Zenith D952X - Page 15

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Chassis 25WDR50, (Zl), 29CT20,
29CT21, (ZD, (Z2), 29CT30,
35WDR50
Chassis29AT24, (Zl), (Z2)
Micro
Reverse Micro
AGC Voltage
Volts
AGC Voltage
Volts At 1st.l.F.
Input
At Gate 2 of FM RF Input
(See Note)
0
5.7
0 1.50
25
4.5
25
1.40
JOO
2.8
100
1.10
200 2.2 200
.95
500
1.5 500
.80
1K
*-0.96
1K
* .75
5K
-0.22
5K
.65
50K
-1.10
50K
.52
100K
-1.20
100K
.40
Block
Stage
Example
1- 99
FM Tuner
R1.C1.L1.
101 • 199
AM Tuner
R101,C101, L101.
201 ■ 299
IF
R201, C201, L201.
301 - 399
Multiplex
R301, C301, L301.
401 ■ 449
Audio, Right Channel
R401, C401, L401.
451-499
Audio, Left Channel
R451, C451, L451.
501 ■ 599
Power Supply
R501.C501.L501.
601 - 699
Switching Circuits
R601.C601, L601.
701 - 799
Special Applications
R701, C701, L701.
801 - 849
Audio, Right Back Channel
R801, C801, L801.
851 - 899
Audio, Left Back Channel
R851, C851, L851.
AGC ADJUSTMENT NOTE
Chassis 29AT24, (Zl), (Z2) - Adjust AGC control so that
under no signal conditions a voltage of 1.5 volts is measured
across the 820 ohm resistor connected from 1st IF emitter to
“ B- ” (-12.4V) when in A.M.
TROUBLE SHOOTING AND SIGNAL TRACING
The old technique of “ screwdriver testing ” is definitely not
recommended while trouble shooting any solid state product.
In that method various circuit points were touched or shorted
to ground to cause a hum or click in the speaker. This must be
avoided because a solid state component can be destroyed if
excessive voltage or if wrong voltage polarity is applied. Only
standard point to point signal tracing with the proper RF, IF,
and Audio Signal Sources should be used.
RESISTANCE MEASUREMENTS
It is recommended that you use an ohmmeter having a very
low voltage in the meter circuit. If the voltage is too high at
the meter terminals, it could bias solid state devices into
conduction, resulting in misleading readings. As an alternate
procedure, remove transistor, or other solid state devices from
the circuit before making measurements.
SPEAKER PHASING
It is important that coded speaker leads be connected to coded
terminals on speakers for proper polarity within each speaker
group. It is then important that the speaker groups be in phase
with each other. One excellent method is to play a monaural
record with the volume of each speaker group equal. The
sound should appear to come from a point midway between
the speaker groups. If the sound comes from other than
midpoint, you should check polarity. One of the easiest
methods of checking polarity within the speaker group is to
momentarily place a 4% volt battery across the speaker feed
terminals. All the speaker cones should simultaneously move
in the same direction.
POWER AMPLIFIERS
Power transistors and their circuits are unique in operation,
therefore, repair procedure differs from those steps followed
when repairing tube type-circuits.
1. Each channel of the following amplifiers use a pair of
matched power transistors in the final output stage.
Therefore, should one transistor fail, both transistors must
be replaced simultaneously, since they will not perform
properly unless matched. (In chassis using complementary
symmetry circuits a matched pair consists of one NPN and
one PNP transistor.) (In chassis using quasi-complementary
symmetry circuits, the outputs consist of two matched
NPN ’ s. The drivers, which are matched NPN and PNP
should also be replaced as matched pairs): 3AT20, (Zl)
4WDR50X, (XI), 6AT24, 15WCA10, 15WDR5OZ1,
15WDR51, 16CT21, 25WDA10, 25WDR50, (Zl), 29CT20,
29CT21, (Zl), (Z2), 29CT30, 35WDR50, B545, D546, (1),
D583, D742, (1).
2. When a power transistor is replaced the insulator (when
used) between the transistor and the heat sink should also
be replaced. On the following be certain to apply Dow
Corning No. 340 heat conductive grease between the
transistor and the insulator. Also between the insulator and
the chassis. The Dow Corning grease can be obtained in 1
c.c. quantities by ordering Part No. 205-51: 4WDR50X,
(XI), 6AT24, 15WCA10, 15WDR50Z1, 15WDR51,
16CT21, 25WDA10, 25WDR50, (Zl), 29CT20, 29CT21,
(Zl), (Z2), 29CT30, 35WDR50.
3. On the following place the heat conductive grease in the
clamp, or on the chassis, and all around the transistor:
B545, D546, (1), D583, D742, (1).
4. Do not operate these amplifiers without their proper
speaker load.
5. Do not short out the audio output of either channel when
the amplifier is operating.
6. Should a power transistor fail (short) be certain to replace
the emitter resistors for the specific channel. Also be cer
tain to check the condition of the silicon diode rectifiers,
and driver transistors.
7. Remove plug-in transistors from their sockets before doing
any soldering to the socket lugs.
CIRCUIT BOARD COMPONENT IDENTIFICATION
As a special feature to aid the Service Technician, Zenith has
identified the location of components which are mounted on
certain circuit boards. This information is printed on the
circuit boards and also appears on the schematic. We also have
prepared a two-color drawing of the foil side of the circuit
board showing .the relationship between the components and
the foil. This will aid the Technician in quickly tracing circuits,
as not only are the components shown, but also the voltages at
various check points. Components are identified by a letter/
number combination. A letter prefix to indicate the type of
component: C=Capacitor, L=Coil, R=Resistor, CR=Diode, etc.
The numbers are assigned in blocks to identify the circuit, in
which it is used, as follows.
CIRCUIT BOARD SERVICING
Servicing circuit board sets is, in general, much the same as
servicing ordinary receivers. However, certain tools and tech 
niques are helpful for this type of work.
1. Good pair of long-nose pliers.
2. Sharp wire cutters.