Vintage Synth Repair

So the presets that use the filters have a problem. However it seems
unlikely to me that both filter chips have failed (though one never
knows...). This in turn would make me search first for circuitry that is
common to both filters.


I take it you have already checked the switch Matrix? This makes many
connections that affect the general behaviour of the synth (like which
envelope is used, which waveform is routed to the Synthe filter and so on).


You mentioned that the envelope works on Synthe 1's volume; does it also
work on Synthe 2's volume? Both use different envelopes (Synthe 1 has no
Attack; Synthe 2 no Decay, just Attack and - essentially - Release even
though it is called Sustain here).


I'd probably start looking around IC5 (this is the final opamp between
filters and switch matrix). However IC5 is (I believe) also used for the
Wood presets.


The filter IC itself is the famous Korg 35 chip:


http://www.timstinchcombe.co.uk/index.php?pge=korg