[sdiy] Minimoog tuning questions

[Eric Frampton]

Hi all -

I’ve had another Mini come across my path. This one’s S/N is in the late 4000’s. It came to me with a bunch of problems, including some well-intentioned but (ultimately) incorrect mods done by previous techs that I’ve been going through and fixing/un-doing. I’m down to, I think, the last problem, but it’s a biggie: all three oscillators, at the center of their Range pots, to want to speak roughly an octave too low.

This is the older oscillator board, not the newer one.

I’ve checked the PSU, got a solid +/- 10.0V at the osc. board
Cleaned all edge connectors
Checked the output of the keyboard, and it starts at 0V on the low F and works up by roughly 1V/oct from there, as it should
All 3 oscs scale correctly at any octave setting (it’s the ranges that are off)
The octave switch transposes correctly with all 3 oscs (they’re still all an octave too low, but switching the footage makes a 1 octave jump as it should)
This one does not have the deadband mod on the pitch wheel, but I’ve double checked and the pot measures 15K to ground at center detent
This one does have the buffer board installed
This one does have the R/C network installed on each osc.
One of the mods the previous tech had done was to install sockets for all the 741’s. Before removing all the sockets, I tried swapping 741’s around to see it made any difference; no change.
One SG3821 is socketed; I tried swapping with a 3046, same kind of problem

I’m seeing 5V on pin 6 of IC1, IC4, and IC6.

Fortunately I have a working Mini of my own to compare against (s/n in the 1700’s, also with the older board). However, mine -does- have the deadband mod but -not- the R/C network, which seems to be making some of the voltages - notably, the outputs of IC3, IC5, and IC10 - not match between good and bad boards at all.

Putting the bad board in mine, it speaks roughly an octave low. Putting my good board in the bad machine, it sounds at roughly at the correct octave. So I think it’s safe to say the problem is on the board itself.

Since all three oscs are low, I know I could just offset the pitch wheel until I got in the ballpark then calibrate from there, but that seems like cheating - I’d really like to figure out what’s going on. I’ve tried using the techniques you all taught me on the last go-around with another mini, but apparently my current-fu is not good yet.

Help?

e