Cgs synth

Hi,

I built two cgs65 from the rev. 1.1 pcb and with one vco
they function fine. But as soon as i connect a second vco
to the powersupply, the vca starts oscillating on it's own.
The audio of the vco is not even connected a that point
and the vca audio is not routed to the vca input.
Perhaps this has something to do with inadequate grounding?
Any suggestion how i could tackle this?

kind regards