Like I said previously, I apologize for the double-posts first time here. I felt the one about the mystery circuit board needed it’s own post, and that mixing it up with a plea for advice could make it harder for future seekers to find.
I am stuck on a Micromoog repair, and I’m finally seeking outside advice. I can probably fix two of it’s problems, but I think the one I need help with is beyond my skill. It generally works flawlessly, if a little off from time to time.
- Extreme scaling issues. It will not scale to spec. You can get it close within about one octave, but it won’t make it across two. And across an octave switch setting? Forget it. See below…
- Weak modulation range, but only on SH/Noise. The pitch variation is very narrow. Thinking about changing one op amp and one cap. All other mod as normal (except filter remains on in “off” positions).
- There is a suspicious amount of oscillator audio bleed in bypass mode. I may have a loose ground. I can’t remember if the factory-suggested shielded wire upgrade was done, and I also can’t remember if I had this working when it was last opened up.
I reacapped the PSU and got +/-15 up to spec several years ago. In the last several years, I’ve also had to repair the VCA (it was stuck open), and the filter. I also had to replace an OSC trim pot (I think it was on the Micro, might have been on an MG-1 I did at the same time, I haven’t opened either of them up in like 6 years).
On to the scaling problem:
I got it in 1989. It never scaled close until I started teaching myself how to work on stuff sometime in the 2010’s.
In the mysterious dark ages of the past, I tried to install a Synhouse™ MIDIJACK (cue comedy trombones).
Blame placed squarely on my shoulders and choices here, It went poorly. The less said the better. This was immediately removed again, and all wiring was returned to stock.
During the course of that misadventure, I decided it was a fair idea to remove an undocumented circuit board from the Micromoog and match it to the schematic before installing the Synhouse™ MIDJACK as per their very specific instructions.
At the time I was able to turn up information claiming this was a scaling modification of some kind, but since the Synhouse™ MIDIJACK provides scaling correction of it’s own (if yours is calibrated properly, which, granted mine was not yet), it seemed best to only have one thing doing it. And my information was not documented, only a rumor.
It would turn out that I didn’t have the PSV correct until after the MIDJACK debacle in the first place, so, there’s that.
The pitch remained pretty much right where it always had been: off.
The only issue I can find (before and after the MIDIJACK debacle) is that the oscillator temperature will not quite stabilize where it should. The only thing I could use to calibrate it at the time was a laser IR thermometer, which is probably not the best tool, but it’s what I had.
I chose to stop at that point.
The punchline: The synthesizer was covered in beer and thrown down some stairs in 1985. It’s done surprisingly well, considering. I have gone over the board for hours and hours and hours since 2010 looking for cracks, cleaning things with alcohol, touching up solder joints… nothing.
The only time it ever stopped working was in 1993 or so, when we lost -15v briefly. The little jerk is a tank. “That droid has seen some action”. Only more like C1-10P than RT-D2, if you’re familiar with Star Wars. This Moog probably smokes cigars with my dog when I’m not home - and I don’t even OWN a dog.
It needs to be fixed, and gone.
It’s life story is available upon request.