Chevrolet 985432 Service manual

CHEVR.OLET R.ADIO SER,VICE
AND SHOP MANUAT
985432-PUSH BUTTON RADIO
985455-CHEVY II PUSH BUTTON RADIO 98543r -MANUAT RADTO
985453-CHEVY II I/IANUAL RADIO
985449-COR,VAIR PUSH BUTTON RADIO
985447-CORVAIR MANUAT RADIO
98539ó-CORVETTE RADTO
985471-fnANUAt TRUCK RADTO
985443-SERIES 95 MANUAT TRUCK RADIO
98551 9-GUIDE.MATIC HEADLAÍNP CONTROL
FOR
I I6 3
FOR,EWOR.D
The informqlion in lhis service ond shop mqnuql covers o generol description
of Chevrolet Rqdios qnd Guide-Mqtic Heodlomp Control for l9ó3 qnd
thorough doto on operolions, specificolions and procedures for lesling ond
servicing Chevrolet Rodios qnd Guide-Mqtic Heodlomp Control.
CHEVROIET MOTOR DIVISION
GENERAT MOTORS CORPORATION
DETROIT 2, MICHIGAN
Copyr¡ght I 9ó2
Chevrolet Motor Division
Generol Molors Corporotion
ffi
Litho in U,S,A¡
P&A l9ó3
8.5M

INDEX
GENERAL INFORMATION - ALL MODELS
Electrical DescriPtion
Procedure for Checking Inoperative or Noisy Radios Procedure for Checking and Servicing Printed Circuits
Procedure for Checking Transistors
Service Procedure for the Push Button and Manual Radios
Procedure for Servicing the Corvette Radio
Procedure for Alignment of AII Chevrolet Radios
Capacity and Inductance Alignment Procedure for All Chevrolet Radios.
985432 PUSH BUTTON RADIO
General Information
Transistor Complement and Function. .
Page
1
1
2 2
5
I
L2
13
Push Button Setting
t4 t4 t4 14
16
18 t7 19
15
Condensed MechanicaL Operation of the Push Button T\rner
Mechanical Adjustment of Tuner
Procedure for Parts Replacements Radio
Circuit Diagram (Schematic Drawing)
Service Parts List
Troubleshooting the Push Button Radio
985431 MANUAL RADIO
General Information
Transistor Complement and Function . . . .
Circuit Diagram (Schematic Drawing)
Service Parts List
Procedure for Part Replacement. . . .
Troubleshooting the Manual Radio . . .
985449-985455 DELIIXE PUSH BUTTON RADIOS FOR CORVAIR & CHEVY II
General Information
Transistor Complement and Function. . . . Push Button Setting .
Condensed Mechanical Operation of the Push Button T\rner and Radios
Tuner Adjustments for 985449 - 985455 Radios
Circuit Diagram (Schematic Drawing)
Service Parts List
Procedure for Part RePlacement
Troubleshooting the Push Button Radio
98544?-985453 MANUAL RADIOS FOR CORVAIR & CHEVY TI
General Information
Transistor Complement and Function.
Circuit Diagram (Schematic Drawing).
Service Parts List . . . . .
Troubleshooting the Manual Radio . . . .
2t 2l
22 2l
2L 22
2t 27
27 28 31
29 33
31
28 38
38
40
38 38
46
General Information
985396 CORVETTE RADIO

985396 CORVETTE RADIO (Cont'a. )
Tube and Transistor Complement and Function
Push Button Setting
Operation of the Wonder Bar Tuner Mechanical Adjustment
Procedure for Part Replacement. . .
Circuit Diagram (Schematic Drawing).
Service Parts List
Troubleshooting the Wonder Bar Radio.
985471 TRUCK RADIO
General Information
Transistor Complement and Function. .
Circuit Diagram (Schematic Drawing). . . .
Service Parts List
Troubleshooting the Manual Radio
985443 SERIES 95 TRUCK RADIO
General Information. .
Transistor Complement and Function. .
Circuit Diagram (Schematic Drawing).
Service Parts List
Troubleshooting the Manual Radio
985519 GUIDE-MATIC HEADLAMP CONTROL
Adjustments and Tests
General Description
Circuit Diagram (Schematic Drawing)
Functional Operation
Sensitivity Tests
Service Parts List .
Trouble Shooting Procedure
Vertical Aim. .
Page
46 46 47
47
51
52 55 55
66
66
68 67
66
60
59
63 60
59
74 73 78 74
76
79
74
?6

CHEVROLET RADIO SERVICE
AND SHOP MANUAL
Electrical DescriPtion
The circuit used in the Chevrolet receivers are
of the superheterodyne type that use no regenera- tion. The tuning circuits are of the permeability
type and are tuned by varying iron cores in and
out of the antenna, radio frequency, andoscillator coils like pistons. This is shown for the push
button models in Figure 1.
Figure I
The intermediate frèquency stages are tuned by
means of two iron cores in each transformer
as shown in Figure 2, and are adjusted from the
top and bottom of the transformer. Both the
first (input) and second (output) intermediate frequency transformers are tuned bythis method.
Figure 2
The antenna circuit is coupled to the antenna by means of an adjustable antenna trimmer to take
care of normal variations in antenna and antenna
cable capacity. The antenna trimmer is located
on the front of the radio chassis under the dummy knob on the manual tuning shaft, and is adjusted by means of a small screwdriver.
This trimmer permits the receiver to be ad-
justed to any of the Chevrolet antennas for
maximum sensitivity and performance. This
adjustment is very important, and station mixing
may occur if it is not adjusted.
Procedure For Checking Inoperative Or Noisy
Radios
Receiver Completely Dead
Check for a blown fuse, blown fuse could be caused by one of the foliowing:
1. Short in 12 volt circuit of radio. It will be
necessary to remove radio from carandcheck
the 12 volt circuit.
2. Solenoid remaining energized. (Wonder Bar
Radio only).
Check speaker for open circuit. Check antenna for open or short circuit.
Weak - No Volume
A weak receiver can be caused by the failure to
adjust the antenna trimmer or the speaker not
being completely plugged in. Check these before
removing the radio for servicing.
IMPORTANT: Adjusting the antenna trimmer for maximum volume on a weak station or noise will provide maximum performance and
prevent weak & fading radios.
Figure 3
To prevent weak or fading reception, remove
right-hand tuning knob and adjust "Antenna
Trimmer" screw. See sticker on radio for instructions. CAUTION: THIS \VILL NOTWORK IF TUNED TO A STRONG STATION.
1

NOISY OPERATING RADIOS
The noise can be caused by one or more of the following:
1. TIRE STATIC is caused by friction between
the tires and pavement, and is almost a
continuous roar while car is in motion, and
does not vary appreciably with car speed.
The intensity of the noise is greater on a
dry sunshiny day, and not so noticeable on
humid or rainy days. To eliminate this type noise be sure that the front wheel static
collectors have been installed, being sure that
they are free of grease and are making good
contact to front wheel spindle. If the static still persists, install tire static powder in
all five tires.
2. NOISY ANTENNA can be located by turning
on the radio receiver, tuning in a station and
by tapping the antenna with a scre!'¡/ driver
handle. If noisy, a crashing sound will be heard in the radio each time you tap the antenna. The antenna lead-in can also cause noise in the radio if the shield is broken or
unsoldered from the ends, or if the lead-in
r¡¡ire in cable is loose or broken. This can be
checked by shaking the antenna lead-in cable.
If you can cause a crash in the radio while
shaking lead-in, replace lead-in.
3. MOTOR INTERFERENCE in Chevrolet radios
is usually caused by poor grounds when in-
stalling the antenna or receiver. Check to
make sure all required suppression material
has been installed and that aII grounds are
free of paint, grease, or rust and are tight.
4. GENERATOR INTERFERENCE is a whining
noise similar to a siren, and increases or decreases with speed of the engine. InstaII
or replace generator condenser.
5. Delcotron Interference is a whining noise
most noticeable with the radio volume very
Iow. This noise is usually caused by a defect in the Delcotron itself. However, in the radio it is necessary to keep the "4" lead (battery
input) away from other leads which would pick
up noise, particularily the volume control
leads.
Procedure For Checking and Servicing Printed
Circuits
Atl stages of the 1963 Chevrolet radios use printed circuit boards, the latest method used in wiring electronic products. The printed circuits
reduce the possibilities of shorted or broken
wires and loose connections that the wired sets were subject to.
The servicing of printed sircuits is not difficult
but a few precautions must be observed when
trouble develops in the component parts mounted to the printed circuit board and repairs or re- placements are made.
In the servicing of the printed circuit portion of
the radio, EXCESSIVE HEAT applied toanyof the
soldered terminals can cause the printed circuit
to lift from the circuit board which results in
the necessity of replacing the entire printed cir-
cuit board. A 25 to 50 Watt soldering iron is
recommended for work on the printed circuit
board. Care should be taken not to place the
soldering iron tip directly on the printed circuit
board. The iron should be placed on the lead or terminal being soldered to the printed circuit which will allow the heat and solder to flow down
the lead or terminal to the printed circuit.
When removing or replacing component parts
mounted on the printed circuit board, it is im-
portant that the heat be applied to the wire lead
or terminal and not directly to the printed cir-
cuit. A small wire brush is most helpful in the removing operation. Care shouldbe exercised not to crack or break the circuit board as any break in the board will necessitate replacement of the circuit board.
The printed circuit has an insulating and sealing coat placed over the entire board after the cir- cuits are put in place on the board, and for
any soldering that is required on the printed
stripes, the insulation must bescrapedoff - NOT BURNED OFF with a soldering iron as it wiII
also burn the printed strip. If a portion of the
circuit on the board is broken, it can be repaired
by scraping off the insulating coating and solder-
ing a piece of wire across the break.
Resistance and
at the soldered
any soldering
make sure to
solder from the
voltage reading should be made
point on the circuit board. After
operation has been performed,
remove any loose particies of
printed circuit board.
Procedure For Checking Transistors
Transistors have a very low failure rate, how-
ever, they do fail occasionally so a transistor
checking procedure is very valuable. The transistor which is most likely to fail is
the large power transistor called the output
transistor. This coutd cause no "Thump" to be
heard as the radio is turned on, but remember
that a defective speaker or blown fuse will also
prevent the t'Thump".
2

DOT BY
ùs-221
D6-24 LTRANSISTOR
DS-25 I renmlraus
6- 26J
COLLECTOR
(SERVICE TRANSISTORS ONLYI
&I-OOATING LUG
BASE
TERMINAL EMITTER LEAD
INAL OR BASE
TERMINAL
-BASE
YELLOW LEAD
OR BASE TER
-EMITTER
ooLl-EoTo
rs cAsE ÍvIOUNTING STUD
OOLLECTOR
Dg-5O3 TRANSISTOR TERMINALS DS€OI TRAT.ISISTOR TERMINALS
Figure 4
There are many co.mmercial transistor checkers
on the market also newer type tube checkers
can test transistors. These checkers have their
own testing procedure so the followingparagraphs wilt be devoted to testing transistor with an
ohmmeter.
Checking Power Transistors
The DS-501 and DS-503 leads must be unsoldered
and disconnected from the circuit. Allow the transistor to cool to room temperature before
checking.
Place an ohmmeter on the Rx1 scale and "zero"
the meter. Connect meter between the emitter
tead and the cotlector mounting stud, see Figure
4 leaving the base open. After taking this read- ing, reverse the meter leads and read the meter again. The lowest of these two readings must
not be belorv 50 ohms.
If the transistor does not read S0ohmsor higher,
the transistor is defective and should be re-
placed.
Most defective power transistors read '(O"
ohms. This will cause the fuse resistor on the
circuit board to open and often causes the 10 ohm resistor connected to the fuse resistor to
increase in value, These must be checked and
replaced if necessary.
Checking Small Transistors
SmaII transistors, such as the D522, DS24, DS25,
and DS26 usually open inside the transistor when
they fail. When this happens, no signal can pass
through that stage.
If a stage is found to be dead by the signal
injection or "Trouble Isolation" procedure, the
transistor can be checked by bridging a good one
across it, just like an open condenser is checked:
1. Connect a good transistor of the same or
equivalent type across the one in the circuit, make sure that all three leads are making
a good connection at the proper solder points on the circuit board - Base lead to point B;
Emitter lead to point E; Collector lead to
point C. See Figure 5.
2. IL the radio plays, remove the defective
transistor from the circuit and solder the new one in.
3. If the radio doesn't play, the
something else is at fault. Go
cedure for Trouble Isolation".
chances are
to the "Pro-
NOTE: The above test cannot be made on the
large transistors, DS501 and DS503, because
they usually short instead of open.
3
\

OIRCUIT BOARD
o
c
*r.oïI?3ro'
OHMMETER TEST
Small transistors can also be checked for open
Ieads by using an ohmmeter. However, different
meters give different readings, depending on the
range sélected, the battery used inside, and the
meter resistance. The table in Figure 6 shows the readings ob-
tained when a Simpson Model 260 Volt-Ohm-
Milliameter' or RCA Model 1VV-??A was used'
The procedure is:
1. Set the meter on the RX100 range (radio
power suPPIY off).
2. Connect the ohmmeter leads to the Base and
Emitter, B and E, solder points on the cir- cuit board. Now reverse the meter leads
and connect again to B and E' A definite
change,in resistance should be noted, and one
of the readings should be less than 500 ohms'
3. Connect the ohmmeter leads to the Base and
Collector, B and C, solder points on the cir- cuit board. Now reverse the meter leads
and connect again to B and C' A definite change in resistance should be noted, and one
of thã readings should be less than 500 ohms' NOTE: In Figure 6 that the power to the
radio must be turned off, and the radio vol-
ume control turned completely clockwise be-
fore taking ohmmeter readings.
NOTE: That the DS22 and DS26 have a 5:1
ratio or better when the highreadingisdivided
by the low reading. The DS24 and DS25 have
a 10:1 ratio or better. If the transistor is
removed from the circuit and tested, the low readings will be about the same as shown in the chárt, but the reversed readings will be
much higher.
NOTE: Adequate tests are provided to de-
termine if transistors are functioning prop-
erly. It is not necessary to remove and
"eplace transistors that have passed these
te sts.
Figure 5
Figure ó
4
IN-CIRCUITTESTINGoFSMALLTRANSISToRSUSINGoHMMETER
Radio Power "Off", Volume Control Set at Maximum
Meter Scale RX100
Values may vary with different meters. Above measurements were made with a Simpson
Model 260 or RCA Modet WV-774 meter'
**Reading will be lower if volume control not turned completely clockwise.
Transistor Meter Reversed
BandC
Meter on
BandC
Meter on
BandE Meter Reversed
BandE
DS22
DS24
DS25
DS26
250 Ohms
250 Ohms
200 Ohms
200 Ohms
1,900 Ohms
50,000 Ohms
?,000 ohms
2,000 Ohms
200 Ohms
200 Ohms
200 Ohms 200 Ohms
2,000 Ohms
50,000 Ohms
3,000 Ohms
**1,500 Ohms

Service Procedure For The Push Button and
Manual Radios
Turn on signal generator and set in audio posi-
tion to obtain a 400 cycle audio signal. Ground
one lead of signal generator to radio chassis.
A .1 mfd, capacitor should be placed in series
with the remaining lead to block D.C. current,
The lead with the capacitor will be the probe for
signal tracing. Keep radio volume controlturned
to maximum for all tests.
PRELIMINARY TEST (IMPORTANT) - Turn radio
on with ear next to speaker. As this is done a
"thump" should be heard in the speaker. If
O.K. go to Step 1. If no "thump" was heard,
check:
a. Speaker connections and speaker for proper
hook up.
b. Power connections and fuse for proper hook
up.
c. DS501 power transistor collector voltage by
measuring the voltage between the transistor
case and chassis. (IMPORTANT: DO NOT
CONNECT METER,TO THE TRANSISTOR HEAT SINK OR FINS.)
d. A reading of about 11 volts from collector to
ground means that something is open between collector and ground. Check output trans-
former.
Note of Explanation: The signal generator is
now put into use, beginning with Step 1. The
letters in parenthesis are found printed on the
circuit board. For example, (AF) stands for
"Audio Frequency" amplifier and refers to the
DS26 transistor. (C) stands for collector. When
the signal generator is applied at thrat point, if
nothing is heard the output stage should be re-
checked as described in the preliminary test
above.
The test points - Step 1 through Step 7 - are
shown in Figure 7. STEP 1. Touch generator probe to DS26 (AF.)
collector (C) anA adjust generator output to pro-
duce weak tone. If weak tone cannot be heard,
check the DS501 output stage. Without changing
generator controls, go to Step 2.
STEP 2. Apply generator probe to DS26 (AF) base terminal (B). An increase in signal should be noted, indicating DS26 transistor gain. If
O.K. go to Step 3. If no gain was heard, check:
a. DS26 transistor without removing it from the
circuit. See "Procedure for Checking Small Transistors" page 3. Volume control must
be completeiy clockwise during all tests.
b. Check the voltage at the DS26(AF) collector
(C), by measuring between (C) and chassis.
The voltage should be about .5 volt. A read- ing of "0" volts or near tt0" means that no current is flowing in this stage, and one of the
following items is probably defective: Open
resistor or poor connection in the Base (B) circuit, or in the Emitter (E) circuit. Trace
the printed circuit from those points and check for open solder connections.
c. A very high voltage of 10 or 11 volts from
DS26 (AF') collector (C) to ground means that there is an open between collector and ground.
Check input transformer resistance, andcheck
for bad solder connections.
d. Bridge a 100 mfd. capacitor across item 33.
If the gain comes up, replace the capacitor.
Change signal generator from audio position to
generate an intermediate frequency signal. Set signal generator lo 262 kilocycles.
STEP 3. Apply generator probe to base (B) of
DS22 (IF) transistor. A loud signal should be
heard without turning the generator controls to
a very high level. This usually takes less than half the maximum settings on the signal gen-
erator, as wiil be learned by practicing with
your generator on a good radio. If O.K. go to
Step 4. If no signal or a very weak signal is heard, check: a. DS22 transistor without removing it from the
circuit. See "Procedure for Checking Tran- sistors".
b. Voltage between collector (C)andgroundinthe
DS22 (IF) stage. Shoutd be "0" volts. If
voltage .is high, near 10 or 11 volts, the
trouble is due to: Open connection in the (IF)
collector circuit (C), or open IF transformer,
item 6.
c. Check DS22 (IF) conduction by measuring
voltage across the 1000 ohm resistor, item
54. Measure this by putting the positive
Iead of a d.c. voltmeter on conductor 2 on
the circuit board, and the negative lead on the
emitter (E) of the DS22 (IF) transistor. The
voltage should read about 1.0 volt.
If the voltage is low or near "0", check for: Open connection on the circuit board in the
5
\

SIGNAL GENERATOR
fU NI NG
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FREOUEl{CY
fONE
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AUDIO FqEOUE^¡CI A
STEP
BAND
-- ouTPUf lo @ o ol l¡uo'o-tx ¡lr-q.r -t¡ J
ac
\D output coñfRoi
STEP
gTEP WEAK OR
NO SIGNAL
fONE B C VOLUME
BEIWÉEN.5 I I
¡uDro rnEouEt¡cv a-.- u ourput co¡tnou
EANO
_OUTPUT lo @ o ol
louoro-¡r ¡rr - n.r - t.¿ J
SIGNAL GENERATOR
ON
off
U¡MOOIJLAIEO
fUNING
FREOUENCY
EAND
-oUTPUf - lo @ o @l louoro-¡x ¡rt- n.F. - r.Ê J
D output oo¡.¡fnoL
fUNING
MODULAfEO\ ON
oFf
VOLUME
FETWEEN A
FREOUÉNCY
SIGNAL GENERATOR
FREOUÉNCY
fONE BC
7
6
5
4
3
I
z
F
E
c
A
o
@
Figure 7 STGNAL TRACTNG LOCATIONS ON CIRCUIT BOARD - ALL PUSH BUTTON AND MANUAL RADIOS
(IF) base circuit (B) or emitter circuit (E)'
Check IF transformer' item 5, for open.
If the voltage is very high instead of "0",
check for leakage or shorts in the Base
(B) anO Emitter (E)circuits, including: Shorted .047 mfd. condenser, item 27.
Shorted.04?mfd. condenser' item 29. Shorted
IF transformer, item 5.
d. Bridge a ,047 mfd. capacitor across capacitors'
item 2? and item 29. If gain increases, the
capacitor Paralleled is oPen.
e. If trouble is still not located, turn signal
generator volume control to maximum. Apply
it collector (C) of DS22 (IF) transistor. A weak signal is usually heard, depending on the
signal generator. If no signal is heard, check or replace: I.F. transformer' item 6; detector diode DS27.
STEP 4. Appty generator probe to DS25 con-
verter collector (C) and adjust generator output
to produce weak tone. Without changing gen-
erator controls, go to SteP 5.
STEP 5. Apply generator probe to base (B) of DS25 converter transistor. An increase insignal
should be noted, indicating DS25 transistor gain. If gain is not present, check:
a. DS25 without removing it from the circuit.
See "Procedure for Checking Small Tran-
sistors".
b. Vottage between collector (C) and ground in
the DS25 converter stage. Should be rr0" volts. If voltage is high, near 10 or 11 volts'
the trouble is due to one of the foilowing:
Open connection in the collector (C) circuit
in the converter stage. OpenIF transformer,
item 5. Open oscíllator coil, item 4.
6

m-.e::-"ilÞ]
SIGNAL GENERAIOR
o ourrur co¡trcu
luNlNc
ac
&ND
fÂEOUENCY
roNt
oro'o r¡¡ouENct a
m-å::::þlr
SIGNAL GEN€RAÍOR
o outrut otrcu
ÌUNIN6
FÊEOUENCI ac
&ND
fotE ÎONE A C V4UME
Âuoro FiFqu€Ncl  o oulPur coilrrcL
SND
f- oulPuf -------l lo o o ol l¡o'o-¡¡ ¡tr-¡¡ -r¡ J
SIGNAL GENERAIOR
0s 503
OUfPUf
I, E AMP
{sñvrcE wrfH os25l
os 22
r¡ L!
I I I
o
R.Ê AÍP
lsñvrcE wrtH 6251
2C
DS 24
I I I
J
621
A6
o€f
os 25 Ds !è
r-- MEô
I
fRot
fusÊ
alæx
650
Figure 8 SIGNAL TRACING PROCEDURE - 985432 - RADIO
c. Check DS25 converter conductionby measuring
voltage across the 3900 ohm resistor, item 51.
Measure this by putting the positive lead of a
d.c. voltmeter on conductor 2 of the circuit
board, and the negative lead on the emitter (E)
of the DS25 converter. Thevoltageshouldread about 1.0 volt.
If the voltage is low or near "0", check for:
Open connection on the circuit board in the
converter base circuit (B) or emitter circuit
(E).
If the voltage is high, about 10 or 11 volts'
check for: Shorted .000220 condenser' item
25. Shorted .0047 condenser, item 24. Shorted
trimmer, item 218.
d. If alt above tests pass, align lst I.F. coil. If
coil fails to peak sharply replace it. See alignment procedure.
Change signal generator from intermediate fre-
quency setting to radio frequency signal. Re-
move the .1 mfd. condenser from the probe lead
of the signal generator. Place a.000082 mfd. con-
denser in place of the .1 mfd. just removed. Set signal generator to 1100 kilocycles and tune
radio receiver to 1100 kilocycles (11 on dial
scale). A slight retuning of the radio dial may
be necessary, once the signal is injected into
the radio, to provide maximum signal through the radio. STEP 6. Apply the generator probe to 'DS24
(RF) collector (C), and adjust generator output to
7
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DS-50r
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t.¡lv.
I
to.!v.
0s-26 c
ov. DS-22
93V. c
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9.7V.
9.tv. 9.9V.
2.3V
Bto.4v
to.ov. E
ro.ov.
Fisure 9 VOLTAGE CHART - ALL PUSH BUTTON - RADIOS
Fisure l0 PARTS LAYOUT ON CIRCUIT BOARD '985432 - RADIO
8
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6
5
64
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4
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36 +

produce weak tone. Without changing generator
controls, go to Step 7.
STEP 7. Move the generator probe to the an'
tenna socket. A tone of equal or slightly less
volume will result in the speaker. If signal at
antenna socket is not heard, check: a. DS24 transistor without removing it from
the circuit. See "Procedure for Checking
Small Transistors".
b. Check the voltage between the coltector (C)
and ground of the DS24 (Rf')transistor. Should
read about 2.3 volts d.c. with antenna dis-
connected from the radio.
If voltage is high, near 10 or 11 volts, there is an open circuit between the collector (C) and ground. Check:
RF coil, item 3, and resistor, item 47. AIso check for bad solder connections in that area. If voltage is low, near "O" volts, check:
Check for opens in the DS24 (RF) base circuit
(B) and emitter circuit (E). Check the antenna
coil, item 1, for open. Check trimmer capacitor, item 16 and .0022 capacitor, item
23 for short.
c. If (RF) stage is dead but voltages areall O.K,
check:
Antenna coil, item 1, for open. There are two
windings on this coil, both at rear of tuner.
Check antenna choke, item 2, for open. Check
antenna trimmer, item 16, for short.
This completes the tests for a weak or dead
radio. Below are additional hints which may help you find the trouble if it has not been
located:
If noise can be heard in the speaker when the antenna is plugged in, but no stations can be picked up, the converter is probably not oscil-
Iating. To check for normal oscillation, mea-
sure the voltage across the 3.9K resistor, item
51 should be about 1.0 volt. Tune the radio from
one end of the dial to the other while watching
this voltage. If the voltage does not change
slightly, the converter is not osciilating. Common
causes of this are: Open condensers in the DS25 converter circuit.
Check by bridging them with good capacitors of
the same value.
Open oscillator coil, item 4.
Defective trimmer, item 21.
If the radio plays loudly but is muffled on very
strong stations, check the voltage between (RF) collector (C) and ground. This voltage should
drop to a low value when turned to a strong
station. If it doesn't, check: DS27 AGC diodes, item 201 and item 202. When
checked on the RX100 scale of an ohmmeter,
there should be 10:1 ratio or better. Also check to see that those diodes are not mounted back- ward.
Check for open in conductors 22-36 and 39.
If the radio is very weak and distorted when tuned to strong stations, check: a. The speaker and connections.
b, ,47 ohm or' .33 ohm fuse resistor, item 64,
for open.
Check the clear mica insulator between the
power transistor and the heat sink or fin.
The fin is grounded to the radio chassis on
some models, but transistor is insulated from
the fin by an almost invisible piece of mica.
Other models have the fin insulated from the
radio and the transistor connected directly to
the fin.
Procedure For Servicing The Corvette Radio
All circuits, except the trigger circuit, are very
similiar to the push-button and manualreceivers, so the troubleshooting procedures outlined for
these sets may be used for the wonder bar
radio. Figure 10 shows the schematic diagram
for the Corvette radio and the various points of
signal injection.
It is very important that the first three stages
of the radio are working and aligned properly before any attempt is made to troubleshoot the trigger circuit.
CONDENSED ELECTRICAL OPERATION
OF ELECTRICAL TUNER USED ON
ÍWONDER BAR RADIO . 985396
The purpose of the electrical components asso- ciated with the tuner is to control the relay so the operator may start the tuner sweeping cycle
by merely depressing a station selector bar
switch so that the sweeping operation will con-
tinue until a signal is received. At that time' it
I
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Fisure ll - SIGNAL TRACTNG PROCEDURE - 98539ó _ RADTO
is the function of this circuit to accurately tune
to the frequency of the selected station. It also
provides the necessary conditions to keep the
tuner on the station until a change is desired.
ELECTRICAL OPERATION CYCLE
1. To start the tuner, the station selector bar is
momentarily depressed actuating the station
selector switch No. 89 which energizes the relay.
A. Switch No. 89 completes a circuit from
ground through the station selector switch
(89) and relay (84) to the 12 vott suppty.
B. The current through this circuit energizes
the relay (84) and removes the relay arm
from the stopping disc - thus starting the tuner, and opening contact No. 2andclosing
contact No. 1 on the relay switch (84).
2. To keep the tuner seeking after the station
selector bar is released, the relay is held energized by a holding circuit.
A. Contact No. 1 being closed supplies voltage
to pin 3 of the 124L8 tube which causes plate current through the relay.
3. To stop the tuner on station, the relay is
de-energized by an electronic triggering cir-
cuit actuated by an incoming signal.
10

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zND I.E
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84 86
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TRIGGER ß
55 56 PUSH
BAR
GIRCUIT 89
90 30
SENS. CONTROL
R.F, STAGE
A. The incoming signal develops a voltage in
the primary and secondary of the 2nd I.F. transformer.
B. The I.F. signal voltage in the secondary
of the 2nd I.F. transformer is coupled
through the condenser (32) to the grid of the trigger detector section of the 124L8
tube. This signal voltage is developed across the resistors (62) and (65).
C. The trigger detector section functions as
a plate detector. The I.F. voltage appears on the grid and the tube conducts. The I.F. component is removed in the plate
circuit by the .000100 condenser (34).
D. The plate current flow in the trigger de-
tector section develops a biasing voltage
across the 2.2 Meg. grid resistor (?1),
making the grid more negative than the
cathode. This causes the plate current in
the relay control sectíon to stop.
E. Stopping the current flow de-energizes the
relay (84) and the relay arm engages the
Fisure l2 - TRIGGER CIRCUIT - 98539ó - RADIO
stopping disc, stopping the tuner onstation, opening contact No. 1, and closing contact No 2 of the relay switch.
4. To hold the tuner on the new station until
another station is desired, the relay is held de-energized until the starting circuit is again actuated by the operator.
THE TRIGGER DETECTION CIRCUIT
The purpose of the trigger detector circuit is to
take the input signal voltages of various amplitudes
and trigger the relay tube so that the accuracy
will be the same on all stations regardless of
the signal strength. The grid of the plate de-
tector (trigger-detector) is tied into the AGC line.
A portion of the AGC voltage is used to vary the
bias on the plate detector in proportion to the
strength of the incoming signal. rüith a strong incoming signal, the bias voltage is high and triggering doesn't take place until the tuner gets
very near the resonant frequency of the station
providing the incoming signal. At this point the
IF signal is great enough to overcome the bias
11

and the plate detector is caused to conduct, stopping the tuner. Likewise on a weak incoming signal, the AGC voltage is small and the bias
on the plate detector is small, therefore a
smaller amount of IF signal will stop the tuner
at a point very close to the peak of the in- coming signal. In other words, the AGC voltage raises and lowers the threshold over which the
IF signal has to climb in order to stop the tuner. The circuit is so designed that the IF signai
voltage is high enough to overcome the bias and
stop the tuner only when the tuner has reached
the station frequency.
SENSITIVITY CONTROL
The sensitivity control g0 is a step switch with resistors of various values between each step.
The control is inserted into the AGC circuit of
the RF amplifier during the tuning sweep when
contact No. I of the relay switch is closed" It is the means by which the operator controls the
number of stations on which the tuner will stop.
Typical Complaints and Remedies
I. Tuner stops when bar is released
A. Check or replace 124L8 trigger tube
B. Check or replace relay
II. Tuner runs as soon as set is turned on
A. Check for a shorted foot switch
B. Check for a short at the wonder bar
selector switch
C. Check for a sticking relay
III. Tuner will not stop on stations
A. Check the antenna and 124L8 tube. If
radio is dead, check R.F., Conv, and I.F. Stages
B. Ground the negative end or a 1.5 V. flash-
light battery and with the tuner cycling
touch, the positive terminal on pin 8 of
the 12AL8 tube. If tuner does not stop
check:
1. Capacitor item 32.
2. Capacitor item 35. 3. Associated resistors around trigger
tube
4. Check or replace relay
C. With a VTVM, check for the negative
AGC voltage on pin 8 of the 12ALB tube
while tuning manually through A station. Iï no voltage appears
1. Check AGC system, mainlyDS2?diodes
D. With VTVM, check for the small negative
pulses on pin I of the 124L8 trigger tube
with the tuner seeking. If no pulses are
present -
1. Check sensitivity control circuit
2. Align 2nd I.F. coil with a meteracross
the speaker (I.F. coil critical), (always
replace with recommended parts.)
IV. Tuner will not stop on the center of stations.
A. Check or replace relay 84.
B. Set the clearance of the relay when ener-
gized, so it barely misses the teeth of
the governor gear.
C. Align the 2nd I.F. coil and replace if it
does not peak sharply.
PROCEDURE FOR ALIGNMENT OF
ALL CHEVROLET RADIOS
All receivers are properly aligned at the factory
and should require no further adjustments, ex-
cept adjusting the receiver to the antenna when installation is made unless the adjustments have
been tampered with, or new coils, intermediate
frequency transformers or tuning cores have been installed.
To properly align the receiver, it will be neces-
sary to have an output meter and signal gener-
ator.
NOTE: If any one of the tuning coils or cores
have been replaced, see "Capacity and In-
ductance Alignment Procedure" before pro-
ceeding with alignment of the receiver. If
only the adjustments have been tampered with or an intermediate frequency transformer has been replaced, proceed with the alignment
as follows:
1. First hook up an output meter to the radio
receiver. Any volt meter which will read
"A.C." can be used. Set the volt meter in
the 2.5 or 3 volt "4.C." range position, and ground one lead of meter to radio chassis.
Place the other lead from volt meter on the
speaker terminal.
t2

2. Turn on signal generator and set adjustments
to obtain a 262 kilocycle signal. Connect one
lead of signat generator to radio chassis for ground. Attach the other lead of signal gen- erator to the base of the converter transistor.
3. Adjust signal generator volume control so
that the volt meter will tead about half scale.
NOTE: Radio receiver volume control must be turned to the maximum position so that the
automatic volume control circuit will not affect the alignment of the receiver.
4. Adjust in sequence cores "4, B, C and D"
as shown on circuit diagram and parts lay- out for maximum meter reading. Repeat
adjustments to get maximum meter readings.
Keep the signal generator volume turned
down so that during adjustments the meter
does not read more than half scale. This
will result in a better alignment of the re-
ceiver.
5. Next change signal generator setting to obtain
a radio frequency signal and tune signal
generator to exactly 1615 kilocycles. Place a
.000082 mfd. condenser to antenna connector
and attach signal generator lead. Tune the
radio receiver to the "Stop" on the 1600
kilocycle end of the dial. Keep the signal
generator volume control adjusted so that
output meter reads at about half scale.
6. Adjust trimmers "E, F and G", on circuit
diagram and parts layout, in sequence for
'maximum readings on output meter. Repeat
for maximum meter readings.
7. After the receiver has been installed in the
car, turn on receiver and tune in a weak station near 1000 kitocycles with the radio
volume control turned to maximum position
and the antenna extended to full height. Re- adjust trimmer '(G" ONLY for maximum
volume.
CAPACITY AND INDUCTANCE ALIGNMENT
PROCEDURE FOR ALL CHEVROLET RADIOS
This alignment procedure is to be used only when any of the following parts have been re'
placed in the radio; antenna coil, radio fre-
quency coil, oscillator coil, or any of the tuning
cores.
The intermediate frequency alignment at 262 kilocycles is the same as outlined in "Al'ign
ment Procedure" operations l through 4. After
completing the intermediate frequency alignment,
proceed as follows:
1. Connect signal generator tead to a .000082
mfd. condenser and connect to antenna termi-
nal of antenna socket. Mechanically align
Figure l3
iron core t'H", on circuit diagram and parts
Iayout to measure t-3f8" for aII radios ex-
cept 985396 and it is 1-5/8" in coil form
from rear mounting edge of coil with radio tuned to stop on 1600 kitocycle end of dial.
2. With signal generator still adjusted to exactly
1615 kilocycles, adjust trimmers "E, F and
G" on circuit diagram and parts layout in
sequence for maximum output meter reading.
3. Tune signal generator and radio receiver to
600 kilocycles and readjust iron cores "J
and K" ONLY, for maximum outPut meter
reading. Repeat the adjustment for maxi-
mum meter reading.
4. Reset signal generator to exactly 1615 kilo-
cycles and tune radio receiver to stop on
1600 kilocyôle end of the dial. Then readjust
trimmers "F and G" ONLY, until no further
increase in output meter reading can be obtained.
5. After the radio receiver has been installed in
the car, turn on the receiver and tune in a weak station near 1000 kilocycles, with radio volume turned to maximum position and an-
tenna extended to full height. Readjust trimmer
"G" ONLY, for maximum volume.
13

CUSTOMER SERIES 95 TRUCK RADIO 985443
The radio consists of a radio reciver unit with
an external speaker. This type of design is
Figure ó8
advantageous for both installation and service as all component parts of the receiver are readily
accessible for quick efficient replacement when
service is required. Using an external type speaker affords the advantage of having a larger
type speaker in a limited space area. The
speaker is coupled to the instrument panel by a
special type gasket, thereby using the entire
instrument panel for unusually good tone re-
production.
TRANSISTOR COMPLEMENT AND FUNCTION
DS.24 DS.z5 DS-22
DS-26 DS-503
Fisure ó9 VOLTAGE CHART - 985443 - RADTO
66
Radio Frequency Amplifier
Converter
Intermediate Frequency Ampiifier
Audio driver transistor Audio output "HI-POWER" transistor
GENERAL INFORMATION
Tuning range 540 - 1615 kilocycles
Intermediate frequency - 262 kilocycles
Maximum power output 6 watts
Undistorted power output 3.5 watts
Current drain 1.2 amperes at 12 volts
Speaker-Alnico V permanent magnet type 4" x
10,,
DS-503
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Fisure 70 ISLAND NUMBER AND SIGNAL TRACING OF CIRCUIT BOARD - 985443 RADIO
Voice coil impedance 10 ohms at 400 cycles
AII circuits use a printed circuit board
Fuse protection 2.5 amperes
SERVICE PROCEDURE
Check voltage for correct voltages as shown in
figure 67. If voltages are correct and radio
does not play proceed as outlined in Service
Procedure for Push Button and Manual radios
on page 5.
SERVICE PARTS LIST
985443 - RADrO
NOTE: Atl Chevrolet radio service parts
are available to dealers through General
Motors Parts Division Warehouses. Orders
for radio parts requirements to be placed
with warehouse in the usual manner.
Illus. Service
No. Part No.
1 2 3 4
5
6 7
?281 71 6
728t946 7282042 728L7L7 7282313
7282323
122t623
Description
ELECTRICAL PARTS
Coils
Antenna, tuning
Choke, antenna series
R.F., tuning Oscillator, tuning
1st I. F.
2nd I. F.
Choke, 12 volt supply, input
Capacitors
7271564
7282272 Spark Plate
Electrolytic, 3 - section
67
39 37

o,
æ
DS 24
R.F. AMP
(s€RVTCE Wrf H 625) os 25
coNv. DS 22
I. E AMP.
(SERVTCE Wrf H OS25)
DS 26
A.F. AMP. DS 503
OUT PUT
A
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3V 57 MEG.
O!
to 2v
6A
t2v.
çFROM
FUSE
BLOCK
ì
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t6
I
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t__ L_l ¡
aI
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L-
3900
26
cÉ27 37A
AGC
oEf. 7
65C
Voltages measured termfnal to chæsig
yith a volt-ohm meter - no signd ild
l2 volta app¡ied to radio.
Total battery drãin 1.2 mps ât l2volts.
Tolerilce on voltage lú.
Before meering tra¡rsiator voltagea, a
l0 ohm spealer must be comected to
radio.
Vo¡tage
placed,
applied
shou¡d be meeaured from Irover traDaiator cæe to grannd. If pover t¡a¡aiator ia re-
(¡tem *66) to obtåin prõer cotlectör voltege with 12 volts
adrust bias Iþtentiometer
to redio.
I Iten #64 is a fwe resiator for the pover tranalator. Seryice rit¡r enct rep¡aceEent.
O Pr¡ntea oñ circuit bord.
ô O Wi¡t not appeu in aU radios.
CAUTIOI¡I: Onfy a l0 ohm speeler should be r¡sed on t¡ia radio.
1.5^
E
ìf rF
260
1C.
4
7
t4
B
æ27
AGC
OET
D
49
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Fisure 7l CIRCUIT DIAGRAM - 985443 - RADIO