Vintage Synth Repair

>> 1. Q7 turns off a little too late. This allows C8 to discharge for a very small time. Different possibilities:

>> 1.1. Q7 itself is faulty.

Indeed. This is a new component now. At least that sweep is gone. It does appear to work but I'll need to investigate that possible delay you describe.

>> 1.2. The various keyboard contacts break contact in the wrong order. Usually the gate buss should break contact first, then the CV buss. Is the behaviour uniform for all keys? Is the behaviour identical when releasing the key slowly or quickly?

The behaviour does occur on each key but I have to say that the amount of swing sounds inconsistent. It definitely happens on each key but sometimes it's more dramatic. I'll apply some external CV / gate and see if it produces the same problem. That should help me troubleshoot the kbd assembly.

>> 1.3. Other things in the path from the gate buss to Q7.

On key press CR2 and R21 show that the gate is active. The opposite side of C7 correctly shows -15V.

2. The circuit finds some kind of "balance" after Q7 turns off.

>> 2.1. Are +/-15V exactly identical? Is there possibly ripple on one of the rails?

They are, yes. +/-15V. All new caps on the PSU board to be 100% sure. I checked for any ripples but I'm finding any so far.

>> 2.2. Perhaps something's wrong with Q8.

I'm beginning to think that the 3958 is faulty too. I'm not clear on what measurements I should find from each pin though. I also don't have a direct replacement so I'd have to concoct a version with two transistors.

>> At this point I would probably first check the power supply voltages and the try to see (possibly with a two channel scope) if the busses are switching in the right order and if there is some delayed switching of Q7 (compared to the actual gate buss).

I'll have to see if I can set up a dual arrangement. I'm working 
on this at another location and will have to come back to this one.