Vintage Synth Repair

I've run across a problem in the past, with a piece of equipment that 
used a single DAC fed thru a demultiplexer.  Hopefully it is similar 
to your synth - I have not seen the schematic for the PS3200.

The output of the DAC is a sequence of voltages, generated from the 
stored memory data.  The demultiplexer is used to send each of the 
voltages to the circuit it controls, where it is stored on a 
capacitor.  Since the capacitor discharges over time, the DAC 
constantly cycles thru the sequence of voltages, recharging each 
capacitor before it can discharge enough to be noticeable.

Now for the problem.  If one of the capacitors is leaky, or the 
circuit it is connected to is discharging it, its voltage will drop 
significantly before the DAC gets back around to recharging it.  This 
will cause a noticeable modulation in the circuitry it controls.  The 
voltage across the capacitor will appear as a sawtooth waveform if 
you look at it on an oscilloscope.