Zenith J584W - Page 10

GENERAL INFORMATION
THEORY
From time to time Zenith includes the use of new components
and circuit applications in product design. Theory and ex 
planation of such components and circuits is included in
various manuals. Refer to the inside front cover for further
information.
CIRCUIT BOARD COMPONENT IDENTIFICATION
In order to assist the Service Technician, most circuit boards
are marked to identify the location of components, test points,
etc., using the schematic reference symbols and numbers. We
have also prepared a 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:
POWER AMPLIFIERS
When servicing these products, the Service Technician must
consider the following:
1. Each channel of the following amplifiers use a pair of
matched power transistors in the final output stage. There 
fore, 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.): 3WJR50, 50Z, 3WJR51, 3WJR52,
52Z, 8WJR56, 8WJR57, 15WJR29.
2. When a power transistor is replaced the insulator (when
U£0d) between the ttengigtBF and the heat gink shsultl alsa
be replaced. On the following be certain to apply Castall
No. 832M heat conductive grease between the transistor
and the insulator. Also between the insulator and the
chassis. The Castall grease can be obtained in quantities by
ordering Part No. 205-303: 8WJR56, 8WJR57, 15WJR29.
8
3. Do not operate these amplifiers without their proper
speaker load.
4. Do not short out the audio output of either channel when
the amplifier is operating.
5. Should a power transistor fail (short) be certain to replace
the emitter resistors for the specific channel. Also be
certain to check the condition of the silicon diode rectifiers,
and driver transistors.
6. Remove plug-in transistors from their sockets before doing
any soldering to the socket lugs.
7. Check bias adjustment control (on chassis so equipt) if any
components have been changed in the pre-driver thru out 
put stages. See schematic for added information.
SIGNAL STRENGTH CHART
There are certain minimum voltages necessary for proper stereo
FM reception. To help determine if there is sufficient signal
available, the following developed AGC voltage versus micro 
volt input voltage charts have been compiled. Since the desired
FM Station may not always be operating in the stereo mode
when an installation is made, these AGC voltage measurements
have been taken with a monaural FM signal. The point"* 0 of
minimum AGC voltage necessary for good stereo FM reception
has been indicated on these charts.
AGC voltages are to be measured with a V.T.V.M. connected
to the following Test Points.
Chassis 3WJR51 — Test Point "C" at base of Q1; located
between Transistors Q101 (A.M. Converter) and Q201
(1st. I.F.).
Chassis 8WJR56, 8WJR57 — Test Point at junction of R2
and R229; either end of orange wire at pulley end of gang.
Block
Stage
Example
1 - 99
FM Tuner
R1,C1, L1.
101 - 199 AM Tuner
R101, C101, L 101.
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.
Chassis 3WJR51
Chassis 8WJR56,
8WJR57
Micro
Volts
Input
Voltage
AGC Voltage
at Test Point "C"
Micro
Volts
Input
Reverse
AGC Voltage
At Gate 2 of FM RF
0
1.23
0
5.4
25
1.10
25 4.5
100 0.88
100
3.3
300 0.78
200
3.65
500
0.71
500
2.5
1l<
*0.67
1K
*2.1
5K
0.60
5K
1.22
50 K
0.12
50K
0.15
100K
0.06
100K
-0.08