Zenith J584W - Page 34

FIGURE 21 - CHASSIS 8WJR57 WITH MAGNETIC PREAMP
CHASSIS 8WJR56, 8WJR57 AND 15WJR29
MAGNETIC PHONO CARTRIDGE PREAMP
As indicated in Table A, Chassis 8WJR56, 8WJR57 and
15WJR29 are basically the same as the "H Line Chassis
6WHR56, 6WHR57 and 12WHR29 but are equipped with a
new preamp circuit to provide the gain and RIAA equalization
required for the magnetic phono cartridges installed in record
changers used in models having these chassis. (Chassis 8WJR56
is installed in Models JR684W and JR684W1 which do not
contain record changers but a "Magnetic Phono/Aux Switch"
is provided so that an accessory record changer with either a
magnetic or a ceramic phono cartridge may be connected.
Model J9026W is a current accessory that is equipped with
a magnetic phono cartridge.)
Circuitry for the preamp is located on a separate shielded
circuit board assembly mounted adjacent to the tuning capaci 
tor pulley on Chassis 8WJR56 and 8WJR57 (See Figure 21),
or mounted piggy-back on the RF/IF/MPX assembly of
chassis 15WJR29. A photograph of the circuit board assembly,
with the shield removed, is shown in Figure 22. Figure 23 is
a schematic of the circuit while an illustration of the cartridge
appears in Figure 24.
As you will note from the schematic in Figure 21 the preamp
(right channel shown) consists of two NPN transistors in a
conventional AG coupled oirouiti Gellewr current i?
R704 and R706, while bias is established with R703 and R707.
Emitter resistors R705 and R710 are selected to provide both
a low Total Harmonic Distortion (THD) and also low Tran 
sient Intermodulation Distortion (TIM). Between base and
emitter of each transistor are RFI (audio rectification) sup 
pression capacitors C703 and C705.
At the input, resistor R701 and capacitor C701 are selected
to provide the proper matching for the low impedance of the
magnetic phono cartridge. The series/parallel combination
of C706, C707, R708 and R709 is a feedback path and equali 
zation to provide reproduction using the established RIAA
equalization curve.
Output of the preamp is via capacitor C708 to either the band 
switch directly in Chassis 8WJR57 and 15WJR29, or to the
bandswitch via the "Magnetic Phono/Aux Input" Switch on
Chassis 8WJR56.
Due to the low output level of a magnetic cartridge, and
high gain of the preamp, in comparison to a ceramic cart 
ridge, it is necessary to provide the magnetic cartridge preamp
with a de supply voltage that is much cleaner (less hum ripple
or noise) than would be true for a ceramic cartridge. There 
fore two additional RC sections (C709, C710, R711 and
R712 or R713) are included in the B+ decoupling network.
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