Vintage Synth Repair

This calls for a more thorough debugging.

Set the front panel controls as follows:

- Cutoff 0
- Resonance 0
- EG Intensity 0 (center)
- Key Track 0

Now check with a scope the situation at pin 3 of IC23. You should get a
proper DC voltage of +5V. If you move the Cutoff control to max the voltage
should change to -5V. Same should apply to the outputs of IC23.

Next we check the multiplexing of the keyboard CV. For this we monitor pin3
of IC25. Put the synth in Unison mode and press the highest note on the
keyboard. You should get a reading on the scope that looks like DC with a
single periodic 0v "dip".
The keyboard CVs for the six channels start immediately after this "dip".
You can check if they correspond to the keys pressed.

Now we set Key Track to Full, Cutoff to zero, again use Unison mode and
press the highest key. Check at pin3 of IC23. You should basically get a
"mirrored" reading from the one at IC25 - a weak DC signal with a single 5V
"peak". Again the channels would start immediately after this peak. Check
if the signals there make sense. By playing different notes you can check
if the voltages on the input (pin 3) correspond to the voltages on the
output of IC23 or if - for example - the voltage for channel 1 shows up on
the wrong output pin.