1v per oct keyboard is....microtonal and droopy? FIXED?

Europanaut · Last edited by Europanaut on Mon Mar 02, 2015 3:58 pm; edited 2 times in total

Certainly not a desirable situation. Well not for me at least.

So it's the Single Buss 1V/Octave Keyboard project. Gate works. Trig works. The keyboard's 100ohm resistor array checks out on the meter. I've tried calibrating the circuit, but it doesn't seem to effect much at this point.

The problem is this. It takes 2.5 octaves to make a minor third. This is somewhat limiting. lol.

Any suggestions on where in the circuit that I should point my diagnostic attention? I've built around 25 MFOS modules, and they all work great, usually first try. But this one is being cranky.

Thanks!

Europanaut · Posted: Sat Feb 28, 2015 9:05 am Post subject:

Gonna dig into this issue tonight. I'll trace the circuit backwards from the keyboard.

No last-minute ideas from the forum?

Europanaut · Posted: Sun Mar 01, 2015 1:03 am Post subject:

So, I have no idea why, but the microtonal issue just sort of "went away". And didn't come back all night. So who knows what that was about.

We then replaced R4 with a 27k, and the calibration locked right in. Sounds great.

Except.....the dreaded droop. And the glide doesn't work. Not sure if the two problems are related. EDIT: Glide works. Don't ask.

So, MASSIVE long droops. That's the repair priority. I'm hosting a modular synth meet next weekend, and I'm REALLY hoping to have this fixed by then.

I've found a bunch of references to droops on this forum, but there doesn't seem to be a definitive opinion on the source of the issue. I've read that the 444 and the big poly cap are important to the sample and hold circuit that is supposed to correct the droop, but I don't have replacement caps of that type. I do have a couple of spare 444's, but I'd like to mine the collective experience here on the forum before I start swapping stuff out. A bit of prep can save hours.

Oh, the other thing that we noticed is, the pitch stays solid, no droop, when the synth is first powered up. It's rock solid for about 30 seconds, then starts getting progressively droopier.

I just swapped out the 444. No change.

Running +- 15v.

So any ideas?

PHOBoS · Posted: Mon Mar 02, 2015 7:41 am Post subject:

I don't have any experience with this particular circuit and I doubt I can help
but what kind of capacitor are you currently using for the S&H section ?

Europanaut · Posted: Mon Mar 02, 2015 9:35 am Post subject:

PHOBoS · Posted: Mon Mar 02, 2015 10:28 am Post subject:

hmm Polystyrene isn't bad for this purpose, unless there is something actually wrong with the cap. At least it's not a ceramic capacitor.
The S&H section is pretty straight forward so if it's not the cap it might have something to do with with the FET Q3 (or maybe Q2).

Europanaut · Posted: Mon Mar 02, 2015 3:34 pm Post subject:

So here's a weird one. I tried a bunch of different .01uf caps as swaps for C19. No change.

Then, for no reason in particular other than it was on the bench, I tried a .22 film cap. Worked perfectly. No droop. Well, after about 30 seconds or so there's a VERY slow drop off, which from what I understand, is expected.

So I'm happy, and have a perfectly functional single-bus keyboard controller.

But....wtf?

PHOBoS · Posted: Mon Mar 02, 2015 4:28 pm Post subject:

That tells me that somehow the cap gets discharged too fast and there are only 2 things attached to it:
The FET Q3 and the opamp U3c. You mentioned that you already replaced the opamp without any change.
You could check the PCB around the FET for any solder bridges (I doubt that would cause just a bit of a droop though)
and measure if the resistor values (R17/R14) are correct. Or try replacing Q3.

It's nice that you at least got it working now but something is still wrong, unless the caps you tested with were all faulty.

Europanaut · Posted: Mon Mar 02, 2015 4:46 pm Post subject: