Plug-In Board Af 12 - Preamplifier - Wersi DX 400 Assembly Manual

3. Plug-in Board AF 12 - Preamplifier

Plug-in board AF 12 contains analog processing circuitry for
all the organ's audio tone signals as well as tone signals from
external sources. AF 1 2 apportions the signals between the
two audio channels as shown in the following table:
Signal From Input Via IC To Channel
(pin no.)
1)
Slave DE
2)
Slave NDE
3)
Slave SDE
W-Voice NDE
W-Voice DE
VCF NDE
VCF DE
Rhythm A
Rhythm B
Reverb A
Reverb B
Microphone
Mic. Reverb
Tape Playb. I.
Tape Playb. r.
Tape Record
Tape Record r.
1) D E - Deemphasis = Lowpass
2) NDE - Nondeemphasis Practically linear
3) DE - Superdeemphasis - Super lowpass
Principles of Operation
Fig. 6 shows the component layout of plug-in board AFl2and
Fig.7is its schematic diagram. All the audio tone signals
developed in the organ enter AF 12 via pins 3 thru 9 and go
to the two sections of control amplifier 1C2. The three slave
signals (pins 3, 4, 5) are initially
AM 334-01-502
3 3 A+B
4 3 A
5 3 A+B
6 2 A+B
7 2 A+B
B 2 B
9 2 B
10 4 A
11 4 B
12 6 A
13 6 B
15 A+B
16 5 A+B
17 1 A
18 1 B
19 - fmA
20 fm.B
amplified by the four sections of 1C3. The gains of 1C2's
amplifiers are dependent on a DC control current from Q3
applied to pins 1 and 16. This control current, in turn, is
dependent upon the position of the swell pedal (pin 23); the
phototransistor in the reflex coupler of PC board OS 2
produces an output current whose level is determined by the
pedal position. Trimpot PS determines the point of
maximum volume (visually indicated by the LED fed by
IC5b). The minimum volume level with a fully retracted
pedal can be set via P6.
The signals from 1C2 pins 8 (channel A) and 9 (channel B)
go via summing buses A and B to summing amplifiers lC4c
and d. (Microphone audio is applied to the summing buses
via R1Ol and R103, rhythm via R98 and R105, and reverb
via R99 and R104). The summing amplifiers are quieted
when field effect transistors (FETs) Qi and Q2 are
conducting. This occurs when, for example, the organ is
turned on and (as explained in PS 12) a reset pulse via D4
turns off the normally conducting Q6; FETs Qi and Q2 then
"see" a negative (less positive) potential via Dl and R134;
this drives them in-to conduction and they effectively
become low resistance shunts across their respective
amplifiers. The processor system also quiets the summing
amplifiers, via D3, during processing runs which may cause
noise in the audio system.
The outputs of the summing amplifiers go via R4 and R6 to
the tone control network built around IC1 - Audio to be
recorded is taken off before the tone control network via R51
and R52. Tape playback audio is applied to the network via
R31 and R5. Trimpots P1 and P3 control the bass, P2 and P4
control the treble. The outputs on Ici1's pins 8 and 14 are the
processed audio signals to be sent to the power amplifiers
(cf. Fig. 36).
The rhythm signals (cf. Fig. 15) are amplified by lC4a and b,
and go via control amplifiers 1C7 as well as R98 and R105
to. summing buses A and B. This portion of the signal is
independent of the swell pedal. In order to achieve a certain
amount of swell pedal control over the rhythm, however,
portions of the rhythm signals go to control amplifiers 1C2
via R13 and R60. Rhythm control amplifiers 1C7 receive
their control current from QS. as is driven by ICS, which
processes control voltage impulses sent via pin 24 from the
rhythm volume control as well as the processor system.
15