[sdiy] Moog 904A "issues"

[Paul Schreiber]

I am poking around the insides of a couple of Moog 904A LPF modules. If you have one (or more),
please go to:

www.synthtech.com/moog

and look at the 2 pictures.

According to the Moog service manual, there is a "Selected Resistor for Regeneration" (i.e..
self-oscillation) that is "approx. 2.2K". In both these examples (there were 2 different pcb
layouts of the 904A) this resistor is 1.8K.

What I am seeing in these 2 filters is that *neither one* self-oscillates! I can modify the value
to get it to oscillate, but find it "strange" that the filters were shipped out (about 3 years
apart) with this behavior.

Now, for part #2:

Once I get them oscillating, the "onset of oscillation" (when Resonance is on 9) on the '2' range
is about 425Hz. That is to say, the module *won't* self-oscillate until you set the Fixed CV to
about '0', and then the frequency out is 425Hz. For all 3 ranges, the Fixed CV *must* be 0 to +6V
settings. Past the '0' tick, the frequency goes up 1V/Oct as expected.

So, what I'm wondering is:

a) do all the 904As have 1.8K resistors?
b) does your 904A self-oscillate at all?
c) if it does, is the Fixed CV knob have to be on 0 or above (i.e. does it stop for settings
below '0')?

Paul S.
yes I work on Moog modulars