Vintage Synth Repair

>>What could be the problem ?<<

sounds like the mic pre-amp is just sending dc to the filter stages;
this is the equivalent of a constant noise replacing your voice-signal.

do you have schematics? could you read them if you did?

I just fired mine up to remind myself.... what do you see on the meter
when it's switched to look at the mic level? does the level control do
anything?
also, remember that in it's unaltered form, the vc10 lets it's
white-noise generator straight through to the output anyway; only the
keyboard sound is processed by the filters. there is a mod for this,
published on the net someplace.... google will find it.

duncan.