ARP 2600 TZ-581 replacement?

[WeAreAs1 at aol.com]

Hello John,

I took another look at the ARP 2600 mono keyboard diagram, and at the pinout
for the 6-pin connector that goes from the keyboard to the synth.  If your ex-
girlfriend really did plug it in backwards, pin 3 and pin 4 (+15 and -15
volts!) were reversed.  I don't know of many opamps that could withstand such
an insult for more than a few seconds.  That opamp module is probably some
kind of discrete-FET-input-feeding-into-a-741 thingy, since bifet opamps
weren't yet widely available at that time.   After what happened, I'm
surprised that it still works at all.   (Conversely, I don't quite see how
reversing the connector cable would have affected the TZ581, but I guess it
did!)  BTW, ARP didn't make those opamp modules - they were buying them from
General Dynamics, who were likely selling them as some kind of "precision
instrumentation amplifier".

I think it would be very easy to replace the opamp module, if need be.  You
could use a simple, more modern high-impedance single FET opamp, such as a
CA3140 or 3130.  These are available at most electronics stores for less than
two bucks, and probably have better performance than ARP's original opamp
module as a CV follower/memory (less CV droop after releasing the keys - the
2600's droop rate is horrendous).  You'd just need to make a little perfboard
to mount it on - or simply drill eight holes right into the PC board (in the
footprint of a DIP-8 IC), and attach some wires.  I think you might even still
be able to use the same offset trimmer (R19) for zeroing the low C.  I don't
remember if the 3140 has pins for offset trim - if not, you could try a TLO81,
which has offset trim connections.

The later model two-voice keyboards, the Odyssey, and the Axxe had keyboard CV
followers made with one of those trick little dual FET's (IMF3958) feeding
into a garden-variety LM1458.  I don't think such an elaborate circuit is
necessary, though.  The IMF3958 might be impossible to procure, and the CA3140
as a non-inverting unity-gain voltage follower should work just fine.

Feel free to write me if you have any questions.

Michael Bacich