[sdiy] Minimoog tuning woes, Op-amp swaps?

[Eric Frampton]

Hi Tony and Tom and list -

Tony wrote:

>> the tuning between high A and lowest A requires a different scaling than, say, the middle two C's.
> 
> OK, this is the first problem you need to solve before doing anything else. It could be a keyboard problem or that, somehow, all three of the VCO expo circuits have been affected by the new resistors. To find out which you need to check the keyboard voltage. Hook up a DVM to the keyCV signal that goes to the VCOs from the glide circuit. As already stated it's not going to be 1V/octave, but you should still get a linear change across the keyboard.

I do get 1.02V +/- .005V or so across octaves. Also went down the whole keyboard resistor chain and they're all still matching at 10.3 Ohms.

> It's worth pulling off the keyboard connector and cleaning both the plug and socket with some De-Oxit D5 and then wash out with IPA.

Did that. No change.

> If the keyboard is producing equal volt jumps for any octave played then you need to now look at the VCO board. Was it playing more or less in tune before you changed the resistors?

The new resistors made no difference.

> If so, suspect that you have perhaps a poor connection on the gold contact strips on the VCO board. It's worth cleaning the board side contacts with IPA - or maybe a wipe down with a bit of D5 and then wash off with IPA.

Did that as well, no change.

Tom wrote:

> A long while back I got a response on this list on how to rescale the
> keyboard for 1v/oct and I happen to still have it:

I had a copy of that same email from Michael Caloroso back from in the day, and I had actually installed that trimmer in my machine some time ago. Thinking it might be part of the problem, I just removed it today. No change.

I'm not sure what else to do until I get the buffer board installed in there. Waiting on parts from Mouser.

Perhaps I just need to lower my standards for tuning accuracy. :-(

--

Oh, and the general consensus was indeed to not try to improve on the 741's. So I removed the sockets and soldered them to the board, which did help out with some random pitch jumpiness I was getting.

e