[sdiy] Fixing non-resonant 904As

[Paul Schreiber]

I have verified that the Moog 904As were in fact designed to self-oscillate, around A440 on the
'2' range, with a Fixed CV of '0' and a Resonance setting of '8'.

Component aging over 30yrs can cause the filters to stop oscillating.

The fix is to solder a 3K3 1/4W 5% resistor across R11, which is 1K8 (pictures at
www.synthtech.com/moog). This increases loop-gain to cause oscillation. The actual "onset of
resonance" frequency is module-to-module dependent.

Also, the 50K reverse log pots used for Resonance control vary 20% unit-to-unit, so your knob
setting may not be exactly on '8'.

The output waveform will be a fairly nice sinewave of 1V pk-pk. You can check your module's
1V/Oct cal by setting the Fixed CV to about 1.5 and applying *positive* CVs 1V apart.

Paul S.