Vintage Synth Repair

Ahoi- have a Mini here that will pass the A440 tune
signal but has no audible signal from VCF.

If I plug into the high output and crank up the volume
and emphasis you can hear a tiny whistle from the VCF
but you cannot hear it's output. It's also possible to
hear the oscillators very faintly as well so I know
they're still there. Power supply looks good.

On the filter board I've changed Q1, Q12, and Q17
since I had the 2N4058's but I don't have any TIS97's
to try in the rest of the circuit. I am trying to
establish where I'm loosing signal. I hooked into C27
on the filter board the other day and heard nothing.
Where else should I look? cheers! [w]