Moog Music Forum

I have a MiniMoog D with the early osc board. I am able to calibrate OSC 1 and 2 with no issue following the Moog service manual for older osc boards.
HOWEVER, Osc 3 will not reach 3520 Hz (the high A on the 2' setting) using the range trimpot. AND, even though it's not in tune, as long as it is on the 2' setting the notes sound "normal". If I select anything else (4', 8', 16', 32') while holding down the same key that sounded OK at 2', it goes into a wierd "ring modulator" kind of sound with noise like more than 1 oscillator is firing out of tune (clangorous). As I play a scale the sound changes from sort of "normal" to ring modulator to 2 beating oscillators. Again, it sounds fine at 2' (just not in tune with the other osc's because of the range trim limit).
I have calibrated the voltages, cleaned the osc board, cleaned the contacts, calibrated osc 1 and 2. Nothing helps.
Any help would be appreciated.
Thanks.

Not sure this will help, but depending on the manual, it can have errors:

http://www.synthfool.com/docs/Moog/mini ... y_vcos.jpg

I know this is probably obvious but... Is the Oscillator Modulation switch to "off", and is the modulation wheel at minimum ?

Also, is the weird behavior of osc 3 the same with all waveforms ?

thealien666 wrote:I know this is probably obvious but... Is the Oscillator Modulation switch to "off", and is the modulation wheel at minimum ?

Also, is the weird behavior of osc 3 the same with all waveforms ?

Yes. Mod switch is off. Mod wheel at min. When I set Osc 3 up to modulate Osc1 or 2, it works as it should. I can use the mod wheel to add vibrato to osc 1 and 2. Only when I'm using it in the "audio range" does it do the weird things (except at the 2' setting, where I can play every note up and down the keyboard and they sound normal). Switch to 4' and "ring mod" sound.
Yes it does it with all waveforms.
Thanks.

I would suspect that maybe one of the 741 op amps for Osc 3 might be starting to fail ? And that could maybe cause it to "feedback" or "oscillate", or something like that, and introduce rapid CV variations at certain voltage/frequencies ?

I'm not an expert, mind you, so TIFWIW.

Maybe someone else, with lots more experience than me, has a lead on this ?

Might be a transistor array (3046 or SG3821.)
These parts are often shared by oscillators, so bleed-thru could be possible.
I really don't know.
If he sent the board to me, I'd probably replace them all if I couldn't immediately find the problem.

Check to see if your oscillator board has these installed

http://www.synthfool.com/docs/Moog/mini ... lay_lines/

I put a short video up on youtube showing the problem. If anyone wants to take a look and see (hear) if you have any suggestions I would appreciate it.
thanks,
Charles
http://www.youtube.com/watch?v=lc9BsH6P ... e=youtu.be

Had a look at that video (although the sound level is rather weak, I could still hear the behavior).

First, the bleed thru of the mod wheel is normal (maybe a tad higher than normal) but it is common to Minimoog D. It is caused by the fact that the mod wheel is actually reducing the amount of 100% level modulation when moving it down. And that the Allen-Bradley potentiometer for that mod wheel still has some resistance at minimum position instead of shorting out (effectively removing all modulation). Just make sure that the pot is not offset by the plastic wheel mechanical stopper and really moves to its absolute minimum position when fully down. There will still be a very faint residual amount of modulation, but usually not as much as in your video.

Now to the more important issue. From what I can hear, Osc 3 is still tracking the keyboard although noisily, and since you stated that it does this with all waveforms it is clearly not an oscillator section problem but rather a pitch CV problem for Osc 3 only. I would first try to reseat the CA3046 for Osc 3 (IC7) in its socket. If that doesn't fix it, than I would replace that same CA3046 with a new one. If that still doesn't fix the issue, I would then replace IC6, IC8, and IC10 with new ones (all 741 op amps used for Osc 3 pitch CV).