Vintage Synth Repair

Hi everybody,

I have a SH-101 that was very noisy (hiss and hum). I replaced all the
electrolytic filter capacitor and that really fixed the noise problem. There was
also a problem with the portamento that was changing the pitch when I turned it
ON. I resoldered TR1 on the bender board and that fixed the portamento problem.

Now there is still some problem with the ADSR in Gate + Trig mod.
When all the oscillators, VCF and ADSR sliders are at zero, I can hear a mid
frequency tone when I play any key. It's always the same frequency unless I
play many keys at the same time.

If I raise one of the oscillator slider and the S slider, I hear the sound from
the oscillator but it is kind of modulated with the constant mid frequency tone.

Everything is fine in Gate mode.

I checked the trig signal at the output of IC5 and it is not very clean. I've
got some constant triangle spikes going up to 1 volts. It is the same
frequency than the tone I hear in Gate + Trig mode.

I replaced IC5, IC4 and the diodes around but that did not help.

Any help would be greatly appreciated.

Thank you!

Nicolas