Zenith J584W - Page 37

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FIGURE 26 - CHASSIS 8WJR57 AUDIO OUTPUT CIRCUIT
not shown in Figure 24B, the molded stylus grip, is shown in
Figure 24A. A customer can easily grasp the stylus grip,
without touching the stylus tip, should they desire to replace
the stylus.
COIL ASSEMBLY
Figure 24C illustrates the coil and pole piece assembly required
for one channel of a stereo moving magnet cartridge. You will
note that two coils and two pole pieces are required for each
assembly. When two of these assemblies are combined for
stereo, the four pole pieces form a square. The moving magnet
of the stylus assembly will be positioned so it will move
between the pole pieces, inducing voltage in the coil.
Output of a magnetic cartridge is extremely low (approximately
5 to 10 millivolts) compared to the much higher level of
ceramic cartridges (approximately 300 to 500 millivolts),
therefore the need for the added preamp described above.
CHASSIS 8WJR56 AND 8WJR57
AUDIO CIRCUITRY
In addition to an increase in rated power output and wider
bandwidth incorporated into Chassis 8WJR56 and 8WJR57
(over their "H Line" counterparts), these two chassis incor 
porate Low Cut (SW5) and High Cut (SW6) switches.
Figure 25 is a partial schematic of the preamp and audio
control section of Chassis 8WJR56. The preamp is a emitter
follower circuit, followed by the tape output jack, Loudness
(R408) and Tone Controls (R410, R412). Connected at the
Loudness control is a Low Cut Switch (SW5). When the
switch is in the "OFF" position, C422 is shorted, connecting
the " — " side of C404 to the high side of the Loudness Control,
resulting in normal operation. When in the "ON" position,
C422 will be connected from the " — " side of C404 to the
high side of the Loudness Control providing low frequency
cut. The Hi Cut Switch (SW6) is connected from the balance
control to ground. With SW6 in the "ON" position, high
frequencies are shunted to ground via C423.
Figure 26 illustrates most of the audio output circuitry used
in Chassis 8WJR56 and 8WJR57. This circuit is somewhat
similar to the basic complementary symmetry circuit shown
in Figure 16 for Chassis 3WJR52. Amplifier transistor Q402
is now direct coupled to the pre-driver Q403. Q403 and Q404
are effectively a class A Darlington circuit. Resistors R415 and
R427 serve as collector loads for Q402 and Q403 respectively.
Collectors of Q403 and Q404 are de coupled to the base of
Q406 (directly) and also to the base of Q405 (via resistor
R421 and bias diode CR401). AC signals will see CR401 as a
short circuit. Boot strap capacitor C417 is now connected
directly to the emitter of Q405, instead of at the center point
(junction of RX422 and RX423. At the center point are:
1. Output coupling capacitor C401 and bleeder resistor
R428. The bleeder resistor provides protection should
the set accidentally be operated without any load.
2. A feedback circuit to the emitter of Q402, consisting of
voltage divider R416 and R417, determines the AC feed 
back to Q402. Capacitor C415 is included for de insola 
tion because the circuit is direct coupled.
Voltage at tta juMtiBR 8f and R436, determines the
center point de voltage. C418 provides added high frequency
stability. Operation of this circuit is the same as explained for
Chassis 3WJR52 shown in Figure 16.