On this nice JP-8, which was here for a re-capping, there were issues with the VCF bend fader on the bender board. The fader had no effect; it was pinned at "0" no matter what. Consequently the bender had no effect on the VCF. My first thought was to make sure the VCF pedal jack had no issues; the jacks are one of the most vulnerable areas on a keyboard, so it was worth testing that jack to make sure there were no shorts there, especially since it is so easy to test. There were none; and in retrospect it is possible that this would have an effect on the JP VCF bender because the microprocessor would probably ignore the jack input if it was unchanging.
The next thing I checked was the fader; faders and controls are also a very vulnerable part of all keyboards. In this instance I noticed that pins 1 and 3 were shorted (no resistance). Looking at the schematic, (figure 1), I could see that there was no way that pin 3 would be connected to ground unless (1) the fader was shorted (how could that possible be?), (2) the op amp, IC2, was somehow shorting pins 6 and 7 to ground, or (3) there was a piece of solder or a solder bridge touching the traces or some other type of mechanical issue.
I investigated for number 3 first, a solder bridge or piece of debris, since it seemed most likely, but to no avail. I then investigated for number 2, the possible "op amp short". There were some connections from that op amp to ground, so it was possible that it could be the culprit, but after removing it, the issue remained.
I couldn't imagine how a fader could short out, but I removed it anyway. When I did, I realized that it was soldered in wrong... the orientation was upside down, and consequently pins 2 and 4 (shorted by design) were in the holes designed for 1 and 3.
This is the second time I've encountered faders soldered in wrong, recently. The last time was on an EMU. I turned the fader the other way, soldered it back in, and all works fine!
As technicians, we look for failed components; but there are occasions where the components were never installed correctly in the first place, and sometimes the problem was ignored for years and years.