Heathkit GR-78 - Page 34

CIRCUIT DESCRIPTION
Rfer tothe Block Diagram fol ut from Page 99) ant
the Schematic Disgram (ol-out rom Page 117) while
reusing the description. The Banditch wil be considered
tobe in Band A poston
RF AMPLIFIER,
‘The AF sgl ecole from the bul whip tenn,
‘extra anton, are couple to the int tn rel for
Band A. This tuned cicuit consists of antenna oil
trimmer capacitor C1, and the thst ston of tuning
‘apioy C5OIA, Tho input sgl then coupled rom a
{ap on coll L110 gate ¥ ofl effet transistor O10, Blas
voltage for Q101 is abana from the AVC amplifier nd is
‘plied 0 ite 2 Source eistor RG and RF Gain eontoh
FREDY provide he rope ran cure for normal apmaion
[with AT fully clockwise. Turning the AF Gal control
hang the sount of current flowing, hereby varying the
‘ain. The amplified sgral from 101 fe fed to tp on AF
foil L101, which slong with trimer capacitor C1OY and
{unog capacitor CEOTE form the ing tuned eet of fat
rina stage 0201,
FIRST MIXER
Traitor 0201 Iso cal ge slim fd fat wart
(FET), The mise featur exalt freedom tom ero
‘modulation o oelding and eit “pling” on sta
sionals. The ampli ig! fry Q101 appli o gate 1
‘f G201 trough capacitor C111. Th cl oeilator sgt
from
0301
i
api
to
gate
2
of
A201,
which
ibs
by
‘istors R208 ane 20K, Sour rssor R205 tables
the DC operating point. The incoming signal snd the
‘scar sgl ae heterodyne (mised in 20, cetng
I the 4F trequeney and mixer prods. On ‘Bande A
‘tough E, the outs of 20 i tot tp on elt L201
hich resonates with cpuctor C20} to provides 455 KH
‘utp gal. On Band F, the ovput trom O20} I fed to
‘oi L202, which resonats with eapaitor C202 provide a
‘$034 MH outs
‘SECOND MIXER/IF AMP
‘The output thom the fit miner, 201, i coupled trough
capacitor
C206
Yo
gate
of
fold
effect
transistor
OT,
heh operates as nF ampiiron Band trough € and
alse providing double conversion on Band F
‘prating bis is provide on gate 2 by rss RAO and
‘402, When the bandovtch i in Band F position, 2 3.579
INE injection signa trom Conversion Osa 811 ie
pli to gate 2 of O01, This sara combines with the
‘3.034 Miz sgl fom the fist miner, 201, sd rele in
fn output signal of 456 AHe plu’ he mixer product
frequen, This output slg coup though epson
CAO to ceramic pabret firs FLADY through FLAK
Thwse fies shape the gaurd and attonaste all
freavecis, encopt the IF Mequeney of 486 ki Ths TF
sina is cued tothe base of the fst IF Api stags,
ner,
Blt for 0402 is obtained tom a vlane divider ratwork,
consiting of esos RATS. ond ANB, Resior Mash
supplies an AVC (automatic volume controll voltage trom
‘the AVE Amplifier, Q404, which coreponds to change in
1h incoming in arent
IF amplifier stage 0402 ats contsng the ecu for the
Feative signal strength mate, The mete is connected
tween the omit of 402 and the mete zoo adrt
‘onto! R408, Control ADB is connected toa positive OC
‘upp voltage, and It can be adjusted to ge 9 269
Indication on the meter. When the average va of the
Inesring RF signal inreies, the AVC vol
‘aug the positive voltage at th bate of ana
‘cre, This cave the current towing Hough O402 to
‘crac and caves corresponding voltage drop aos the
trite rsisor R17, Ths varying voltage Head On the
rete, whlch monitors AV ction and provides a via
‘metvod of nesting relative sal erength,
9