Vintage Synth Repair

Hi, Johnny:

The service manual's description of the noise circuit mentions that Q9
is specially chosen "for optimum avalanche characteristics", so it may
be that the transistor you've installed is not breaking up in the same
way.

Try listening to the noise output at pin 6 of A3 and comparing it to
the output at pin 6 of A4.  The first should be white noise and the
second pink noise.