VCO 1V/Oct Tracking Problem

Marco Piatti · Joined: Aug 08, 2018 Posts: 7 Location: Argentina

Hi.

I made a VCO with a LM394 (metal can package) and it's respective 3300ppm 1K tempco resistor.
I'm calibrating the 1V/Oct Tracking with my Arturia Keystep's CV output and i can only manage to get just one properly tuned octave before it starts drifting off. By the point i get the second octave right the first octave i trimmed before goes out of tune.

This left me astonished since I expected to get it to track at least 3 or 4 octaves and I'm really clueless at this point. Maybe my LM394 isn't working properly?

Are there any reference voltages i should check around the expoconverter?

Thanks in Advance.

JovianPyx · Posted: Thu Aug 09, 2018 9:19 am Post subject:

Link to schematic?

Marco Piatti · Joined: Aug 08, 2018 Posts: 7 Location: Argentina

http://yusynth.net/Modular/Commun/VCO/new/VCO-V2.gif

JovianPyx · Posted: Thu Aug 09, 2018 1:25 pm Post subject:

I see a note on U7A output that should be 5 volts +/- 2%, so I'd check that. I don't see any other reference voltages. I'd measure it both at the source (U7A pin1) as well as the points where it's used.

I'll guess there are good instructions for adjusting pots T1 and T2 and that you've followed them.

There are notes regarding resistor changes if +/- 12v PSU is used. These are important if you didn't use +/-15volts. Any other PSU arrangement such as +/- 9v won't work without appropriate recalculation of those resistors.

Anywhere that a resistor is marked with a precision such as 1% - that should be strictly followed.

Yusynth is respectable stuff, so I'd trust that the schematic is accurate. But you don't mention the construction technique - if this is a purchased board you populated, have you checked that all the component values are correct? Soldering quality can be an issue. I've fixed more than one thing by reflowing every solder connection. Bad solder connections can add resistance and some cases, if there's dirt, oil or corrosion (even if you can't see it) involved resistance or even a crappy diode can be formed.

Marco Piatti · Joined: Aug 08, 2018 Posts: 7 Location: Argentina

Yeah, I'm 100% sure I've caused the problem somewhere, I've strictly followed every notation and step but there's always a way to jinx it.

It came to my mind just now that my on-board voltage regulators are not exactly outputting the calculated 14.2V reference voltage marked on the site but a bit less (around 13.7V) that made the 5V voltage drop to 4.5, so i changed resistor values to make it 5 again.

Could this also be altering something at R20? Since it's the pull-up resistor LM311 Comparator.

This happens in all 3 VCOs I made, so I don't really think it's a "Situational" build problem like a bad solder point (I wouldn't discard it though).

I'll check if all my resistor values are correct just in case

JovianPyx · Posted: Thu Aug 09, 2018 4:10 pm Post subject:

I don't see how R20 or it's function would be affected.

I agree that it's likely not a situational problem if the problem is copied identically into 3 units.

14.7 volts isn't out of spec for a 15 volt regulator which is probably what the +/- 2% note on the 5 volt reference.

If it won't track several octaves (as it should do) then something else is wrong.

Marco Piatti · Joined: Aug 08, 2018 Posts: 7 Location: Argentina

Hello, sorry for reviving an old thread, I'm not sure if this is breaking the rules.

Its been 2 years since I made my yusynth modular, it's all assembled and most modules work as intended, but I still have this problem with the VCOs and I can't play at all, It's pretty sad so I decided to fight against it again.

While trying to trim the octaves I kind of recognized a pattern, If i trim it so I get A1 at 0V CV, and A2 at 1V CV

0V: A1 | 1V: A2 | 2V: G#3 | 3V: G4.... and so on, looks like every octave after the first one starts to fall by 1 semitone. Naturally I tried to trim so I would get A3 at 2V, but as a consequence at 1V now I had a A#2, and still all the following octaves would fall off by 1 semitone... and it just goes on, if I get A1 and A4 right, A2 and A3 would be too sharp, and A5 would be too flat

the fact that its almost exactly a ±1 semitone jump per octave makes me think something is systematically wrong, but I still can't figure out what is causing it. I've tried replacing my LM394 for a pair of matched 547s I had lying around, thinking it may be a fake or broken IC, but the result is pretty much the same.

Could anyone more experienced tell me some reference voltages around the expo converter that I could check? I don't really know what else to do about this, I've tried everything that came to mind...

Thanks for reading, Marco

Gingoism · Joined: Feb 01, 2017 Posts: 14 Location: Singapore

hi,

personally i have not built this vco, but would like to due to its simplicity. here is my trick to calibrating vcos...

so i always start with 50hz (nice round figure) by setting the coarse pot to about 40hz initially then using the fine tune to bring it up to 50hz..this gives you the 'play' amount in the fine tune for later when you start calibrating.

ok now i start to apply 1V and the freq jumps to (hypothetically) 104hz...i now adjust the v/oct trimpot to read about 108 - 110 roughly then go back to 0V to check how much the freq has deviated from 50hz..for example it has now become 52hz...i now back up my fine tune to 50hz again and restart the process....till i get to 100hz with 1V and move on up from there...i just use a scope to do this

i must add this lacks abit of precision but it gets the job done..and to my ears the vcos track mostly well through most octaves..even with uncompensated components like tempcos..etc..

but you might know all this already...but i hope it helps Wink