Sequential Pro One - Service Procedures; Removing the Printed Circuit Board (PCB)

Details of the 3320 and associated components can be found on its Data Sheet in the
Appendix,
2-6 FINAL VCA
The Final VCA U109-12 uses 112 of the RCA CA3280 dual operational transconduc-
tance amplifier QOTA). (The other 112 is used in the Glide Circuit, see para. 2-13) The
OTA is similar to the typical op amp in input and open-loop gain characteristics but
the output is current rather than voltage. The magnitude of the output current is equal
to the product of the transconductance (rather than the voltage gain) and the input
voltage. The transconductance is adjusted by the amplifier bias current (labc) at pin 6
(see Appendix). A second terminal (Id) for biasing linearizing input diodes, controls the
transfer characteristic as shown on the Da-ta Sheet (See Appendix--Figure 12). R1141
sets Id, which determines the input impedance of the OTA. As Id increases, the
effective resistance between pins 9 and 10 decreases.
Since the 3280 is controlled by current, the term CY is not really accurate. The 3280
r7CV" from the Amplifier Envelope Generator is converted to a control current (CC) by
4104, 2N4250 PNP transistor. R1143 limits the amount of current which can be
developed.
R 11 38 VCA BALANCE trimmer cancels OTA off set, ensuring that even with maximum
Iabc, there is no output voltage in the absence of an input signal. R1122 develops
suitable drive voltage from the OTA's current output.
U108-7 5532 voltage follower can drive 600-Ohm loads, so is not bothered when R1125
3s grounded by a mono plug connected to 38.
2-7 ENVELOPE GENERATORS
U116 Filter and UJ 17 Amplifies Envelope Generators are based on the CEM 3310 IC.
Its output is 0 to +5 Vdc changing over the time periods set by the input timing CVs.
When applied to the VCF and FINAL VCA, the transient voltage determines the
frequency and amplitude contours of the voice.
The circuitry around both generators is minimal. The knobs simply set the desired
control voltages.
Note that the ATTACK, DECAY, RELEASE timing controls are all
wired
from -5V to ground, while SUSTAIN is wired from ground to +15V.
If it is desired to extend the maximum ATTACK or RELEASE periods, decrease the
1.8M resistors (e.g. R1196, R1200, R1206, R1205) ta l&M, which will extend the range
to
20 seconds. (Note: such modifications may conflict with published patch diagrams.)
The TRIGGER and GATE si'gnals are generated by circuitry discussed below. C157
forms the needed pulse for the trigger.
'I
R1211 adjusts VOLUME by attenuating the CV to the FINAL ~~~~'~1168 contributes a
negative voltage to the inverting input of U 1 14- 14, creating a positive off set required
to compensate for the drop across the current-converting transistor 4104 and R1143.
Details of the 3310 and associated components can be found on its Data Sheeet the
Appendix.
TMIOOA 12/81 2- 9