Chevrolet 985203 User manual

CUSTOM SERIES 95 TRUCK RADIO 985203
This radio has four tubes and two hi-gain tran-
sistors and is designed for the Series 95 trucks.
The radio consists of a radio receiver unit with
••t •• • ~
',\ I :I I' 14 of
Figure 82
an external speaker. This type of design is ad-
vantageous
for both installation and service as
all component parts of the receiver are readily
accessible for nuick 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 speak-
er is coupled to the instrument panel by a special
'.'
Figure 83
89
type gasket, thereby using the entire instrument
panel for unusually good tone reproduction.
ELECTRICAL DESCRIPTION
The
circuit ' used in this receiver is ' the super-
heterodyne type that uses no regeneration. The
tuning
circuits are of the permeability type and
are tuned by varying the iron cores in and out of
the antenna, radio frequency and qscillatot. coils
like pistons. The ' interm ' ediate frequency stages
are tuned by means of two iron cores in each
transformer and are adjusted with an insulated
screw driver from the bottom and top of each
transformer; both the first (input) and second
(output) interItlediate
frequence transformers are
tuned by this method. See Figure 83.
The antenna
circuit Is capacity 'coupled to the
antenna by means of an adjustable antenna trim-
mer condenser to' take care of normal variations
in antenna and antenna coil capacity. The antenna
condenser is adjustable by means of a small
screw driver and is located at the lower front on
the right side of the radio ' case.
The automatic volume control is of the delayed
signal type and is very capable of main~aining a
constant level of volume at all times. Very high
frequency
filter chokes are used in the radio
frequency grid circuit to discriminate against
ignition interference In the receiver, thus elim-
inating the use of spark plug and distributor
suppressors.
TUBE AND TRANSISTOR COMPLEMENT
12DZ6
12AD6
12EK6
12AE6A
DS26
DS503
AND FUNCTION '
Radio
frequency amplifier
Oscillator - modulator
Intermediate frequency amplifier
Detector -Automatic volume control
and first audio
Audio '
driver transistor
Audio output "HI-POWER" transistor
GENERAL INFORMATION
Tuning
range 540 -1615 kilocycles
Intermediate frequency -262 kilqcycles
Maximum power output 4.2 watts
Undlstorted power output 3.2 watts
Current drain 1.4 amperes at 12 volts
Speaker-Alnico V permanent magnet type 6 x 9
, Inch
Voice
coil impedance 4 ohms at 400 cycles
All cl!: cuits ,!se a printed circuit board

II , 6V.
o
11.4V
AV ,
10 , ZV ,
IZV,
OV, 10,ZV. 1.0'1,
{o??\o 0
V DS~26
10.3V.
·-1.9V,
Figure 84 VOLTAGE CHART -985203 -RADIO
PROCEDURE FOR CHECKING THE
VOLTAGES OF 988336 RADIO
Hook up radio on the service bench to a "12"
volt
power supply unit. It is important that you
have
12 volts at the spark plate of the radio, or
the voltage readings will be correspondingly
lower. All voltage readings have been taken with
V. T.V.M. Set the volt-ohm meter in the "30"
volt position to read "D. C." voltage. Ground one
lead of volt meter to radio chassis and with
other lead check all tube pins marked "H" with
a voltage
reading of 12 volts as shown in Figure
84. If incorrect or no voltage, check or replace
the following:
1. Check
or replace "On and Off" switch, Item
55C on Circuit diagram and 55 on parts
layout.
', '
,
,
2. Check or replace condensers, Items 27 and
28 on
circuit diagram and parts layout.
Next
check the transistor collector element
marked "C" of the DS503 output transistor
which should read 1.3 to 1.7 volts D.C. If high
or no voltage, check or replace:
1. Check or replace emitter fuse resistor, Item
50 on the circuit diagram and parts layout.
2. Check or replace speaker, Item 60 on the
circuit diagram and parts layout.
90
3. Check
or replace output transformer, Item
58 on the circuit diagram and parts layout.
4. Check
connector, Item 59 for ground to chas-
sis. This is a shorting type interlock switch
and must be orrened by speaker plug when
testing radio.
5. Check or replace potentiometer, Item 57 on
the circuit diagram and parts layout.
Next
check the resistance of the base element of
the
transistor marked "B" of the DS503 to
ground with
the radio turned off. Make this
check with meter on the R x 1 ohm scale. The
re sistance should be between 2 and 15 ohms. If
incorrect or zero, check or replace:
1. Check or replace the input transformer, Item
56 on the circuit diagram and parts layout.
2. Check or replace condenser, Item 19B on the
circuit diagram and parts layout.
3. Check or replace reSistor, Item 51 on the
circuit diagram and parts layout.
Next
check the resistance to ground of the tran-
sistor emitter element marked "E" with the
radio turned off. Set the ohmmeter on the RXI

c B
18
11151
F E
Figure 85 PARTS LAYOUT ON CIRCUIT BOARD -985203 -RADIO
scale . The resistance should be between 1 and 4
ohms. If inc orrect or zero, che ck or replace:
1. Che ck or re place fu se re sistor, Item 50 on
the c ir c uit diagram and parts layout.
2. Che
ck or replace condenser, Item 19B on the
circuit dia g ram and parts layout.
Next, with
th e radio turn ed off, check the re-
,.,
s istance be tween the collector element i marked
ue" and the emitter element marked ~ teE" of
the DS503. If less than 1 ohm, the transistor
may be short ed. Disconnect th e . l ~ ads and check
the resistance again. If still less than 1 ohm,
replace the transistor.
Next check the transistor collector element
marked "c" of the DS26 transistor which should
re ad .5 to 1.0 volts D. C. If high or no voltage,
check or replace:
1. Check resistor.
2. Check or replace audio transformer, Item 56
on the circuit diagram and parts layout.
91
3. Check or replace condenser, Item 19A on the
circuit diagram and parts layout.
4. Check or replac e 12AE6A tub e.
5. Check or replac e re sistor ; Item 47 on the
circuit diagram and parts layout.
6. Check or replace resistor, Item 48 on the
c ircuit diagram ' and parts layout .
Next
check tube pin No. 7 marked "P" on the
12AE6A which should read 10.2 volts D. C. If
incorrect or no voltage, check or replace:
1. Check or replace transformer, Item 56 on
the circuit diagram and parts layout.
2. Check
or replace resistor, . Item 52 on the
circuit diagram and parts layout.
Next
check tube pin No . 2 marked "K" on the
12AE6A which should read 1.1 volts D. C. If in-
correct or no voltage, check or replace:
1. Check or replace resistor, Item 42 on th e
circuit diagram and parts layout.

o
58
67
TUNER
78
10
5503
1
56
CIRCUIT BOARD
(TUBE VIEW)
~-0526
Figure 86 PARTS LAYOUT -BOTTOM VIEW -985203 -RADIO
Next check tube pin No.5 marked " P" on the
12EK6 I. F. amplifier which should read 11.4
volts D. C. If incorrect or no voltage, check or
replace:
1. Check or replace 1.F. transformer, Item 7
on the
circuit diagram and parts ' layout.
'.
•
Next check tube pin No. 6 marked "S" on the
12EK6 which should read 11.4 volts D.C. If in-
correct or no voltage, check or replace:
1. Check or replace electrolytic condenser,
Item 19A on the circuit diagram and parts
layout.
2. Check or replace resistor, Item 52 on circuit
diagram and parts layout.
Next check tube pin No. 5
marked "P" on the
12AD6 which should
read 11.4 volts D. C. If in-
correct or no voltage, check or replace:
1. Check or replace 1. F. transformer, Item 6
on the
circuit diagram and parts layout.
92
Next check tube pin No. 6 marked "S" on the
12AD6 which should read 11.4 volts D.C. If in-
correct or no voltage, check or replace:
1. Check or replace oscillator coil, Item 5 on
the
circuit diagram and parts layout.
Next check tube pin No. 5 marked "P " on the
12DZ6 R.F. amplifier which should read 11.4
volts D. C. If incorrect or no volta ge, check or
replace:
1. Check or replace choke, Item 3 on the circuit
diagram and parts layout.
2. Check or replace condenser, Item 17A on the
circuit diagram and parts layout.
Next check tube pin No. 6
marked "S" on the
12DZ6 which should
read 11.4 volts D. C. If in-
correct or no vo lt age, check or repla ce:
1. 'check or replace re sistor, Item 52 on the
circuit diagram and parts layout.

P
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Lw
~
fOs503l
b 0
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I @
= 8 I
56
" ,'Y
~ l
CI RCU IT BOARD
(CIRCUIT VI
EW)
TUNER
J
78
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-
II
0
Figure 87 PA RT S L AYO UT - TOP VI EW -985203 - RAD IO
PROCEDURE FOR SIGNAL TRACING
RADIO 985203
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 Qhassis.
NOTE:
To protect the signal generat yr from
"D.C." voltage, place a 0.1 mfd. condenser
in signal generator lead between the signal
generator and the end of the test lead. Adjust
signal generator volume about 3/4 open to
obtain a strong signal.
With signal generator lead, touch DS26 collector
marked "C". If no signal, check or replace:
1. Check or replace fuse resistor, Item 50 on
the
circuit diagram and parts layout.
2. Check speaker interlock socket, Item 59.
3. Check or replace speaker, Item 60 on circ u it
diagram and parts layout.
93
4. Check or replace the output transformer,
Item 58 on the circuit diagram and parts
layout.
5. Check or replace input transformer, Item 56
on the circuit diagram and parts layout.
6. Check or rep l ace DS5 03 transistor output
stage.
With signa l generator lead touch tube pin No. 7
maoked "P" on the 12AE6A tube. If no signa l
check or replace the following:
1. Check transistor.
2. Check or replace · 12AE6A tube.
3. Check or replace resistor, Item 47 on circui t
diagram and par ts layout.
4. Check
or replace resistor, Item 48 on circuit
diagram and parts layout.

Figure 88 CIRCUIT BOARD , PATTERN - 985203 -RADIO
5. Check or replace condenser, Item 19A on
circuit diagram and parts layout.
With signal
generator lead touch tube pin No. 7
marked "G" on the 12EK6A tube. If no signal,
check or replace the following:
1. Check
or replace 12AE6A tube or tube socket
on circuit diagram and parts layout.
2. Check or replace resistor, Item 46 on circuit
diagram and parts layout.
3. Check or replace resistor, Item 42 on the
circuit diagram and parts layout.
4. Check or replace condenser, Item 23 on the
circuit diagram and parts layout.
Next
turn volume control all the way open and
touch tube
pins No's. 5 and 6 marked t 'DP" on
the 12AE6A tube.
If no signal, check or replace
the following:
1. Check
or replace 12AE6A tube.
2. Check or replace volume control, Item 55A
on
diagram and 55 on parts layout.
Now change signal generator from audio position
to generate an intermediate frequency signal.
Set
signal generator to .262 kilocycles, leaving
0.1 mfd. condenser in signal generator lead to
protect the signal ' generator from "D. C." volt-
age. Keep signai' generator grounded to radio
chassis.
94
Next touch tube pin
No.1 marked "G" on 12EK6
intermediate frequency amplifier tube. If no
signal, check or replace the following:
1. Check or replace 12EK6 tube.
2. Check or replace intermediate frequency
transformer, Item 6 on circuit diagram and
parts layout.
3. Check or replace 12EK6 tube socket.
Next touch tube pin No. 5
marked" P" on 12AD6
tube.
If no signal, check or replace the following:
1. Check
or replace intermediate frequency
transformer, Item 6 on circuit diagram and
parts layout.
Next touch tube pin
No.7 marked "G" on 12AD6
tube.
If no signal, check or replace the following:
1. Check
or replace 12AD6 tube.
2. Check
or replace oscillator coil, Item 5 on
circuit diagram and parts layout.
Next change
signal generator from intermediate
frequency setting to radio frequency signal. Re-
place the 0.1 mfd. condenser in the signal
generator lead with a .000082 mfd. condenser to
protect signal generator from "D. C." voltage.
Set
signal generator to 1100 kilocycles and tune
radio receiver to 1100 kilocycles (11 on dial
scale).

S IGNAL GE NERA TOR
SIG NA L GENERATOR
SIG NAL GENERATOR
.1 VGlIIgtS mllsu rtd I.rmlnlll to ttllnls with I
VlVM - ~ sign. 1M 12 .0 lOlls It IIlus. 11 .
Ost lll l!o r grill lO l l.,. IIbn IfIIh ut t uned a;
] 0Xi I(e.
Tot ll " A~ c ln ln It ltv. - 1.4 IIIIpl .
Toit r.nct on I'OII IgH ! ]0"0.
....
11 0"
• Indt t• ., ltid Irom tu ntr coli .n 'y .
••
lItfor. """url ng trlnslltor 1011.,.,. th e shorting t)llt sPlIk.r socktt mUll III.,*,
.nd • 4 oIMn IPI .... conlMdtd . II trlnslslof II ,.««1 . • llnt til. pottntlorMttr
tlllul. 51] to aMlin _optr colltctor ~ 'filth It .... 111 InpIA .., ,do .
t lII ul . :!O Is I '''''' ,..slslor lor tilt IrlM l stOf . Stn1c: . wIIh oid ''filament.
e outputlr'nslo,,,,,r 'filII .If II'Iorttd II shorting IyfIIlpttMr IlfI tth II not htld aptn .
POINT SIGNAL STOPS - CHECK OR REPLACE ITEMS LISTED
,
,
,
,
,
- ~ - -
TO tel 0 10
'1111 'LOCI!
No s ignal at Point 1 - Check or rep la1e ite ms No
signal at Point 6 -Check or r eplace
19C- 51-50- 56 -58- 60- 69-DS503. .
item 6.
No
signal at Point 2 - Check or rep l ace ite ms No signal at Point 7 - Check or rep l ace item
19A-47-48-12AE6A tube-DS26. 5 -1 2AD6 tu be or socket.
No signa l at Point 3 - Check or rep l ace ite ms No
signa l at Poi nt 8 - Check or replace items
23-42-46-12AE6A tub e. 4-5 - 17A-17B-2 1 A-2 1B.
No
signal at Point 4 - Check or replace ite ms No
signal at Point 9 -
Check or vepl ace item
42-55A-55B-12AE6A tu be or socket.
3-12DZ6 tube or socket.
No signa l at Point 5 - Check or rep l ace it em 7 No signal
at Point 10 - Check or rep l ace items
or 12EK6 tube or sock et. 1- 2- 16.
Figure 89 SI GNAL T RAC I NG PROCEDURE - 985203 - RAD IO
95

Next touch tube pin No.5 marked "P" on 12DZ6
radio frequency amplifier tube. If no signal,
check or replace the following:
1. Check or replace radio frequency coil, Item
4 on circuit diagram and parts layout.
2. Check or replace oscillator coil, Item 5 on
circUit
diagram and parts layout. .
3. Check
or replace condensers, Items 17A,
20A,
21A and 21B on circuit diagram and
parts layout.
4. Check or replace resistor, Item 37 on circuit
diagram and parts layout.
Next touch tube pin
No.1 marked "G" on 12DZ6
radio frequency tube. If no signal, check or
replace the following:
1. Check or replace 12DZ6 tube.
2. Check
or replace 12DZ6 tube socket.
3. Check or replace choke, Item 3 on circuit
diagram and parts layout.
Next touch
antenna terminal at antenna socket.
If no signal, check or replace the following:
1. Check or replace antenna coil, Item 1 on
circuit diagram and parts layout.
2. Check or replace condenser, Item 16 on
circuit diagram and parts layout.
3
.. Check or replace choke, Item 2 on circuit
diagram and parts layout.
This completes the entire checking procedure of
the receiver, and if the procedure has been fol-
lowed
as outlined, the failure will have been
located.
After repairing "failure, receiver will
operate and shoul(! now l).e "aligned for proper
performance as follows and which is part of the
service operation when' repairing radios.
PROCEDURE FOR ALIGNMEN1
OF 985203 RADIO
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 tran ' sformer, or tuning cores have
been installed. '
To properly align the receiver, it will be neces-
sary to have an output meter and' signal generator.
NOTE : If anyone of the tunlllg coils or cores
have been replaced, see "Capacity and In-
96
Figure 90 IR<?N CORE ALIGNMENT -985203 -RADIO
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 align-
ment 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 voltmeter in the
t, 3" volt "A. C. " range position and place
voltmeter leads across speaker voice coil
terminals. Speaker is Item 60 on circuit
diagram and parts layout.
2. Turn on signal generator and set adjustments
to obtain a 262 kilocycle signal. Connect one
lead of signal generator to radio chassis for
ground. Attach the other lead from the signal
generator to the tube pin No. 7 marked" G"
on 12AD6 tube.
3. Adjust signal generator volume control so
that the volt meter will read , at about half
scale.
NOTE: Radio receiver volume control must
be turned to the maximum position so that
the automatic volume control circuit of the
radio will not affect the alignment of the
receiver.
4. Adjust in sequence cores "A, B, C and
D", as shown on circuit diagram and parts
layout, for maximum meter reading . Repeat
adjustments to get maximum meter read-
Ing. Keep the signal generator volume tuned
down so
that during adjustments the meter
does not read more than half scale. This

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Figure 91 CIRCUIT DIAGRAM -985203 -RADIO

Figure 92 TUNER AND DIAL CORD VIEW - 985203 -RADIO
will result in a better alignment of the
receiver.
5. Next change signal generator setting to obtain
a
radio frequency signal and tune signal
generator to exactly 1615 kilocycles. Connect
a .000082 mfd.
condenser to antenna con-
nector and attach signal generator lead. Tune
radio receiver to the "stop" on the 1600
kilocycle
end of the dial. Keep tlte 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 kilocycles, with radio vol-
ume control turned to maximum pOSition
and
the antenna extended to full height. Re-
adjust trimmer "G" ONLY for maximum
volume.
98
CAPACITY AND INDUCTANCE ALIGNMENT
PROCEDURE FOR 985203 RADIO
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 "Alignment
Procedure" operations 1 through 4. After com-
pleting the intermediate frequency alignment,
proceed as follows:
1. Connect signal generator lead to a .000082
mfd. condensep.-and
connect to antenna ter-
minal of antenna socket. T(me signal gen-
erator to exactly 1615 kilocycles, and tune
radio receiver to "stop" on the 1600 kilo-
cycle end of the dial.
2. Adjust output meter to about half scale, and
then adjust trimmers "E, F and G" on cir-
cuit diagram and parts layout in sequence for
maximum meter readings.