[sdiy] Moog 904A "issues"

[Mike Granger]

Paul,

My 904 goes to the brink of self oscillation at the maximum setting of 
the regen knob, but it will not oscillate. I believe that it was 
designed that way, thus the need for selecting that resistor to trim the 
regen control so that it goes almost all the way, but not into 
oscillation. The minimoog has a trim control to do the same function.

Paul Schreiber wrote:

>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
>
>