive a tubeor transistor into a nonlinear partof the operating curve. This non -linearity' demodulates the carrierand produces audio from theoriginal modulation. After this aud-io is formed and added to thedesired audio in the amplifier, itcannot be eliminated. The onlysolution is to prevent the demodula-tion.

The cure, therefore, is to reducethe level of the modulated carrieruntil it is too weak to causeaccidental detection. RF signalscannot be heard, so there is noproblem so long as the RF carrieris not detected.

Here is an effective new method of identifying and removinginterference coming from modulated RF carriers in audioamplifiers.

Bypassing the RFEliminating RF from an audio

stage appears to be very simple.Just bypass the RF with a capacitorof small value that will not affectthe normal audio. Although theidea seems plausible, the addedcapacitor seldom helps enough, andoften makes the interference worse.

High -frequency signals are notlikely to enter at the audio -inputterminals. Most inputs haveshielded cables and wiring whichshould act as a bypass capacitor.Additionally, many amplifiers have100pF to 330pF fixed capacitorsconnected directly across the inputs.

These interfering carriers use thepower cables, speaker wires, anyother unshielded wires or even themetal braid of the shielded wires toenter the amplifier wiring. Suchwires act as antennas to bring theRF signals near the amplifierstages.

Next, the unwanted RF signalsuse the chassis or a ground loop ofthe board wiring to flood theamplifier with a strong RF level.Although the RF can reach asusceptible transistor input by straycapacitances of the wiring, it maytravel through other hidden paths.Amplifier wiring has many unsus-pected impedances and resonancesto RF frequencies which couple andeven amplify the RF. When suchstrong carriers reach a transistorbase, demodulation can occur.

Wrong bypassingAn incorrectly connected bypass

capacitor can increase interference,as shown in Figure 1. The internalcapacitor forms a loop to couple

the RF on the wire inside thechassis, and the loop acts as anantenna to radiate the RF energy.

The better way of bypassing anywire that enters the shielded chassisis to use a feedthrough capacitor orconnect a conventional capacitoroutside the chassis (see Figure 2).

The top scope trace is typical ofmodulated RF carriers that are de-modulated to audio (bottom trace) bynonlinearity in amplifiers.

Figure 1 Bypassing RF inside theshielded chassis can form a loop tocouple the RF from the wire to thecircuit wiring.


Shielding and ground wires

One misconception is that ametal shield around an amplifierwill prevent RF signals from enter-ing. But shields are not absolutelynecessary. In fact, the interferingsignals might use the metal chassisas an injection loop.

Providing a true ground at thesehigh frequencies is almost impossi-ble. A chassis grounded to earthmight be hot at some points andcold to RF at others. Also, aground wire probably won't helpbecause every wire with RF has hotand cold points at each quarterwave length. A random -length

ground wire might even increasethe interference.

RF impedancesAs stated previously, the princi-

pal cause of demodulated interfer-ence in an audio amplifier is highence in an audio amplifier is highimpedances to RF (circuit reson-ances). These unintentionally tunedcircuits can multiply the RF levelby many times, and they areformed by the wiring versus thestray capacitances.

For example, a bypass capacitorfrom input plug to ground mightact as a coupling capacitor from anRF-hot spot on the ground direct tothe base. In other circuits, the

WIRE

FEED THROUGH

CAPACITOR

A

CHASSIS

001µFBYPASS

CAPACITOR

OUTSIDE

BETTER RF BYPASSING

CHASSIS

B

Figure 2 Here are two good methods of preventing RF from entering the chassison a wire.

SPEAKER

AM SIGNAL GENERATOR

WITH AUDIO MODULATION

550 kHz TO30 MHz

AUDIO AMPLIFIER

CONNECT TO TWO GROUNDS

Figure 3 A modulated generator signal is connected to two widely separatedgrounds of the amplifier. As the frequency is varied, the generator signalactivates the unplanned RF-resonant circuits so they can be swamped, detunedor the RF removed from transistors by filtering.

shunt capacitors complete a reso-nant Pi network that takes a weaksignal at low impedance and trans-forms it to a stronger signal at highimpedance.

The surprising discovery aboutresonances in audio amplifiers isthat there are so many of them. Asingle input circuit might haveseveral resonances between 10MHzand 220MHz or higher. Within thisrange, the resonance peaks can beeither broad or sharp. Breaking upthese resonances is the secret ofeliminating audio -rectified interfer-ences.

FM audio interferenceFrequency -modulated (FM) sig-

nals often are demodulated inaudio amplifiers by hidden tunedcircuits and nonlinearity.' Thisseems impossible. Normal FM re-ceivers must have elaborate demod-ulation circuits that operate byphase shift to produce audio. Noneof that is found in an audioamplifier.

However, there is a more simplekind of FM demodulation (calledslope detection) where the signalmoves up and down the skirt of aresonant circuit. Thus FM audio isrecovered by conventional AM de-tection.

In audio amplifiers, the frequen-cy slope is formed by the accidentaltuned circuits, and the nonlinearityof an overloaded audio stage sup-plies the AM detection. Both condi-tions are necessary for FM audio tobe produced in an audio stage. Theinterference can be prevented byeither swamping the resonances orreducing the nonlinearity (or both).

These fixes are based on practicalexperience. When I first announceda series of FM broadcasts, I wassurprised and dismayed to findmany record players, tape recordersand sound systems in the vicinitywould reproduce the audio. There-fore, one of my extra jobs was toeliminate the interference.

Silent interferenceEven more puzzling are the cases

where FM carriers cause troublewithout being heard. One suchproblem involved the backgroundmusic and paging system in asupermarket. Intermittently themusic would stop and no micro -


RFI interference

phone paging could be done duringthat time. The 15s to 30s of pagingsilence was accompanied by a loudbuzzing noise in the speakers. Thestore owner found that turningdown the microphone gain wouldbring back the background music.Evidently the problem originated inthe microphone preamplifiers. How-ever, the symptoms would not occurwhile I was there.

Finally, an observant groceryhelper noticed that the problemalways happened when a taxi cameto a nearby taxi stand. The driverwas asked to transmit with his2 -way FM radio as I listened to thestore sound system. His broadcastswere the cause of the mysterioussilences, but he was using the radiolegally and had no responsibility.So, this required elimination of theproblem by modification of themicrophone preamplifiers. Themethod described next provided thecure.

Tracking down interferenceDebugging radio -frequency inter-

ference (RFI) raises many questionsand calls for many decisions. Canthe circuit modifications be madein an electronic shop, or must theoffending transmitter be used as asource while the audio system is inthe usual position? How can atechnician be certain the RFI iseliminated?

In order to answer those ques-tions for myself, I discovered (al-most by accident) a system ofdebugging that is almost infallible.

The only item of equipment is anAM signal generator of any quality.Those with rough and distortedsignals are excellent, so long as theRF output and modulation are veryhigh. The generator applies manyfrequencies to the amplifier as thetesting proceeds.

Create an RF loopAs shown in Figure 3, the

generator is attached so the signaltravels from ground to groundacross the chassis or circuit board.Do not connect the generator signalto the amplifier input or inputs.Allow the input terminals to float.

If the amplifier has provisions fora speaker, connect a test speaker. Ifnot, connect the preamplifier (ortape deck) to a signal tracer orpower amplifier that in turn drivesa speaker. The object is to listen tothe audio coming from the problemamplifier or preamp.

Turn the generator RF outputand modulation to maximum. Turnone channel to maximum gain onthe amplifier being tested. Rotatethe generator dial from one end tothe other of all bands and listen forthe audio tone. Any amplifier thatis susceptible to radio interferencewill produce a generator tone atseveral different frequencies. Prob-ably some frequencies will have alouder tone.

CB interferenceIf the complaint involves CB

radios, tune carefully through the27MHz to 28MHz band. Any tone

INPUT C3

C2

270pE Cl4700

SMALL

CERAMIC

CAPACITORS

AUDIO

AMPLIFIER

TRY BYPASSES

Figure 4 First tack solder ceramic capacitors (having veryshort leads) to these points of the first preamplifiertransistor. Unless the RF interference is worse with themconnected, leave them there.

that's heard indicates the amplifierprobably will reproduce CB audio.

Next step is to stop the audiorectification. From this point, sometrial and error experimentation is inorder, for there are no absoluteanswers.

Generally, the first or secondaudio transistors are prime sus-pects. These usually operate at fullgain without any control betweenthem and the input terminals. Testby listening to the tone as theamplifier gain control (downstreamfrom the suspected stage) is turnedto zero. If turning down the controleliminates the tone, the source ofthe audio rectification is ahead ofthe control.

Try bypasses firstAfter the principal frequency of

the rectification is located, tryconnecting a 470pF ceramic capaci-tor with very short leads betweenbase and emitter of the inputtransistor, as shown in Figure 4.Tacksolder the leads in place. Donot touch the capacitor; a hand canact as an antenna.

A capacitor can be mounted to awooden or plastic tongue depressor(Figure 5) to speed up these tests.

If the 470pF ceramic did notmake the tone louder, leave it inplace and try a 270pF ceramic frombase to ground (or from emitter toground if the input is at theemitter). Try grounding the capaci-tor to various ground points withinreach. If a spot can be found thatradically reduces the interference

TONGUE

DEPRESSOR

DISC

CAPACITOR470pF

/ CAPACITORTAPE

SHORT

LEADS

TEST CAPACITOR

Figure 5 Don't hold a capacitor while temporarilyconnecting it. Instead, tape the capacitor to an insulator(such as a tongue depressor).


Surprised? Don't be. Becausetoday, Ungar has one of the broad-est, most complete lines of sol-dering and desoldering equipmentand accessories in the business.Product by product, you'll findapplication -oriented designs thatlet you neatly handle any job thatmay come along.

Take, for example, theUngar soldering irons. Modularlydesigned to give you a completechoice of handles, heaters andtips. You get the versatility youneed. And fast, easy, economicalon-line replacements. Plus, the

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The OnlyThing We Don't Make

s The Solder,Circle (6) on Reply Card

RFI interferencetone, solder the capacitor there.

Next, leave those capacitors inplace and cut the transistor inputwire (base or emitter according tothe input). As shown in Figure 6,bridge the cut with a 390Q resistorfor the grid circuit (or a 39 Q valueif in the emitter circuit). To checkfor more RF reduction, disconnectthe previously installed capacitors,one by one. Unless the interferenceis reduced by removing one or bothcapacitors, leave them in place.Also, try moving the 270pF capaci-tor to the side of the added 39 Q or390Q resistor that is toward theinput terminals while leaving the

other capacitors in place (Figure 7).If the RF interference remains

excessive, try a 470pF capacitorfrom the first transistor collector toground. Any improvement indicatesthe problem is at the base circuit ofthe second stage. Try the same testsas detailed for the first stage.

A small -value unbypassed resistoradded to the emitter circuit (Figure8) might reduce the RF interfer-ence. Try a .01uF ceramic from theB+ (or B-) supply to ground. If thepower line enters the chassis, trybypasses as shown in Figure 9. (Inone stubborn case it helped tooperated a preamp from batteries.)

PersevereIf the RF interference has not

been eliminated by now, the ampli-fier has a stubborn problem. Butthere are more things to try.

Modify the input wiring bycutting and removing the input wireright at the transistor base. Run aseparate short wire direct to theinput capacitor. If the wire is morethan an inch long, shield it. If ashielded wire is used, ground it atonly one end, trying each end forthe best result. Or ground bothends at separate points. Sometimesa different or smaller input cou-pling capacitor will reduce the

CUT

COMMON

EMITTER

COMMON

BASE

C4

A ADD SUPPRESSOR RESISTORS

INPUT

Figure 6 If the bypass capacitors do not eliminate enough of the audiorectification, cut the wiring as shown and add parasitic -suppressor resistors.

CI

-1700-

CUT ---""

C3

ADD

39

B

Figure 7 Test alternate connections of C2 and choose theone giving the least audio demodulation.

Figure 8 An unbypassed small -value resistor added to theemitter circuit might reduce the RF interference.


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WE'RE AT YOUR SERVICE.Tube Products Department-Owensboro, Kentucky 42301

GENERAL©1979, General Electric Co

ELECTRIC


RFI interference

interference. If it is an electrolytictype, make certain it is not leaky.Ground the input terminal; if thecollector de voltage changes andholds the different reading, thecapacitor is leaky.

What about RF chokes?RF chokes should be used only

as a last resort. The inductance canresonate with stray capacitance andcreate a new resonance. For exam-ple, I once completely removed CBinterference from an amplifier byusing chokes, only to find theamplifier now received the local FMstation!

The Figure 10 circuit suppressedRF rectification in a Bogen pre-amplifier that handled low -impe-dance microphones. The chokeswere VHF types wound on ferritecores. Addition of a 330 n I/4Wcarbon resistor across each chokemight be necessary to break upother resonant points.

Final testsAfter the amplifier seems to have

an acceptable level of RF interfer-ence, again use full generatoroutput and sweep through allavailable frequencies. If no frequen-cies can be heard very loudly, theamplifier is ready for the ultimatetest. Hold a 2W or 3W CBwalkie-talkie with the antenna nearthe amplifier wiring and press thetransmit button. There should beno acoustic feedback and the CBaudio should not be heard in theamplifier speaker.

After the amplifier is reinstalledin its regular location, repeat thetest with the CB walkie-talkie. Ifthe amplifier can withstand thissevere test, it should keep out allunwanted RF signals.

SummaryThe secret of eliminating the

demodulation of RF in any audioamplifier is in breaking up all RFresonances and hot spots by bypassdetuning, with parasitic -suppressingresistors or by rerouting and re-dressing the wiring of sensitiveinput stages.

Use of an RF generator tomonitor the effectiveness of each fixgives assurance that the interferencewill still be missing after theamplifier is delivered.

FEED

THROUGH

CAPACITORS

USE TERMINAL

STRIP

- -CHOKES

METAL BOX

POWER LINE FILTERS

CHASSIS

120V

TO POWER

SUPPLY

CHASSIS

120V

TO POWERSUPPLY

(A)

(B)

Figure 9 These two methods are recommended for preventing RF on the line -voltage cable from entering the chassis.

FERRITE VHF

CHOKESLI L2

LOW IMPEDANCEC3

INPUT _p_r_f 0 0 0 0 0 0

10/IF I

C5_6.1 - C4

470pFAUDIO AMP

C2 1800470 470pF0 -r

BRAID

CURING SERIOUS RF I

Figure 10 A stubborn case of RF interference required this elaborate filtering ofthe low -impedance input to the transistor emitter. If the chokes cause seriousresonances at other frequencies, parallel them with 3300 carbon resistors.


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BECKMANCircle (7) on Reply Card


MRO industrial

By George Laughead, publisher

Over the past year the staff ofthis magazine has researched, stud-ied and planned our response to agrowing area of jobs and applica-tions for electronic servicing. Theone area that stands out is thatwhich we label MRO (maintenancerepair operations) industrial elec-tronic servicing.

It is with pride and excitementthat I can announce to you thestart of a regular group of articleseach month that apply to both the

independent technician and serviceshop, and also to the industrialelectronic serviceman. Forest Belt, aleader in coverage of electronics,servicing technology and techniques,is now a consulting/contributingauthor for Electronic Servicing whowill work exclusively on writing anddirecting the editorial in the MROindustrial electronic servicing area.Forest will enlist the aid of manywell-known experts in the MROfield. This new section will measureup to Electronic Servicing's stan-dards of editorial excellence inelectronic technology.

Occupation, title,job descriptionof respondentSource: Electronic Servicing Market Study

Engineers15.5%

Includes engineering duties indesign, production, operations,industrial controls, calibration,quality control, and operation

and/or maintenance and serviceof electrical/electronic

instruments and equipment, andother engineering titles

TechnicalEngineeringManagement21.0%Includes plant service and/or maintenancemanagers, plant supervisors; departmentheads in service, repair, calibration,design, and quality control: installationsupervisors; and line chiefs

Why MRO?

Because it is the responsibility oftrade magazines like ES to guidetheir readers into new areas fortheir skills and techniques to beused-perhaps even more profit-ably-and because we have a largegroup of readers who are alreadyworking in the industrial sector ofthe electronic servicing field.

Last year ES completed a surveyof our subscribers who classifythemselves as MRO. The surveywas closed after receiving a 31%return. This article features the first

Other1.4°/0

Technicians62.1%Includes service and/or maintenancerepairmen, line electricians, industrialcontrols repairmen, troubleshooters,installation and calibration electricians, andother technician titles


electronic servicing____information on the survey to bereleased in the magazine.

The report revealed the type ofequipment and knowledge requiredin the MRO industrial servicingfield. For one, more than 92% ofthe respondents indicated that aworking knowledge of solid-statecircuitry is required for their jobs,and more than 84% report aworking knowledge of digital cir-cuitry is required. Additionally,approximately 40% indicate thatknowledge of microprocessors andminicomputers is required. Thisillustrates that the level of technol-

ogy implementation has risen to anew high.

Test equipment plays a majorrole in the MRO field. Indicative ofthe increasing sophistication of thefield is that 47% of the respondentsreport that they use logic analyzersand probes as part of their workingequipment. Other sophisticated de-vices, including distortion analyzers,curve tracers, and spectrum analyz-ers, were listed by a substantialpercentage.

The growth of the MRO field hasonly begun. Starting in this issuewe touch only one area with an

In-house facility 86.5%Independent outside facility 13.5%

article on bio-medical electronicrepair and techniques. In Septem-ber, Forest's first article as part ofthis new thrust will appear. Forthose of you not involved in theindustrial area, ES will continue tocarry all the regular departmentsand articles that deal directly withthe problems and concerns of theindependent electronic technician.Our expert in this area, CarlBabcoke, editor of ES, will continueto personally monitor, write andreview. Between Forest and Carl,Electronic Servicing will now offeryou the best of both areas. CI

Type of facilityin whichdepartment is located

Source: Electronic Servicing Market Study


Typical medical equipment

By Joseph J. Carr, CET

Electronic medical equipment ismore durable and usually lesscomplex than comparable consumerelectronic merchandise. Therefore,typical repairs of medical equip-ment are not complicated, but moreimportance must be placed on.competent work because of thehuman lives involved.

Some common and recurrentproblems with medical equipmentinclude component failures as wellas defects caused by nurses anddoctors.

Diagnosing and repairing medical electronics equipment is nomore difficult than servicing home -entertainment machines.

60 Hz interferenceVarious muscular activities of

human hearts produce correspond-ing electrical signals that can bemonitored on the skin's surface.These waveforms are recorded onelectrocardiograms (ECGs). Onepossible problem is an unwantedmixture of ECG signals with 60Hzpower -line waveforms.

Amplitude of the chest signalsmonitored by ECGs is very small,typically about lmV, while the60Hz signals radiated from powerwiring might measure several volts.For example, a conventional servicescope will show a volt or more ofdistorted sine waves when the

RIGHT

ARM SKIN ELECTRODE

RIGHT

LEG SKIN ELECTRODE

DIFFERENTIAL

AMPLIFIER

BALANCEDHUM

CANCELS

TO

VERT

AMP

operator's finger is touched to thescope probe.

Fortunately, the lmV heart sig-nals can be acquired in a differen-tial fashion, while the offending60Hz signal is common to bothECG inputs.

Figure lA shows the three ECGleads during normal operation. Thedesired signal is obtained betweenthe right arm (RA) and left arm(LA), with the right -leg connectionserving as a common ground. TheIC differential amplifier acceptsout -of -phase RA and LA signalsfrom the electrodes as though theywere the only inputs. The 60Hzunwanted signal is picked up

RIGHT

ARM SHIN

BAD

CONTACTELECTRODE

DIFFERENTIAL

AMPLIFIER

TO

VERT

AMP

RIGHT UNBALANCEDLEG SKIN ELECTRODE HUM

B

Figure 1 Differential amplifiers accept signals between the inputs while cancelling anyinputs. (A) Correct connections cancel the 60Hz hum and allow the heart signals tocontact upsets the balance, thus reducing the cancellation of hum and permitting humWith an ECG pulse simulator signal, normal balance of attached electrodes minimizespulse waveform results from bad electrode connections.

in -phase signals applied to bothbe viewed. (B) A bad electrodeto be mixed with the signal. (C)the hum signal. (D) Hum on the


repairs

in -phase by both RA and LAelectrodes. This commonmode sig-nal is cancelled inside the IC. (SeeFigure 1C.)

A poor connection at one elec-trode (RA or LA) adds resistance inseries with that input signal (Figure1B). This reduces the RA/LA signalslightly, but it completely unbal-ances the 60Hz at those electrodes;therefore, the 60Hz interference isnot cancelled (Figure 1D).

During five years of servicingmedical equipment, I have seen nocircuit problems that caused 60Hzinterference. However, the 60Hzpattern is found almost daily in anylarge hospital. It invariably origi-nates in wiring connections betweenthe ECG and the patient.

A typical ECG patient cable ispictured in Figure 2. The instru-ment end of the cable has a 5 -pinmilitary -type connector, and a tip -jack terminal block is at the otherend. The various electrode wires areshort (16 to 24 inches) and theyplug into the terminal block. Mostof the breaks occur in these leaderwires. They can occur at either end,but most often give trouble at thepin -tip end.

First step for troubleshooting the60Hz problem is to short the ECGamplifier input and notice if theinterference disappears. If it does,the problem is external to the ECG.Some technicians prepare a five -pinmale connector with all leadsshorted together so it can serve as a"deadhead" plug. With this testconnector substituted for the cable,the technician operates the leadselector on the ECG. If the baseline remains stable and without any60Hz disturbance at all positions,then the trouble is in the cable.

Next, the cable is tested byconnecting it to the ECG, shorting

Figure 2 A typical patient ECG cable can operate with as many as five electrodeleads. Opens usually occur near the ends. (Courtesy of Electronics ForMedicine)

together all leader wires, and thenobserving the screen while trying allpositions of the lead selector. Nocombination of leads should show60Hz interference.

If the 60Hz pattern persists afterthe ECG and its leads have beenchecked, there are still two possibil-ities: a bad connection is betweenan electrode and the patient's skin;or a bad power -line ground existsat the ECG machine, an electricbed, or any other instrumentsattached to the patient.

Adhesive on the disc electrodestends to dry up, causing a poorconnection to the skin as theelectrode pulls loose. This is asimple matter to correct, but it isnot the technician's duty to replacean electrode on a patient.

In fact, never touch a patient Inany way, even when asked to do so

by a hospital staff member. Thereis a principle of privacy here, and apatient might sue the hospital if anon -medical person touched him orher.

If the interference is gone afteran electrode is changed, but reap-pears about the time the techni-cian's tools are packed, then thenurse must be informed that eithera better electrode or a more -effectiveskin preparation is needed. Someelectrodes (especially the low-costpaper disc types) don't hold proper-ly on moist skin. The heavier typewith foam -rubber backing is recom-mended.

The third wire of a power cableshould ground the ECG instrument.But a poor connection in the acoutlet is a common cause of 60Hzinterference. Hospital -grade outletsare marked with a color dot, and


Medical repairs

are more rugged than mass -mer-chandised types. If a separateground wire is connected temporar-ily to the ECG and it reduces the60Hz pattern, the house electricianshould be instructed to change theoutlet.

Scope trace is too highIn the intensive -care unit (ICU)

areas of a hospital, the ECG ofeach patient is monitored continu-ously on a medical oscilloscope (astrip -chart recorder is used occa-sionally to provide permanent rec-ords).

When such a scope malfunctions,the complaint might be that thetrace can't be moved into the upper(or lower) area of the screen, or thatthe upper (or lower) peaks of thewaveform are clipped.

If either Q1 or Q2 of Figure 3Abecomes open, then the trace canbe moved only in the top or bottomof the screen, depending on whichtransistor is open. Both transistorsare mounted on the main chassisnear the filter capacitors in thisHewlett-Packard scope (Figure 3B).

Another problem involving thesame vertical -output transistors isevidenced by amplitude that variesas the position control is adjusted.In dual -trace mode, one positioncontrol often moves both traces.The solution is to test and replaceQ1 or Q2. In fact, 2N5294transistors are not expensive, andthe shotgunning replacement ofboth is a practical compromise.

Fuzzy ECG chartsA fuzzy or smeared tracing is the

most common complaint againststrip -chart recorders. Often themedical person who reports theproblem will assume the trouble isa worn or "bad" stylus. Of course,this is possible, but a frequentproblem is that the tracing paperhas been loaded incorrectly.

Paper in an ECG machine isparaffin -treated so it turns blackwhen heated, and the writing stylusis an electrically heated shaft (ahollow tube with a resistance heat-ing element). The paper is pulledover a straight writing edge (Figure4A) where the stylus contacts it.The paper should be pulled taut atthe edge, but if someone loads the

paper incorrectly, the reverse ten-sion is defeated and the trace issmeared. Incidentally, it is a simplematter to load the paper correctly,but a longer, harder job to do itincorrectly. It is advisable to bevery tactful when telling a doctor ornurse that the paper was not loadedaccording to factory specifications.

No beam on scopeMonitor scopes are used where

patients are seriously ill, and ascope that's out of order is acritical matter to the medical staff.Without vital information from amonitor scope, they cannot antici-pate a crisis or effectively respondto a crisis after it occurs. Therefore,any severe malfunction in a CCU orICU monitor scope usually justifiesan emergency service call. Fewhospitals have back-up units.

Complete loss of the scope trace

(assuming that strip -chart recordersin the system continue to operate)usually is caused by problems inthe power supply. In fact, thepower supplies of any medicalequipment should be tested firstwith voltmeter and scope. Afterseveral breakdowns of each model,a technician begins to anticipatewhich supply is most likely to fail.

Partial sweepStandard sweep rate for ECG

and arterial blood -pressure monitorscopes is 25mm/S, although otherspeeds sometimes are included. Asawtooth waveform of a 4 -secondrepetition rate (0.25Hz) is requiredto sweep a standard 10cm scopescreen. This deflection usually isprovided by push-pull tubes ortransistors.

Older tube -equipped models havesweep problems usually caused by

Figure 3 Vertical deflection and center-ing depend on push-pull transistors.(A) The vertical -output stage resem-bles those of many audio amplifiers,complete with base diodes to mini-mize crossover (notch) distortion. (B)In model 7803B Hewlett-Packard ECG,the output transistors are mountednear two filters on the main chassis.


weak or dead tubes. Some modelshave zener diodes that limit themaximum amplitude (Figure 5). Ifthe zeners short, the sweep startsnear the center of the screen.

Certain solid-state models usingthe totem -pole type of horizontalamplifier (Figure 3A) have similarsymptoms when the crossover (ornotch) distortion diodes are shorted.The beam is shifted toward themiddle. An examination of thesawtooth waveform by the benchscope will reveal the type of defect(Figure 6).

Wide traceModel 769 multichannel scope by

Sanborn (later Hewlett-Packard)

has sweeps that are reversed fromusual TV practice. The beam isswept horizontally by a 0.33Hzsawtooth, while the vertical heightis deflected at 15.75kHz. Thiswould produce a raster, except forblanking that eliminates the elec-tron beam until it is needed to formone of the traces. A gating amplifi-er generates a pulse that unblanksthe CRT once for each channelduring each vertical sweep. There-fore, a dot of light appears to movefrom left to right across the screenfor each horizontal sweep thatrequires 3 seconds. Actually, thedot of light is made up of manyseparate dots lighted in sequence.Timing of the unblanking pulses is

Figure 4 (A) When strip -chart ECG paper makes a sharp angle near the heatedstylus, clear traces are produced. (B) But when the paper is threaded incorrectly(making a broad curve over the roller), all traces are blurred.

determined by the input signalwaveform.

A band of light across the screeninstead of the expected traces canbe caused by any of several defectsthat disrupt the beam blanking.

Loss of filtering in one of thepower supplies is the usual sourceof these blanking problems. Lookfor excessive ripple and wrong dcvoltages of the +6V, -6V, -200Vand +250V supplies. Failure of aseries -pass transistor in one of the6V supplies can cause a band oflight across the CRT screen.

Another (and more perplexing)cause of the light bands is shown inthe vertical -sweep and high -voltagesection (Figure 7). Notice that the

VIA I2AT7

VERT

OUTPUT

270K

! RM

D2

igh%117 4117

ZENER LIMITERS

Figure 5 Driver stage of the Sanborn/H -P model 769 has zener diodes tolimit the maximum amplitude. Shorteddiodes move the trace to the right.

Ai NORMAL WAVEFORM

iBI CLIPPED WAVEFORM

Figure 8 (A) A 0.33Hz sawtooth is thecorrect horizontal sweep waveform inthe Hewlett-Packard model 769 ECG.(B) Clipped sawteeth are produced bya defect in the horizontal sweep.


Medical repairs

circuit is similar to the horizontalsweep in monochrome TV receivers.(In fact, if a flyback is orderedfrom Hewlett-Packard, the replace-ment will be a Triad D-604, whichis familiar to many TV technicians.)Cl is the first suspect, although C3also should be tested.

Missing section of traceNon -fade medical scopes are

special-purpose digital storagescopes. That is, they have a specialsemiconductor memory (that issimilar to the memory in comput-ers) rather than a storage mesh inthe CRT.

As shown in Figure 8, theincoming analog waveform is digi-tized by an analog -to -digital (A/D)converter. Each amplitude level ofthe analog input waveform isconverted to a binary digital num-.ber that represents the instantane-ous amplitude. Typically, between100 and 300 samples are takenevery second.

The A/D converter can sampleonly one point at a time, so eachwaveform can be represented accur-ately by a table of binary values(which are stored in successivelocations of the memory). Thismemory bank (a shift registercircuit) is scanned as many as 100times per second for display on theCRT screen (the CRT display isrefreshed at every sweep). Then adigital -to -analog (D/A) converterchanges the binary words back tothe original analog levels for thescope's vertical amplifiers.

The memory is updated from theA/D once every few horizontalscans by overwriting the mostrecent data in place of the olderdata. On the CRT screen, thedisplay gives the illusion of areal-time trace that does not fade.

Figure 8B shows one kind ofsymptom seen on the CRT whenone of the shift registers goes bad.The waveform has a void or gap inthe part of the trace that's servedby the defective shift register. Lossof data produces a black space inthe trace.

If one cell (that is, a single flipflop) is defective, only a single dotof trace seems to be missing.Probably it is not worthwhile re-

6CB5

1x2

C4

500pF-r30KV _L

HV

C3

01

OPEN

C2 4j20µF_L 150V

6114

Cl4µF

450V

47

+360V

OPEN

SCOPE VERT SWEEP & HV

Figure 7 Circuit of the Hewlett-Packardthe horizontal -output stage of TVs.

model 769 vertical sweep is similar to

A

CLOCKCONTROL LOGIC

ANALOG

TO

DIGITALCONVERTER

DIGITALTO

ANALOG

CONVERTER

ANALOG

TO VERT

AMP

ANALOG

MEMORY

NON -FADE SCOPE

401011.10

B

4,8

glimmomommo

Figure 8 Medical non -fade scopes haveregister can remove part of the trace.

placing an IC for such a minorsymptom.

A longer gap on the CRT screenis produced when all shift registercells are either set to 00000000 or11111111, and a wrong mark onthe screen results from a few cellsthat are set permanently at eitherhigh or low logic states. To deter-mine the type of defect, compare

a digital memory. A defect in a shift

the incomplete waveform with thesame input signal viewed on aservice scope.

CommentsThese descriptions of typical re-

pairs made to medical equipmentindicate that competent electronictechnicians can perform equallywell in this new field.


A secondlook atwaveforms,part 3Scope waveforms are used toexplain problems with slow -recovery diodes, details of var-istor rectification, some factsabout horizontal drive, plus apassive resistance/capacitancecircuit that seems to give a

gain.

By Gill Grieshaber, CET

Electronic facts, theories andspecific applications are much easi-er to remember when illustratedvisually. Scope waveforms can per-form these illustrations in manycases. Two subject areas that needadditional clarification are the ne-cessity of fast -recovery diodes andthe proper choice of varistors thatare used for voltage regulation.

Fast -recovery diodesAny technician who has installed

a 60 -Hz supply diode as replace-ment for a TV video -detector diodeknows now that those two types arenot interchangeable. Difficultieswith other diode replacements arenot so easy to solve. These prob-lems have arisen from the universaladoption of horizontal -sweep recti-fication. Known variously as pulseor scan rectification, the dc isobtained by rectifying a sample ofthe horizontal -sweep signal.

A 60 -Hz diode will become hotand soon short if used for suchrectification. Fast -recovery diodesare absolutely essential for use insweep -rectified supplies. An expla-nation based on square:wave inputsignal has been given in SamWilson's Technical Notebook ofJuly 1978.

However, the same shortcomingof 60 -Hz diodes can be demon-strated also with sine waves wherethe two peaks are more definite.

Diode switchingThe Figure 1 circuit was set up

to test the switching of variousdiodes at different frequencies.Preliminary tests showed most 60 -Hz power -supply diodes workedproperly up to at least 400Hz. Twofrequencies were chosen for thecomparison waveforms. They were15kHz (near the horizontal -sweepfrequency) and 150Hz, which waseasy to obtain without dial changeson the decaded ranges of the VIZmodel WA -504B sine/square gene-rator.

Results are given in Figure 1.

Two LEDs also were tested. Thewaveforms resembled those of anold damper diode in photo C, butthe voltage yields were much lower.

Clearly, diodes intended for 60 -Hz power are not recommended forsupplies operating from higher fre-quencies. But there are more les-sons to be learned from thewaveforms.

Other diode applicationsDon't use a 60 -Hz diode to

replace a defective one in thesignal -rectifying section of anyVOM or ac voltmeter that is usedto measure audio signals. A slow -recovery diode can cause an erroras large as 50% (-6dB) at 20kHz.

Even more important is theselection of diodes used for verticalor horizontal blanking in TVreceivers. A slow -recovery diode inthose video applications mightcause a black or white line to be

seen at top, bottom or either side ofa raster.

The top trace of Figure 2 repeatsthe 15 -kHz sine wave after it isrectified by the slowest -recoverydiode, while the lower trace is thesame after the generator is changedto square -wave output. Notice thatabout 40% of the negative peakremains there-it was not clippedby the diode. If this diode is usedto replace a horizontal blankingdiode, the blanking would bemissing over about 19% of thehorizontal cycle. A portion of the

All waveforms weremodel T935A Tektronix

taken from a

scope.


Waveforms

left edge of the picture might betoo bright or too dark (dependingon the exact circuit).

Another critical application in-volves the so-called bias -blankingdiodes at the picture -tube grids inRCA CTC36, CTC38 and others.Those diodes appear to pass blank-ing pulses to the CRT grids, thuseliminating any horizontal retracelines. Instead, the negative -goingpulses reduce the grid -to -groundpositive voltages slightly duringeach horizontal -retrace time. If oneof these diodes is slow in turningoff, the positive trailing edge ofeach pulse will force its picture -tubegrid to become more positive, and avertical stripe of color will be seenat the left edge of the raster. Thetint of the color stripe will indicatewhich grid is affected.

Incidentally, any defect in one ofthese diodes brightens that one.color in the raster. In other words,a leaky, shorted or open diode willincrease the brightness of theassociated color. A defective diodeat the blue CRT grid will increasethe blue in the raster (and the colorpicture too, of course).

DC voltage from varistorsVaristors have a nonlinear vol-

tage -versus -current relationship. Ifa varistor voltage is doubled, thecurrent might increase 10 times(varies with type). That seems a bitlike a diode, but a varistor has asymmetrical action that preventsdiode -like operation. Varistors haveno polarity. The same amount ofcurrent will flow for a certain

Figure 1 Top waveform of all photosshows half -wave rectification of a150Hz sine wave. Bottom trace is thesame for 15kHz sine wave. Thepercentages refer to the measured dcvoltage actually obtained versus thepossible voltage. (A) A top -hat -typeconventional 60Hz diode performedvery well at 150Hz (top trace) for a100% dc output. When the frequencywas changed to 15kHz (bottom trace),the dc voltage dropped to only 60% ofthe former reading and the waveformshowed substantial conduction duringthe negative peak. This diode is notsuitable for sweep, scan or pulse

When used in the Figure 1 circuit at150Hz, a slow -recovery diode wave-form shows a tiny overshoot wherethe descending sine wave becomes abase line in the lower scope trace.After the overshoot area is magnified10 times horizontally and 10 timesvertically by the scope, the topwaveform shows that the overshoot isa small area of conduction into thenegative peak before the diode recov-ers and becomes open. Therefore, theproblem of slow recovery exists evenat low frequencies, but it is notnoticeable until the repetition rateapproaches the duration of this unde-sired diode conduction.

positive voltage as for the samenegative voltage.

According to these facts, dcvoltage never should be producedwhen a varistor is placed in arectifier -type circuit. Nevertheless, acircuit that produces a negative dcvoltage by varistor "rectification" isincluded in each chassis of manydifferent TV models.

The answer to these opposingfacts is found in the input -signalwaveform. If sine waves, triangularwaves, ramps or symmetrical square

rectification because any conductionduring the opposite peak producesheat. (B) A fast -recovery diode hadonly a slight pulse at the 15kHznegative -voltage line (bottom trace)and a 96% dc voltage. (C) An olddamper diode gave only 96% at 150Hz(top trace) and 76% at 15kHz (bottomtrace). (D) The base/emitter junctionof a silicon diode gave the bestresults, with identical waveforms and100% dc readings. This is comparableto a video -detector diode. (E) Thesetests were made using only a sine -wave generator, a load resistor andthe diode being evaluated.

B

C

DIODE

150 Hz

OR

15 kHz

SINE

15K

TO SCOPE

AND DC

METER

CHECKING RECOVERY TIMEE


-3V

1 ['leo

FromKiller

When the slow -recovery diode ofFigure 1A was operated from a 50kHzsine wave (top scope trace), theoutput showed conduction of almostthe entire sine wave (bottom trace).

Figure 2 The slow -recovery dioderesponse to a 15kHz sine wave (topscope trace) is compared to a 15kHzsquare wave (bottom trace). Obviously,such diodes are not suitable for use inhorizontal -blanking circuits.

R Y AM P

5GH8A

1 150V

94

R187

01 10%

2. 2

meg

39K 1W

280V

280V

Cath

10000To CRTGrid

CliSparkBias Gap

BlankingPulses

In this dc -restoration circuit from an RCA CTC36 color TV, diode X18 must be afast -recovery type or color stripes will appear at the left edge of the picture.

waves are supplied to the varistorcircuit, no dc voltage will beproduced. Only short -duty -cyclepulses can force a varistor torectify.

Figure 3A is the circuit used totest several parameters of varistorrectification, and Figure 3B showsacceptable results that were ob-tained when the varistor wasmatched to the pulse voltage. Thezero -voltage line is the horizontalline nearest the top, while the lowerline is the average -voltage line. At50V per scope division, there isabout 40VPP between the two lines.Therefore, the dc voltage from thisrectification is about -40V (averagevoltage is less positive than zero

volts); a dc meter measured -38.2V. This dc voltage proves that somekind of rectification has takenplace.

As explained in Part 1 (June), allwaveforms have the positive peakabove the average -voltage line andthe negative peak below it. Thisaverage -voltage line is the referencepoint for any voltage reaching thevaristor.

According to the Figure 3Bwaveform, the positive peak mea-sures 200VPP and the negativepeak has 65VPP. Because thepositive peak has about three timesthe negative peak amplitude, 10 to15 times as much current will flowduring the positive peak as during

the negative peak.Compare those figures with diode

rectification of a 1000 -to -1 currentratio. The varistor operation isabout the same as very inefficientdiode rectification. (A diode usedinstead of the varistor in Figure 3Acould produce about -200Vdc if thepositive peak was rectified.)Matched components-The Figure3A varistor must be carefullymatched against the coupling -capacitor capacitance and the peak -to -peak voltage from the horizon-tal -sweep circuit. A large Cl capac-itance is necessary to prevent anyunwanted tilt and to allow thepulses to pass without much atten-uation. If the amplitude at theoutput of Cl is at least 90% ,of theinput, the capacitance value issatisfactory.

A varistor that is too high inresistance will produce insufficientdc voltage. Another varistor of toolow resistance also might reduce thedc voltage or produce none; howev-er, the reason for this low voltage isdifferent from that of the excessiveresistance. The top scope trace ofFigure 3C is the flyback waveformat the input of Cl, and the lowertrace shows the reduced amplitudefrom Cl when a low -resistancevaristor was substituted. Notice thatthe base -line ringing is not changedmuch, but the positive peak hasbeen reduced substantially. Thiscondition produced less than 1V ofnegative voltage.

Varistor current-The Figure 3Dwaveforms prove that more currentflows during the larger positivepeak than during the negativepeak. Positive horizontal -sweeppulses of 540VPP were connected tothe Figure 1E circuit, producing-74Vdc. The two -tipped pulses withzero and average lines are shown bythe top scope trace. A 1K resistorwas connected between the varistorand ground to provide a waveformof varistor current (bottom trace).Both average and zero lines are thesame, and no dc voltage wasmeasured across the 1K resistor.Notice that the left tip has maxi-mum current and other positiveand negative parts have much less.Therefore, proper conditions wereestablished for varistor rectification.


Waveforms

POSITIVE

PULSES

Cl 265V PP-38V

AND PULSE

WAVEFORML Y BACK 275V PP

470 pF

A

VARISTOR

VARISTOR"RECTIFIES"

PULSES

Figure 3 Definite conditions are re-quired before a varistor can producedc voltage. The input signal must havepulse waveform. (A) The schematic istypical of shunt rectification, but thesignal is horizontal -sweep pulses anda varistor is substituted for a diode.(B) The lower horizontal line is theaverage voltage of the waveform, andthe upper one is the zero line. Whenthe zero line is above the average line,the waveform will measure as anegative voltage. Signal of 275VPPproduced -38V. (C) The same 275VPPpulse signal (top trace) was reduced toonly 85VPP at the varistor when avaristor of insufficient resistance wasused. Less than -1V was measured.(D) Another source of 540VPP pulses

Of course, the varistors arenon -polarized and can be wiredwith either lead toward the signal.Polarity of the rectified dc dependsonly on the pulse polarity. Positive -going pulses produce negative dc,and negative -going pulses producepositive dc voltage.

Horizontal output tubeIt is stated frequently that damp-

er -tube current contributes the lefthalf and output -tube current furn-ishes the right half of horizontaldeflection as seen on the picture -tube screen. Although actual cir-cuits do not have equal damper and

produced -74V. Lines on the top traceare the same as for the previouswaveform. The bottom trace is thecurrent through the varistor, whichproves more current flowed during thehighest tip.

output -tube conduction times, thesaying does clarify the sources ofhorizontal -yoke current. Therefore,this explanation of the grid -drivewaveform will begin with the as-sumption that output -tube platecurrent flows for about the secondhalf of each cycle.

In Figure 4A schematic, the200VPP of oscillator rounded -saw -tooth waveform does not seem tomatch the -50V of bias (-53Vwithout both scope probes). Theseconditions are not possible withclass "A" amplification. However,class "A" operation would havecurrent flow at all times and that is

contrary to the purpose of thecircuit.

Briefly stated, the sawtooth'spositive tip is clamped to ground bygrid/cathode current flow when thegrid becomes a volt or so positiverelative to the cathode. (Comparethis to the Figure 2 schematic onpage 26 in June ES. The twocircuits are the same except thegrid/cathode tube diode is substi-tuted for the solid-state diode.)Therefore, all of the sawtooth(except the positive volt at the tip)is negative. The sawtooth traces aninstantaneous dc voltage that duringeach cycle varies between -200V atthe bottom and about OV at thetop.

Now, the 6DQ5 tube with ascreen voltage of +145V has aplate -current cut-off point between-40V and -50V. Therefore, the tubecannot draw current during thelower -200V to -50V section of theFigure 4B (upper trace) waveform.About 150V is wasted in obtaininga delayed start of plate current.Plate current begins at about -50Vand increases to maximum currentat zero volts (positive tip). Fromthat point the sawtooth dropsrapidly down to -200V and the nextcycle begins.

In the picture of Figure 4B, thetop waveform is the grid sawtooth(50V/division) with the true zeroline added by the scope. Thiswaveform does not show where theoutput tube conducts, but platecurrent is shown by the lower dualtrace. If a line is drawn between thepoint of the lower trace where thepositive sawtooth first begins andthe sawtooth grid waveform above,the intersection of line and saw -tooth shows the cut-off grid voltagewhen measured from the zero lineat the top.

Figure 4C is placed just below 4Bso an accurate visual comparisoncan be made between the two.Figure 4C shows the effects of HVregulation by variation of theoutput -tube grid bias, which isexplained later.

An easier way of proving thecut-off voltage by using a dual -tracescope is shown in Figure 4D. Thegrid sawtooth is forced off thescreen by a 20V/division scope gainsetting and the smaller current


FROM

HORIZ

OSC

200VPP -53V

6DQ5

PULSES AND

-DC

HV

REGULATOR

VARISTOR

POSITIVE

PULSES

FROM FLYBACK

Figure 4 (A) This schematic is typicalof horizontal -output tube grid circuits.Dotted lines show how a varistor HVregulator is connected. (B) Top traceis the 200VPP oscillator signal at theoutput grid, with zero line added bythe scope. The plate current waveformis shown by the lower dual trace. (C)These waveforms are the same asthose in B, but external negativevoltage from a varistor has increasedthe dc grid voltage from -50V to -70V.The instantaneous grid voltage neverreaches zero; therefore, the platecurrent is lower (bottom trace). Both Band C were made with the same scopeadjustments, so comparisons may bemade. (D) To find the grid bias whereplate current begins, the sawtoothwas made taller and then the currentlower trace was moved up until thesawtooth slope crossed the currentline at the corner where plate currentbegan. The vertical position of thecurrent baseline shows the cutoffpoint was -48V. Careful scope mea-surements showed retrace occupied17%, damper current flowed for 28%and output tube current flowed for55% of the whole cycle. But, for thetrace section of the cycle alone,damper current flowed for 34% andtube current flowed for 66%. This isnot the 50/50 ratio usually stated. ifa

waveform is moved up and down bythe vertical -centering control untilthe corner where plate currentbegins is crossed by the sawtoothline. The voltage can be read fromthe scope calibrations. In this casethe cut-off voltage was -48V.

Varistors control HVMany of the latest model tube -

equipped color TVs included varis-tors in circuits that controlled thehigh voltages. As shown by thealternate section of the Figure 4Aschematic, positive -going pulsesfrom the flyback were rectified by avaristor. Then the negative voltagefrom the rectification was appliedthrough isolation resistors to thecontrol grid of the horizontal -output tube where it raised thenegative voltage that was producedby grid/cathode clamping of theoscillator sawtooth. Increased nega-tive grid bias limited the maximumplate current, thus reducing thehigh voltage and slightly narrowingthe picture width. HV regulation bythis method was adequate, but itallowed more HV variations thandid the old 6BK4 circuit.

Figure 4C shows the zero -voltageline in relation to the grid sawtooth.Amplitude and waveshape were notchanged by the added dc voltage.The lower -trace current waveformshows less current than the normalamount in Figure 4B. This wasexpected from the bias increasefrom -50V up to -70V. However, itwas predicted that the higher biaswould move the point (where cur-rent began) to the right on thesawtooth. That point remained atexactly the same location regardlessof bias; only the plate currentchanged. This seemed to be wronguntil the screen voltage was mea-sured. With normal -50V at thegrid, the screen measured +145Vand the HV was 24.5kV. But with-70V, the screen measurement in-creased to +205V and the HV wentdown to 21.5kV.

The increase of screen voltagecancelled the increased negativegrid voltage so the point on the gridsawtooth where plate current beganremained the same in both cases.This explains why the screen -gridvoltage is not regulated (but issupplied through a dropping resis-


Waveforms

Figureleaking coupling capacitor oroutput tube cancels partnegative grid voltage and producesclipping of the sawtooth tip (topscope trace). Plate current (lowertrace) flows for a longer time, thuscausing the output tube to overheat.The measured grid voltage was -40V,or 10V lower.

5 Positive voltage from aa gassyof the

tor); with poor screen regulation,moderate variations of ac and dcgrid voltage have no detrimentaleffect on sweep performance. Butwith good regulation, an increase ofac or dc grid voltages would causea white drive line down through thepicture near the center.

Positive grid-A common problemwith output circuits of this kind isleakage in the grid -coupling capaci-tor or a gassy horizontal -outputtube. Both cases result in a dc gridvoltage that is less negative than itshould be. Figure 5 shows theclipping of the sawtooth tip when apositive voltage was leaked to thegrid, and reducing the grid voltageto about -40V. The flat-toppedwaveform is typical of an insuffi-

0.15SQUARE

fWAVES

A

10K

TO

SCOPE

Figure 6

differentiated by the high-pass filter ofA produces pulses that have almostdouble the total height of the originalsquare waves (B). Can a filter amplify?

A square wave that is

cient negative grid voltage. Ofcourse, the output tube drawsexcessive current and often failsafter a time. Receivers havingbarely enough horizontal deflectionbefore the defect might show a lossof width at the right edge of thepicture.

An allied problem occurs whenthe oscillator plate resistor becomesreduced in ohmic value. Both thedc voltage and the ac waveform atthe output grid are too low. Theoutput tube draws slightly excessivecurrent and the horizontal linearityis distorted, usually with a smallcompression at the right edge of thepicture that appears to be a lack ofproper width. These symptoms arenot very noticeable and they canfool a technician into installing anew set of tubes and hoping therepair will be satisfactory. Ananalysis of the output -grid wave-form is the best method of identify-ing the problem, since the sawtoothis distorted and the amplitude isbelow tolerance.

Can an R/C filter give gain?The conventional way of measur-

ing peak -to -peak voltages with ascope is to measure the totalgraticule divisions between thehighest part of the waveform (tip ofpositive peak) and the lowest part(negative peak). Usually the twopoints that comprise the readingare not one above the other(identical time of occurrence) butone happens before the other.

There is one example of a passivefilter (without tubes, transistors orany amplifying device) that pro -

B

The waveforms were photographedwith half of the scope screen covered.No scope adjustments were madebetween the photographs. Verticallines have been added for clarity.

duces a voltage gain of almost two.When a square wave (Figure 6)passes through an R/C high-passfilter of the proper values, theoutput pulses from one peak to thenext have about twice the ampli-tude of the square waves thatproduced them.

Can you explain whether or notthis is a true voltage gain? If so,write to the Editor.

CommentsA rectifier -type circuit with the

usual diode replaced by a varistorcan produce dc voltage when theinput signal has a pulse waveshape.In order to obtain any usableamount of voltage, the varistorresistance must be carefullymatched to the actual pulse ampli-tude. A diode can supply muchhigher dc voltages than is possiblewith a varistor, so it is a fairquestion to ask why varistors areused in HV regulator circuits. Thereason is dependability. A varistorcan dissipate a strong single pulsetransient (such as a high -voltagearc) without damage. Under thesame conditions, a diode wouldbecome shorted.

Editor's Note: The profitable re-pairs of modern solid-state circuitsdemand a higher level of awarenessabout the behavior of sophisticatedelectronic circuits in addition tohelp from better test equipment.These two can be combined in theanalysis of scope waveforms. Scopesare universal for any type ofelectronic circuits. No longer is itsufficient to know the approximatewaveshape and peak -to -peak ampli-tude of the correct signal. The trueinstantaneous voltage (both ac anddc) must be known. This is particu-larly valuable when part of awaveform occurs in the cut-offregion. A few examples have beengiven in this 3 -part series. Sendcomments to Editor, ElectronicServicing, P.O. Box 12901, Over-land Park, KS 66212.


m Instructing a microprocessorew

A number of different codes areused for converting decimal num-bers into a more simple system forcontrolling a microprocessor. Twoimportant ones are the octal codeand the hexadecimal code.

Machine languageIt is important to remember that

a microprocessor can operate onlywith binary numbers. Most present-day microprocessors require combi-nations of eight Binary DigITS(bits); each 8 -bit combination is

called a byte.The following is an example of a

byte that commands a 6800 micro-processor to add the contents ofaccumulator B to the contents ofaccumulator A:

0 0 0 1 1 0 1 1

An accumulator is a form ofshort-term memory; each accumu-lator holds a byte of data. In theprevious example, accumulator Ahas been loaded with one byte ofdata and accumulator B holdsanother byte. The 00011011 instruc-tion commands the microprocessorto add together the two bytes thatare stored in the two accumulators.

It is not difficult to write a singleinstruction in binary form. Thistype of instruction is called machinelanguage. However, a single pro-gram (group of instructions) mightconsist of 300 instructions. Such alarge number probably would pro-duce too many errors when copyingthe program into a microprocessor.

Octal codeFigure 1 shows how the 8 -digit

byte is converted into the moreconvenient octal code. First, thebinary number is divided into threesections, beginning at the right.Notice that a zero must be added tothe section at the left to completethe third 3 -digit number.

Table 1 reviews the variousnumber systems, and it can be usedfor reference during the conversionexamples that follow.

By Jack Webster

For speed and accuracy, the binary code needed to instruct amicroprocessor is converted into one of the condensed codes.

Notice that binary 000 corres-ponds to decimal 0, and 011corresponds to decimal 3. FromFigure 1, therefore, the digital00011011 can be expressed as octalnumber 033.

In this same way, all instructionsin 8 -bit bytes can be represented bythree octal numbers per byte. It iseasier and less error -prone for anoperator to punch 033 on a

keyboard rather than a byte of00011011.

When 033 is punched on akeyboard, it is necessary for anelectronic system to convert thatnumber to the binary machine codethe microprocessor understands andcan obey. This is illustrated inFigure 2.

After the octal code for amicroprocessor instruction is

0

ADDED

ZERO

O O 0

O

0 I 0 1

1

3

1

I0

1

1

3

THEREFORE, MACHINE CODE 00011011 EQUALS OCTAL CODE 033

BINARY TO OCTAL

Figure 1 An 8 -digit binary machine code byte is convertedadding a zero at the left and arranging in groups of three.

to octal code by

DecimalNumber

Hexa-decimalNumber

OctalNumber

3 -DigitBinary

4 -DigitBinary

0 0 0 000 00001 1 1 001 00013 3 3 011 00114 4 4 100 01005 5 5 101 01016 6 6 110 01107 7 7 111 01118 8 10 10009 9 11 1001

10 A 12 101011 B 13 101112 C 14 110013 D 15 110114 E 16 .... 111015 F 17 1111

Table 1 Conversionsbetween decimal,hexadecimal, octal andbinary are made easyby this table.


Microprocessors

DECIMAL

KEYBOARDENCODER

1 2 3

4 5 6OCTAL TO

BINARY

7 8 9 ENCODER

0 TO

MICROPROCESSOR

ENCODED INSTRUCTION

Figure 2 When decimal 033 is punched on the keyboard, the instruction00011011 is delivered to the microprocessor.

DROP FIRST ,ZERO .

010

206OCTAL

CODE

MICROPROCESSOR

000 110

THEREFORE OCTAL CODE 206 EQUALS MICROPROCESSOR BYTE 10000110

OCTAL TO BINARY BYTE

Figure 3 Conversion from octal instruction 206 to machine code 10000110 isillustrated here.

4F...'. .I., %- .1..,.0

40. -a/ .....

0 1 0 0

HEXIDECIMAL

CODE

1 1 1 1

MICROPROCESSOR

BYTE

THEREFORE. HEXADECIMAL CODE 4F EQUALS MICROPROCESSOR BYTE 01001111

HEXADECIMAL TO BINARY BYTE

Figure 4 This is the method of converting hexadecimal 4F to machine code01001111.

known, it is easy to determine therequired binary byte for the micro-processor. For example, the 6800octal code for loading the contentsof the memory into accumulator Ais 206 (this code is provided by themanufacturer). Figure 3 shows theprocedure for converting this octalcode to the binary byte.

Each number of the octal code isdivided into a 3 -bit binary numberand the left-hand zero is dropped.The maximum allowable value ofthe first digit in the ocatal code is 3(binary 011) so a left -side zero isalways present.

Hexadecimal codeA hexadecimal code may be used

instead of the octal. Use Table 1for a review of the hexadecimalcount. Sixteen symbols are neededfor the 16 numbers of the count.The 10 Arabic numbers of thedecimal count are used in additionto the first six letters of the Englishalphabet (traditionally, only capitalletters are used here).

According to the manufacturer,the hexadecimal code 4F clears thecontents of accumulator A. In otherwords, the number in accumulatorA is replaced by digital lows whenthe hexadecimal code 4F is deliv-ered to the microprocessor.

Figure 4 shows how the hexadec-imal code 4F is converted to an8 -bit byte for instructing the micro-processor.

A flow chart is used to show thestep-by-step procedure needed toplace a program into effect. Adiamond -shaped block in a flowchart indicates that a decision is tobe made by the microprocessor,and Figure 5 shows an example ofthis kind of step. In this case, valueB is subtracted from A and then adecision is made about the answer.

If the answer is equal to or lessthan zero, an alternate path istaken to the right (in this example).Otherwise, the regular program isto be followed.

The alternate path (to the right)is called a branch or a jump.Branching is done with the 6800microprocessor by order of themachine code 00101111. Conversionof this machine code to the moreconvenient hexadecimal code isillustrated in Figure 6.


SUBTRACT

B FROM A

YES

NO

DECISION STEP

IN PROGRAM

Figure 5 A diamond -shaped block in aprogramming flow chart indicates thatthe microprocessor must make adecision based on the previous step.

BaudotcodeThe previous coverage has con-

centrated on codes used for in-structing the microprocessor. Butthe output of the microprocessor isalso in binary machine language.Normally, this output is convertedinto an alphanumeric readout. Thatis, the readout has letters of thealphabet and Arabic numbers.

One method of obtaining analphanumeric readout is to deliverthe microprocessor output codedsignal to a teletype machine. TheBaudot code (Figure 7) is used forthis purpose.

A 5 -digit binary number has 32possible Baudot symbols. If code11011 preceeds the printout, the bitnumbers that follow represent thefigures column. For example:

11011 (FIGURES)10101 (6)10011 (2)

From those three codes, the ma-chine prints the number 62. Also,the following bit numbers wouldprint the word PIN:

11111 (LETTERS)10110 (P)00110 (I)01100 (N)

0 0 1 0 1 1 1 1

IIII ,/I

I

0 I 0 1 0

2

1 1

F

1

MACHINE

CODE

11

HEXADECIMAL

CODE

THEREFORE, MACHINE CODE 00101111 EQUALS HEXADECIMAL CODE 2 F

MACHINE BYTE TO HEXADECIMAL

Figure 6 According to the example, binary byte machine code 00101111 equals2F hexadecimal code.

BIT5 4 3 2 1

LETTERSCASE

FIGURESCASE

0 0 0 0 0 BLANK BLANK0 0 0 0 1 E 3

0 0 0 1 0 LINE FEED LINE FEED0 0 0 1 1 A0 0 1 0 0 SPACE SPACE0 0 1 0 1 S BELL0 0 1 1 0 I 8

0 0 1 1 1 U 7

0 1 0 0 0 CAR. RET. CAR. RET.0 1 0 0 1 D $

0 1 0 1 0 R 4

0 1 0 1 1 J (APOS)'0 1 1 0 0 N (COMMA),0 1 1 0 1 F !

0 1 1 1 0 Co 1 1 1 1 K

1 0 0 0 0 T 5

1 0 0 0 1 Z1 0 0 1 0 L )

1 0 0 1 1 W 2

1 0 1 0 0 H STOP1 0 1 0 1 Y 6

1 0 1 1 0 P 0

1 0 1 1 1 a 1

1 1 o o o o 9

1 1 0 0 1 B ?1 1 0 1 0 G &

1 1 0 1 1 FIGURES FIGURES1 1 1 0 0 M1 1 1 o 1 x /1 1 1 1 0 V1 1 1 1 1 LETTERS LETTERS

BAUDOT CODE

Additional printout codes will be Figure 7 A 5 -digit binary number at the output of a microprocessor can have 32discussed in a future article. possible Baudot code symbols.


asarAfillsoni,Hotebook

By J. A. "Sam" Wilson, CET

This Technical Notebook de-scribes an experiment that anytechnician can perform. The resultswill prove an important point aboutthe dielectric in a capacitor.

Making an electretA permanent source of electric

flux can be produced easily. It iscalled an electret, and it is con-structed by these steps: Attach an insulated wire to ametal coffee can lid, as shown inFigure 1. Spray the inside with a non-stickcoating such as used in frying pans.The reason is given later. Melt some paraffin in the lid,filling it to the brim. A large candlecan furnish the heat. Prepare a flat piece of metal ofsmaller diameter than the lid (toavoid arcs) and attach a length ofinsulated wire. After the paraffin is melted,remove the heat source. Connect a source of about 30kVdc(perhaps from a TV set) to the canlid and the other piece of metal.Taking care to avoid arcs and

Your comments or questionsare welcome. Please give uspermission to quote fromyour letters. Write to Sam at:

J.A. "Sam" Wilsonc/o Electronic ServicingP.O. Box 12901Overland Park, Kansas 66212

shocks, hold the metal so it barelytouches the top of the paraffin as ithardens. Remember: the 30kV mustbe in contact with the paraffin untilit is solid. (See Figure 1B.) Disconnect the high voltage. Remove the top plate (againusing precautions against shock).Then remove the paraffin from thecan lid; the non-stick coatingmakes removal easier.

That piece of paraffin now is anelectret, which is a permanentsource of electric flux in the sameway a permanent magnet is asource of magnetic flux.

Electrets have several importantuses, and some will be explained ina later issue.

Charged capacitorFor the next step, place the

electret (charged paraffin) betweentwo metal plates (Figure 2). Theassembly is a charged capacitor!

That is a very important state-ment because a charged capacitorhas been created without anycharge having been placed on theplates. After the experiment withthe pails (a capacitor was formedfrom two metal and one insulatorpails), several readers disagreedwith the explanation. Their conten-tion was that the voltage (whichremains in a capacitor after thecharging voltage is removed) isproduced by an excess of electronson one plate and a deficiency ofelectrons at the other plate.

In this experiment, a chargeddielectric was installed between twometal plates. Thus a chargedcapacitor was formed. Obviously,no electrons were moved when the

ADD AND MELTPARAFFIN

WIRE

30KV =-

COFFEE CAN LID

-CANDLE FOR HEAT

METAL PLATE

CAN LID

MAKING AND CHARGINGAN ELECTRET

Figure 1 An electret can be made injust a few steps. Paraffin is melted(A), and while it cools (B), a voltage isconnected. After the finished electretis removed, it retains an electric flux.

Figure 2 When an electret is placedbetween metal plates, it becomes acharged capacitor.


electret was changed to a chargedcapacitor.

It is not necessary for electriccharges (such as electrons) to bemoved into and out of capacitor tocharge or discharge it.

Instead, a capacitor is chargedwhen the electric field between theplates causes the dipoles in thedielectric to become oriented. Theprevious electret experiment wouldnot work properly if the chargingwas accomplished by charge car-riers on the plate surfaces.

Wire capacitorEarlier in the series, a capacitor

made of a solid piece of wire wasproposed. It was based on thecapacitance formula:

c ,kxNxAd

When the numerator was madeto equal the value 1 (k x N xA =1), and the denominator equalto 1/1,000,000,000,000,000 centime-ter, the mathematical result is acapacitor of 1,000,000,000,000,000farads!

A few readers stated the denomi-nator should be d2. Not so. Theequation is correct as written.

Several humorous letters sug-gested applications such as inte-grated circuit capacitors and energystorage for solar cells.

Some perceptive readers recog-nized the major weakness of thewire capacitor. Such very closespacing would cause excessive leak-age, and the voltage rating wouldbe so low that the capacitor wouldbe useless.

Solve these problems and you canstart a profitable capacitor busi-ness.

More questionsReaders continue to ask ques-

tions about operation of the dielec-tric in a capacitor. They ask, "Ifthe dielectric determines a capaci-tor's charge, how can a capacitorwith a vacuum dielectric everbecome charged?"

Also, some readers insist that Istated there is no excess of elec-trons on one plate and a deficiencyon the other.

These are the important state-ments: A capacitor made with a vacuumdielectric can be charged, but it willnot store energy. Energy is stored in the dielectricof a capacitor, while the chargesstay on the capacitor plates. When a capacitor is charged,there is an excess of electrons onone plate and a deficiency on theother.

Nothing contrary has been statedin this series.

Ring counterOne reader asks for the schema-

tic of a free -running ring countermade with neon lamps. If anotherreader can help, please send theschematic in care of the editor.

Innovative experimentsStudents of electronics laboratory

courses need to have the workmade interesting for them at times.When I taught university -level elec-tronics, I gave the students severalopportunities to design their owncircuits. They were urged to beinnovative, so they were told whatwas to be accomplished but nothow to do it. Although I as teacherhad a clear idea about how theproblem could be solved, I oftenwas surprised by the unique (andoften more simple) solutions.

Energy control-In an advancedclass about control circuitry, theassignment was to design a circuitthat automatically would reducehouse temperature at night andrestore the usual temperature byday.

It was expected that the studentswould use their knowledge ofcontrol principles and digital cir-cuitry to solve the problem bydevising an elaborate electroniccircuit. Most groups did take thatapproach, but one lazy group chosea non -digital solution.

A shelf that could be adjustedfor height was placed belowthe wall -mounted thermostat. Onthe shelf was a socket and conven-tional 15-W light bulb with a shadeto direct the warm air upwardtoward the thermostat while mini-

mizing the visible light. A 24 -hourtimer was plugged into a duplexpower outlet and adjusted to turnon the bulb during the night hours.

Heat from the bulb raised thetemperature at the thermostat andfooled it into acting as though thewhole room was warmer. Therefore,the thermostat allowed the furnaceto operate less often, reducing thehome temperature all night. Duringthe daylight hours, the timer turnedoff the bulb, and the home temper-ature was restored to normal.

The system worked very wellafter a few preliminary adjustmentsof the bulb height to obtain thedesired temperature differential.

Other innovative experiments willbe given next month.

Water in light bulb?After the story in the January

issue about students who amazedthe teacher with the light bulb thatoperated normally although waterwas inside, a letter was receivedfrom reader John T. Bailey. Hedoubted the technical accuracy ofthe story. Excerpts are given fromthat letter.

John's first job was to conductvisitors through the Westinghouseplant so they could watch lampbulbs being manufactured. "For ageneral-purpose lamp, the leadwires were sealed by fusing theglass stem around the Dumet wires.Then the stem was fused by gasflames to the bulb and leaving a tipfor exhausting the air by blowing inan inert gas after which the tip wassealed off by fusion. Next, the base(brass in those days) was cementedto the bulb with a bakelite cement,and the wires were soldered to thebase. Now, for the part I don'tunderstand. It is beyond me how aneedle worked in between the glassand the metal base could let anywater into the bulb....the stem presswhere the wires are sealed is wellwithin the bulb and thus out ofreach of a needle. I'm beginning tobelieve the water did come inthrough the wires!"

While all readers are concen-trating on this mystery, I will try toget water into at least one of thegross of light bulbs I purchased.


test equipment

Semi -conductor testersThe LTC -905 curve tracer for

testing solid-state device quality, inor out of circuit, in precise voltageor current steps is being madeavailable by Leader. The unit has asweep frequency of 120Hz withsweep voltage selectable in eightsteps from 10 to 100V. It is alsoequipped with a variable horizontalgain control enabling use with anyscope. It is priced at around $200.


ScopesTwo 15MHz dual -trace models are

included in Tektronix' Telequipmentline. Telequipment D1015 and D1016are dual -trace instruments withautomatic, normal and TV trigger-ing; V/division ranges from 5 mV to20V; time base sweep speeds from0.2 ps to 200ms/division (times fivemagnifier increases the maximum

sweep speed to 40ns/division. Sug-gested list prices are $795 for theD1015 and $895 for the D1016.


Regulated power supplyPTS Electronics has developed a

regulated digital power supply/testinstrument for substituting andmeasuring critical control voltages.DG -5 regulated voltage controlcenter/digital voltmeter is a highcurrent power supply with a full 5Aregulated output over the entire 0 to

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40 525 353

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TYPE 25, 10 24 9

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320x 510

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3t2 ,13 , , 3S25C 1000 35 40 452SC 1013 50 64 7025C 1014 50 64 7025C 1018 70 10 9025C 1030C ISO x10 3902SC 1060 70 60 9025C 1061 70 110 9025C 1079 340 355 3 9025C 1080 340 3 SS 3 9025C 1096 45 SS 6025C 10911 SO 64 025C 1111 420 250 21025C 1114 4 20 0.70 49026C 1115 2 50 3 002SC 116 30 3 40 3 702SC 11115A 3.40 355 3.92SC 1124 110 90 I 002.1127 80 90 1002SC 1162 70 80 902SC 1166 30 40 4525C 11725 320 39 299251

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250 3570250 35.2SO

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2SK 40155250 49259 55

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TYPE 22011.-24 1 9

STK 15 50 S GO 560

TA 706 90 0 10TA ;S2...S0. 00 3 tS

TA 70619 90 1 10 1 20TA 70629TA 749 00 220 2 50TA 71291. 30 1 70 190T 720V SO 270 20TA 72049 00 220 250TA 7205P 60 1 80 2 00T 73100 30 14S 1 60MA COS 90 210 240TC 5000PTC 5419T 50.21,UCIC 002H

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Bench/portable DMMData Tech has introduced the

Model 30L 31/2 -digital multimeter.The unit has a basic dc accuracy of0.10/0 and uses 0.43 -inch LED dis-plays. When the battery option isincluded, the meter displays anindication while charging and anindication when battery voltage islow. When batteries are dischargedto a specified low level, the unit isshut off to prevent erroneous read-ings.

The unit sells for $169.


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LCD -display DMMKeithley .Instruments has intro-

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General purpose scopesPhilips has announced the intro-

duction of two general purpose 35MHz oscilloscopes with digital andcomputer applications. The scopes,PM 3216 and PM 3218 have a maxi-mum sweep speed of 10 ns/div anda trigger hold -off facility that elimi-nates double triggering on digitalsignals, making it unnecessary touse the timebase in the uncalibratedmode, according to the manu-facturer.


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Unless you help.THIS ,PA,E CONTRIBUTED BY THE PUBLISHER

producimIN

Drill bitsKlein Tools has announced a new

line of drill bits. The Klein-Unibitsare used for drilling multiple sizeholes in thin material with a single

bit. The bits are available in sevensizes and will drill holes from1/8 -inch diameter to 1 -3/8 -inch dia-meter. Center punching is not re-quired on most sizes.


Music tapesMusic tapes for The TMM-150 in

standard broadcast cartridges ex-ceed 10 hours of non -repetitiveprograms and can be purchased orleased through Telex distributors.Several features make the unitversatile. For remote controlledstop -start, an external switch canbe connected. A front panel jackaccepts any standard paging micro-phone with on -off switch. Duringpaging the music is automaticallymuted. Separate volume controls formusic and paging are provided. TheTMM-150 fits any size installation,according to the manufacturer.


Power transistorsA series of very fast switching

NPN power transistors with a peakcurrent rating of 50A and designedfor use in power supplies and

amplifiers has been introduced byInternational Rectifier. Designated2N6338 through 2N6341, the devicesfeature a rise time of 0.3ps, astorage time of 1.0ps and a fall timeof 0.26ps at 40A. They have sustain-ing voltages of 100 to 150V andcurrent ratings of 25A (continuous)and 50A (peak). In quantities of 100to 999, they are priced at $3.75 to$9.75.


MilliameterSperry Instruments has developed

an improved version of the modelSP -250 volt-ohm-milliameter withtemperature ranges. The meterhousing has been redesigned usingABS plastics for additional mechan-ical ruggedness. The housing alsohas a tapered effect that allowseasy use of the meter when it isstanding in the upright position. Theunit has capabilities up to 1200Vac,1200Vdc, 600mAdc, four resistanceranges, as well as a temperatureindicator that reads from -40F to1200F. The meter comes ready touse with voltage test leads, battery,fuse, operating instructions and6 -month warranty.


Terminal kitVaco has announced the introduc-

tion of their solderless terminal kitNo. 89949. The kit features 18 eachof 20 of the most popular insulatedterminal styles. In addition, the kitcontains the combination crimpingtool, which slices six sizes of bolts,strips wire, crimps both insulatedand non -insulated terminals andcuts wire.



catalogsWO]ETCO's Electronic Ideas Book is a

64 -page publication with items forhobby, industry and education. Itemsare in stock and ready for immedi-ate delivery, according to the com-pany.


B&K Precision has a 48 -pagegeneral line test instrument catalogdesignated "BK-80." The catalogfeatures a broad range of testinstruments.


General Electric offers a free,pocket-size reference booklet forreplacing the original manufactur-er's part with a GE Color TV HVtripler. The booklet features 21different types of GE triplers andalso includes circuit diagrams andspecifications.


Antenna Specialists has a 100 -page catalog detailing more than250 professional land -mobile anten-nas and accessories. Copies areavailable to qualified 2 -way radiospecialists.


C. M. Levit Electronics offers a6 -page tube and price list. Morethan 2000 types of regular andhard -to -find tubes and transistorsare in inventory.


A. W. Sperry Instruments hasissued a revised comprehensive fullline, short form catalog and pricesheet. The catalog, MES-100 IssueD, contains detailed specificationsfor Snap -Around ammeters, multi -testers, insulation testers, voltageindicators and accessories.


Directional RF wattmetersfromDielectricThe new 1000-AWattmeter fromDielectric acceptsplug-in elementsthat permit RFmeasurements from100 mW to 5000 watts full-scale and from 2 MHz to 1GHz. The large 41/2" meter face is easy to read and has -4- 5%full scale accuracy and great resolution.The 1000-A is available in a convenient all-purpose kit thatincludes a SNIFFER adjustable RF signal sampler, spare UHFconnector and patch cable. All contained in a durablecarrying case.

RF Instruments for ProfessionalsFor the name of your local distributor, call Toll Free 800-341-9678.A limited number of distributor locations are currently available; inquiries welcomed.

Gs DIELECTRIC COMMUNICATIONSA UNIT OF GENERAL SIGNALRAYMOND. MAINE 04071 TEL. 207-655-4555 803-341-9678


You Are Not

ALONE Iok ox

IRS OSHA. FCC

STATEGOV'T

41:..110

LOCALGOV'T

37/1NioloEEOC CONSUMER FTC

ADVOCATES

If you think that being in business makesyou INDEPENDENT, think again!

Consumerists and government agenciesare your silent - but greedy - partners;AND THEIR SHARE COMES FIRST; evenif you don't make a profit.

WELL, YOU DON'T HAVE TOFACE THEM BY YOURSELF

JOIN NESDAAND YOUR LOCAL &STATE ASSOCIATIONS

.0-0N

Send for more information to; 21,

NESDA, 2708 W. Berry St.Fort Worth, Texas 76109

SOLDERINGIRONS

Pencil style. Safety light. Two heats - 20w and40w. 6 tips. Unbreakable handle. 2 and 3 wireneoprene cords.

DESOLDERINGIRONS

Pencil style. Safety light. Some operate at 40w, id eat 20w. 8 tip sizes. 2 and 3 wire neoprene cords.

SOLDERING &DESOLDERING

KITS

Everything neededto solder or de -

solder or both. Allin a handy lifetime metal

box with hasp.

See your distributor or write.. .

Enterprise. Development Corp.5127 E. 65th St Indianapolis IN 46220

PHONE (317) 251-1231



Q1 °marketplaceAdvertising rates in the Classified Section are 35cents per word, each insertion, and must beaccompanied by cash to insure publication.Each initial or abbreviation counts a full word.Minimum classified charge $3.00.For ads on which replies are sent to us forforwarding (blind ads), there is an additionalcharge of $3.00 per insertion to cover departmentnumber, processing of replies, and mailing costs.Classified columns are not open to advertising ofany products regularly produced by manufac-turers unless used and no longer owned by themanufacturer or a distributor.

For SaleT.V. SALES & SERVICE SHOP in central NewJersey. Located on main hwy. 19 years grossed$110,000.00 1978. Zenith franchised dealer.Unlimited potential. Owner moving. Write inconfidence to: Electronic Servicing, P.O. Box12901, Dept. 521, Overland Park, KS 66219.

6-79-3t

AM RADIO reception for interiors of buildings,ships, etc. Catalog. EXTRONIX, 64 Gough,Ivyland, PA 18974. 5-79-6t

REPLACEMENT COLOR YOKES-DEALER ONLY.Zenith 95-2501-2532-2638-2667-S-89633 etc.$19.94, Magnavox 361380-1 $21.95, Sylvania, GEetc. $17.95 to $22.95. Request for price list onyour letterhead. David Sims Enterprises, Inc., 665Jericho Turnpike, Huntington Station, N.Y.11746, (516) 549-3925. 5-79-tf

TECH BENCH kit or plans. Designed for the TV/Radio Tech. Test instrument shelf on two powercolumns with 10 fused outlets. Bench has fourfused outlets with C-MOS grounding, four largetool drawers and neon bench lighting. E -Z buildconstruction. Plans and kit information $1.98plus 50 cents handling. Technical Workshop, Box368, Placentia, CA 92670. 5-79-3t

AUTOMOBILE RADIO and tape replacementparts: Delco, Chrysler, Philco-Ford, Motorola,Panasonic and many others. Large inventory.Laran Electronics, Inc., 3768 Boston Road,Bronx, NY 10469, (212) 881-9600, out of New YorkState, (800) 223-8314. 5-79-tf

TV & RADIO TUBES, 36 cents EA!! Free colorcatalog. Cornell, 4221 University, San Diego,California 92104. 8-76-tf

ELECTRONIC SURPLUS: CLOSEOUTS, LIQUIDA-TIONS! Parts, equipment stereo, industrial,educational. Amazing values! Fascinating itemsunavailable in stores or catalogs anywhere.Unusual FREE catalog. ETC0-011, Box 762,Plattsburgh, N.Y. 12901. 6-78-tf

COLOR PICTURE TUBE REBUILDING EQUIP-MENT. Semi automatic electronically controlledprocess. Complete training. Call or write AtollTelevision, 6425 W. Irving Park, Chicago, IL60634. Phone 312-545-6667. 7-79-3t

HEATH SCHLUMBERGER SG57A TV Post Mark-er/Sweep Generator, like new $150.00, alsoapprox. 300 vintage Sams Photo Fact folders$50.00. J. Criley, 633 Herman Rd., Butler, PA16001. 9-79-1t

TV SALES & SERVICE BUSINESS. Also, storagerental income in Central N.D. Nice, clean, smalltown atmosphere. Fossum, Inc., Cooperstown,N.D. Phone 701-797-3221. 8-79-1t

ELECTRICAL BUSINESS in small central Nebras-ka town. TV and radio sales and service and elec-trical wiring. Will sell inventory and lease or sellbrick building. Reason for selling-retiring.Golden Rule Electric, Palmer, NE 68864. 8 -79 -It

TV REPAIR SHOP INVENTORY. Stock of tubes,panels, parts and service literature. Full line oftest equipment, all In excellent condition. Writeto P.O. Box 538, Big Timber, MT 59011, 406-932-3344. 8-79-1t

LIKE NEW Sound Technology 17008 distortionanalyzer with Intermodulation Distortion and AutoLevel Set options. Late model with connectionsfor Stereo Test Panel; in factory carton $2100.Like new Radiometer SMG 1c Stereo RFGenerator. More than 55 dB separation at 1khz.$800. Like new Measurements Corp. Model 940Intermodulation Analyzer, $250. Call day orevening, Ray Valdes, 305-557-2317. 8-79-1t

FLORIDA-growing TV Business -2,400 sq. ft.bldg., on busy road. W/Living quarters-$55,000-nego. terms. Owner retiring Illness. W12 vans& large TV stock-optional-$10,000. RobertButler Real Estate, P.O. Box 308, Crystal River,Florida 32629. 8-79-1t

ESTES PARK, COLORADO Translator TV stationsplus TV Sales and Service business. Establishedwith room for growth. Prescott Agency, Box1935, Estes Park, Colorado. (303) 586-3331.8-79-1t

ESTABLISHED Zenith, Sony, Sanyo Sales andService. 19 years in business. Good reputation.Full shop with modern equipment. Sellingcomponent stereo and auto decks and radios.3300 sq. feet. Double garage doors in rear.Center of town shopping area. Only Sony TVservice In 75 miles. Good Reputation. Jones TV,140 W. Division, Ephrata, WA 98823, 509-754-4082. 8-791t

PICTURE TUBE rebuilding equipment. Operation-al 6 -position ovens and all associated equipmentand supplies. P.O. Box 1913, Springfield, Mass.01101. 8-79-1t

Business OpportunityMEN/WOMEN FOR FUN! Shaklee IndependentWholesale Distributors earn $200-$5,000 monthly,Bonus, New Car, Etc. Details, Golden, RT2, Box392ES, Fair Grove, MO 65648 Phone (1-417)759-2738. 3-79-12t

WANTED FOR CASH! We buy scrap electronicparts containing gold, silver, platinum! Immedi-ate top dollar cash offer by return mail. Ship to:American Metals Co., P.O. Box 30009, Dept. ES,St. Andrews Branch, Charleston, SC 29407.

11-78-tf

Advertising ServicesTERRIFIC SWAP OFFERS NATIONWIDE! 7issues $3.50. "Electronics Trader," Box 73 -ES,Folly Beach, SC 29439. 8-79-tf

WantedNEEDED: Troubleshooting position in Massachu-setts area. Over 2,000 hours training/experiencein AM/FM radio, B&W and color TV, audio amps,tape recorders, and VCR's. Will soon be CETlevel. For more info please write: Kevin Tribute77B 1613, Box R, Napanoch, N.Y. 12458. 8-79-1t

TV TECHNICIAN AND DEALERS, "INCREASEYOUR INCOME THOUSANDS OF DOLLARSYEARLY," "Rent -Lease -Sell TV's with a guaran-teed system," Send stamp for free details, basicplan $15.00, deluxe version $25.00, master plan$40.00...Perry's Rental System, Box 881, MorroBay, CA 93442. 8-79-3t

Help WantedCLOSED CIRCUIT TELEVISION TECHNICIAN:University of Illinois seeking maintenance tech-nician for small format television equipment.Starting $13,270, raises to $17,035 over twoyears, plus annual increases. Two years elec-tronics study, two years electronic maintenancerequired. Contact: Don Swift, Personnel Services,University of III., Champaign, IL 61820, (217)333-3109. Affirmative Action, Equal OpportunityEmployer. 8-79-31

TECHNICIANS WANTED: Television -audio -com-munications. Excellent fringe benefits, salarydepends on experience. Miller's Electronics,Highway 24, Goodland, Kansas 67735, 913-899-2386. 8-79-2t

advertisers' °la

Beckman Industries 21

The Cooper Group IBC

DielectricCommunications 43

Enterprise DevelopmentCorp. 43

Fuji Svea 40

Gamit Enterprises Inc 41

General Electric Tube Div 19

NATESA 42

NESDA 43

Oelrich Publications 42

PTS Electronics 8-9

Sperry Tech 3

GTE Sylvania IFC

Ungar 17

Wahl Clipper Corp 41

Zenith Radio Corp. ...9-10, BC

Electronic ServicingAdvertising Sales Offices

NATIONAL SALES MANAGER/CHICAGOJim Reilly, 1011 E. Touhy Ave., Suite 245Des Plaines, IL 60018Phone: (312) 299-2601

KANSAS CITY, MISSOURIHelen Hull, P.O. Box 12901Overland Park, KS 66212Phone: (913) 888-4664

NEW YORK, NEW YORKJoe Concert, 4 W. 58th St.New York, NY 10019Phone: (212) 888-1977

FT. LAUDERDALE, FLORIDABrinker and Brinker2240 N.E. 53 StreetFt. Lauderdale, FL 33308Phone: (305) 771-0064

SAN FRANCISCO, CALIFORNIAJohn MacKay, 703 Market St., Room 1109San Francisco, CA 94103Phone: (415) 546-1040

LONDON, ENGLANDJohn Ashcralt & Co., 12 Bear St.Leicester Square, London, WC2H 7ASEngland, Phone: 930-0525

AMSTERDAM, HOLLANDJohn Ashcralt & Co., John J. LucassenSloterweg 3031171 VC-Badhoevedorp, HollandPhone: 2968-6226

TOKYO, JAPANInternational Media Representatives, Ltd.2-29 Toranomon 1-chome, Minato-kuTokyo 105, JapanPhone: 502-0656


now 3-strongXcelite family of attaché

toolcases

TC-150/ST

It joins the other members of the family...Model TC-100/ST, the "big daddy" of Xcelite's cased tool sets,with the greatest variety-a total of 86 types and sizes of drivers,wrenches, pliers, cutters, strippers...and Model TC-200/ST,the 37 -piece set that's unequaled in economy and value.

See the new TC-150/ST at your distributor now. And ask forXcelite literature, which will give you a detailed listing of thecontents of all three Xcelite Attache Tool Cases.

COOPERINDUSTRIES

TheCooperGroupElectronics Division

BOKER® CRESCENT® LUFKIN® NICHOLSON® WELLER® WISS® XCELITE®P.O. BOX 728, APEX, NORTH CAROLINA 27502, 919/362-7511


And here's the newestaddition...Model TC-150/ST...containing an intermediate assortment oftools for the technician, serviceman, orfield engineer. It contains 53 items in all,including 24 famous Xcelite "Series 99"interchangeable -blade tools, a broadvariety of other Xcelite Professionalscrewdrivers, nutdrivers, pliers, cutters,strippers, measuring tape, and specializedelectronic tools, plus a Weller®pencil -type, changeable tip Soldering Ironand rosin core solder. Tools are mourtedin rugged pockets on removable pallets ina durable, attractive case with Whiskey -tan

Marvelon exterior and sun -tan vinylining. Plenty of extra space for

additional tools, prints and manuals!Solid brass hardware and padded

handle are additional qualitytouches.

TC-1001ST

TC-200ST

aus/Ii.e.

FOR YOUR OWNREPUTATION

AND INYOUR CUSTOMERS'

BESTINTEREST

EXACT REPLACEMENTPARTS AND ACCESSORIES

Zenith's Instant Parts Program (ZIP) is thesimplest, least expensive Zenith inventorycontrol system ever devised for TV servicetechnicians. Organizes the most needed, mostused Zenith TV replacement parts and acces-sories so they are where you want them,when you want them.

And ZIP keeps these parts organized thruperiodic checks by your Zenith distributor

salesman who replaces slow moving stocknumbers with new, more popular parts. As aresult, your original investment is protected andyour supply of Zenith parts remains current.

And you need never again risk your repu-tation with "will fits!'

Call your Zenith distributor now for allthe details on the ZIP program that best suitsyour individual needs.

ZENITH RADIO CORPORATION/SERVICE, PARTS & ACCESSORIES DIVISION11000 Seymour Avenue/Franklin Park, Illinois 60131


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