MOTM

Another possibility is a cold solder joint somewhere in the control voltage
circuitry, including the Rate input.  This will manifest itself as a high
impedance.  After building dozens of modules I finally found a cold solder
joint in one of my VCAs.  Exponential mode worked, Linear mode didn't.  It
did not take long to solve once I started probing with a meter.

Since you have two 320s, the easiest methodology is to put them side by side
with the same control settings.  Power them both on, then go through the
schematic from the rate pot forward and compare the DC voltages at each node
with a multimeter.  If you find a large difference you will have a clue to
the problem.

A problem with the timing cap seems less likely to me.

John Loffink
The Microtonal Synthesis Web Site
http://www.microtonal-synthesis.com
The Wavemakers Synthesizer Web Site
http://www.wavemakers-synth.com