Vintage Synth Repair

Hi;

I've fixed a Micro-Preset myself last year and made some notes that may be
helpful.

Do you have the schematic? Most of the copies I've seen so far are fairly
hard to read, though.

The good news is that it probably can be fixed; however finding the faults
can be a bit of a challenge.

A few thoughts to begin with:

Basically the Micro-Preset has three waveforms (saw, square and pulse) and
two noise types; it also has two filters.

Strings and Voice use saw and no filter
Wood 2, 4 and 16 use square and no filter
Wood 8 and 32 use pulse and no filter
Brass uses saw and the left filter in the schematic
Synthe1 uses pulse and the right filter
Synthe2 uses either saw or noise (depending on the octave switch) and the
right filter

So, what we can see here is the following:

- saw works
- pulse works
- left filter works

(Let's for the moment assume that triggering and envelopes also work.)

What you need to find out is:

1. Is square working at all?
2. Is any signal getting to the right filter (pin 3 of IC7)?

The first should be comparatively easy to check (with an oscilloscope) and
would definitely give us a clue what could be wrong here.

BTW, on mine some caps had leaked and partly destroyed traces.