chicago il 60659 heathkit gr-78 generai coverage · chicago il 60659 heathkit gr-78 generai...

Elliott S. Kanter W9KX J3242 W. Hollywood A venueChicago IL 60659

HEATHKIT GR-78Genera I Coverage

Receiver Test/Review

109

built-in whi p antenna will suffice for manyapplicatio ns, but provisions are in corporatedinto . the rear panel for an external an tenna.

That a great deal o f thought went intothe GR-78 's develo pment is evidenced bysuch features as a collapsible carryinghandle , ba ttery-saving switching for panellamps, integral 500 kHz calib rator , cera mici-f fi lte rs and an exce llent noise limiter.Plug-in modulator circ uit board s comple tewit h single wafers of the main se lec torswitch aid in simp le , accurate assem bly andcut the time required to wire this kit toarou nd 35 hours, not counting cali bratio n.

The all so lid sta te circuit ry includes 13sil icon transistors, five of which are FET's,two germa niu m transistors (audio stage), a nda separate PET product detector to provideexce lle nt CW and SSB recepti on . The audiocircuit is novel (see sc hema tic); its trans­fo rmerless audio out put circuit ry combinedwith the a ll solid s tate receiver circ uit

DECEMBER 1974

W ' hen I firs t rea d the ad fo r Heath'sGR-78, I was im me diately enth u­

siastic ab out both it s specifications and itsvery reasonable price . Aft er all , it has manyfeat ures not usually found in receivers cost­ing less than $200. I arranged to acq uire o nefrom Heath to give it the skeptic's " I' m from'I ' . h '"n Issou n . . .s ow me . test .

When my G R-78 arrived , the first th ing Inoticed was the way it had been pack aged ,Eac h component relating to a specific stagein construction was in a special box with anumber keyed to a portion o f th e comp re­hensive instructjon man ual.

According to the specs , the GR-78 is asolid state portab le (in tern al nicad ba ttery)or home receiver operating on eith er120j 240V ac. Its coverage span is 200kHz-30 MH z in six divisions. According toHea th , the receiver is designed primarily forama teur or shortwave listener use. A selec­tion as to band spread coverage desired (hamor SWL) is made during construc tion andtwo calib rated band spread scales a re in-'eluded, o ne for ham use, the other fo rSWL'ers. The internal battery is " floated"across a un ique charging circ ui t wh ich in­sures that it will be fully charged p rovidingthe receiver is operated a few hours or so inthe ac mode. This feature means that in caseof power failure, yo u are st ill on the air asfar as your receiver goes, and for field day,hu rrah ! no more generators, you just operatefrom the internal battery.

The receiver is designed to receive AM/CW/SSB transmissions with a fro nt panelselector switch selec ting the mod e desired . A

l\ HI P

" 1

0101 010 I Q401 0 40<' 0 403

If A\\ P fl 401 151 IF 2\ 0 If" 1Io · 1'; 0 TrlR U ~·.I l,o; l llA\IP I A'.\ PA. '.\ P "' I XE R Fl 404

Av e, 0 '" 1 ~

0 404" 1 -AVC 1 Ave

AMP0 412 0301 0 411 AV C 2

X1Al lOC Al CO NV

eAl oS( OSC ·

. BANO F O"-l Y

0401

IbnPE AKER

PH ONES

d withber ofty, we

electe d to use a very fine tip , an Ungar" Princess" soldering pen cil to minim ized amage to heat sensitive componen ts and toavoid solder "bridges."

During the assembly process, I noted thata goodly number of co mponen ts could beheat sensitive and might conve nie ntly changevalu e if o verheated. Being a pessim ist bynature , I asse mb led each of the pc boards o na large , co mmo n household sponge liberallymoistened with water. The board was placedo n the spo nge co mpo nen t side do wn andso lde ring o pera tions carried o u t. The spongeacted as a giant heatsin k and conductedpossibly damaging heat away from the com­ponents. The purist who never uses heatsinks might be made a believer by the so u ndof hissin g emi tt ing from that sponge . Thishint might save a few hou rs o f troublesho o ting la ter. Besides, the sponge ma kes anexcelle nt work su rfac e and holder for the pcboard .

Fig. 1. Block diagram of GR-78.

1101240 VA C

BA TTE RY1RICU ECHARG ER

pro vides excelle n t re product io n wh ile con­serving o n power d rai n (most importa ntwhe n opera ting fro m the battery) .

The majority o f the componen ts allied tothe receiver including crystals, i-f filters, andthe au dio stage mo un t to the receiver circ uitboard . Th is board is wired firs t and con ta ins<I num ber of steps where by t he componen tde nsity is increa sed gradually u ntil youfini sh asse mbly. a t which point you will note

+ 12V OC10

+ 15vnC

AMAN ' ..., OFF

DEl 0 406 0 401 0 408/0 409

.I' " AUDI O AU0 I 0ANl AUOI O- 0 402 PREAMP DRi VER OU TPU T

"\\

0 40 ~ ew-PR OD \ \ B

DEl 5 TSY ReV

0 4\ 0 BA TTERY

B50 1 tha t the board IS litera lly cra rn meBf O

compo nen ts. Because of the nurncomponents and their close proximi

11 0 73 MAGA ZINE

Refe rr ing to th e blo ck diagram (F ig. I )and the schematic ( Fig . ::!). we note that rfsigna ls received from either the wh ip an­tenn a o r an externa l an te n na are co upled tothe input tuned circui t for Band A. T hecircuit consists of anten na coil LI . tr im merca paci to r C I and the fir st sec tio n o f capaci­tor C50 I A. The signa! is then coupled togate I of F ET QI OI via a ta p o n LI. Biasvo ltage fo r Q I0 I is ob tained from the A VCamp li fier a nd applied to ga te 2 . Sourceresisto r R I 0 3 and rf gai n co n trol R50 Iprovide the pro pe r d rain curre nt fo r n ormalope ra tio n.

The amp lified sign al from Q 1-1 is fed to atap on rf coil Ll O! which togeth er wit ht rim me r capaci tor C 10 I and capaci to rCS OIb form th e in p ut tu ned circ u it of t hefirst mixer stage Q ::!O l .

The firs t mixer, Q20 I is a dual gate FETfeaturing excellent freedom fro m c ross­modulation , o verloa d ing and t he pu llingeffect on an oscilla to r tha t a strong signalfreq ue ntly has. T he amp li fie d signal fr o mQ IO I is app lied to gate I of Q20 2 throughcapacitor C I I I . The lo cal oscillator signalfrom Q30 1 is applied to gate 2 of Q20 1bia sed by resistors R20 3-20 4 . The dco pe ra ti ng point is est ablish ed by sourceresistor R 20 5.

The in coming signal and the oscilla to rsigna l arc he terodyned in Q20 I resultingin an i-f frequency and mixer p roduct. Onbands A through E the o u tpu t of Q20 I isfed to a tap o n coil L20 1 reso nating wit hcapac itor C20 I to p ro vide a 4 .034 Mll zo u tpu t signal.

The o u tput fro m Q 20 I is coupledthrough ca pac ito r C205 to gate I o f FETQ40 I , which o pera tes as an i-f a mp li fier o nBands A -E and as a mi xer ( 2d m ixer)providi ng dou ble co nversion o n Band F.Operat ing bias for gate 2 is p rovided byresisto rs R-40 1-40 ::! . When the band switch isin t he Band F po si t io n, a 3 .579 MHzinj ec tio n signa l fro m co nversio n oscilla to rQ4 1 I is applied to ga te 2 o f Q40 I.

This signal comb ines with the 4 .0 34 Mll zsignal from the I st Mixer O::!O I an d result sin an o u tpu t of 4 5 5 k Hz plu s th e mixerproduct fre que nc ies. T his o u tpu t signal isthen coupled through capacito r C4 10 toceramic p assba nd f ilt e rs F L40 I through

DECEMBER 1974

F L404 . These filt ers serve to shape t heba n dpass and atte nuate a ll fr eq ue ncies ex­ce pt the i-f fre q ue nc y of 4 55 k Hz. Th is i-fsignal is coup led to the base of the first i-fampli fier s tage 0 40 2.

Bias for 040 2 is o b tained fro m a vo ltaged ivider net work comp rised o f resisto rs R41 5and R41 8 . Resisto r R4 31 su pplies an avevoltage from ave am plifie r 0 404 which willcorrespon d in value to th e changes in in­co ming signal st rength.

I-f amp li fier stage Q402 also con tains t herela tive stre ngth metering c irc u it ry. Themeter is connec te d between the emitte r ofQ40 2 and the meter 's zero adjust co nt ro lR40 8 . The zero-adj us t con tro l is co nnec tedto a po sit ive d c su pply vo ltage and can headjust ed to give a zero in dica t ion o n theme ter. T he meter also monitors ave act io nan d provides a visual meth od o f ind icatingthe re la tive changes in ave voltage andtherefo re indi cates relat ive sign a l s tre ngth.

The amp lified signal fro m Q402 is cou p­led through ca paci tor C4 15 to the base ofQ403 (i-f amplifier -2d stage). Bias for thiss tage is o b tained via a vo lt age divid er n et­wo rk mad e up o f resis tors R4 19 an d R420 .Tra nsisto r 04.03 is also stabilized by emi tterresist o r R4 22 wh ich is bypassed to gro u ndby C416 .

The receivers' lo ca l osci lla tor Q30 I is asingle-gate F ET . T he osc il la to r tu ned c ircuitconsist s o f co il L301 , trimmer C30 I andtuning ca paci to r CSOI C. Oscilla to r injec tio nvoltage is cou pled through cap acito r C308 togate 2 of mixer Q201 . T his lI artley o scilla toro pe ra tes 455 kHz higher than the receivedsignal o n Bands A, B, C and D and the 4 55k Hz lower in fr equen cy than signals o n BandE a nd 4 .0 34 MHz higher o n Band F.

Det ec tion is p rovided wh en a portion o fthe ou tput signal from Q403 is applied toAM detector D40 I for amp Iitude-modula tedrecep tio n . The ou tpu t signal is a lso appliedthrough C429 to ga te I of produ ct de tecto rQ40 5 for CW/SS B receptio n. Q405 is a du alga te FET wi th ex celle nt isola tion ch arac te r­istics bet ween gate I and 2 to e limina te BFOoscillat o r " p u ll ing" o r o verload ing o n stro ngsign als.

Injection voltage co ming from BF O oscil­lator Q4 10 is applied to gate 2 of Q40 5 fo rCW/ SSB reception . Th e i-f signal and theBFO signal arc mixed , th e resu ltant o utput is

111

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Fig. 2. Schematic, GR-78.

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d - - -, !l- - - - --- --- - ---- - --s- ---- ;l- - - --'1'.._.-_--_-_-_-_--l-[-g--~-=_-~--~-=-=--;-f-;;;-i'<-~'-~~;l- -i - - - - - - -- ---i .- ,' ~ ". ~" -.... :

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an audio signal developed across R441 andcoupled through C433 to selector switchSW503B,

The BFO is a Colpitts oscillator made upof Q4lO, C425 and C426 to provide theproper feedback for oscillation and to forma tuned circuit with T40 I to resonate at 455kHz . Resist ors R437-43 8 form a voltagedivider and apply dc biasing to the base ofQ410, Emitter resistor R436 provides tem­perature stabilization for the transistor. In­jection voltage is coupled to gate 2 of Q405through C424 . The BFO is actuated by theAM/CW/SSB switch in the CW/SSB positionwhich applied de operating voltage throughRFC403 ,

The audio signal coming from R505 iscoupled through C441 to the base of audio­pre-amp Q406, The audio output is de­veloped across the load resistor R454 anddirectly coupled to driver transistor Q407.The output of Q407 is directly coupled to acompleme ntary pair of transistors Q408-409with audio output coupled through C446 toeither the 16Q speaker or a headphone jack,A portion of the output is fed back to theemitter of Q406 to aid in stabilization of thestage and minimize distortion .

Miscellaneous Circuitry

Transistor Q412 , crystal Y I and associ­ated circuitry form a 500 kH z calibration

112 73 MAGAZiNE

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oscillator which is switched on by the frontpanel calibration switch SW504. The oscil­lator provides very accurate calibrationmarkings every 500 kH z for dial calibrationand band edge marking.

The charging circ uit requires that thebattery be electrically divided in half forcharging from ac . When the ac plug isinserted into a power o utlet , the ac voltagefrom the secondary of T50 I is appliedthrough diode D50 I on one-half cycles tocharge one-half of the battery . The otherhalf-cycle of the voltage is applied throughD50 2 to charge th e o ther half of the battery.

Comments

It would take many more pages to fullyexplore and expound upon the virtues and

the features of the GR·7 8, an d for thatreason several portions of the circu it ry wereleft undescribed . It should suffice to say thatwith simple adherence to the manual, cali­bration per the manual and operation with afairly decent antenna , this receiver willsurpass many if not aU of the receiversavailable today in its price class and probab­ly a few of the higher priced models .

I would say that the receiver is not a kitto be wired by a novice, who has not yet wethis " teeth" so to speak on other kits. but,the GR-78 has returned countless hours ofoperat ing pleasure for those few enjoyablehours of construc tion . Like my mu sicianbuddies might say .. ," The GR·78. Man, it'sa gas!"

.. , W9KXJ

DECEMBER 1974 113