Yusynth Standards ~ -13V on all negative semitones

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

Hello fellow electro-musicians,

Went to calibrate my Yusynth Standards module and everything was going well until I got to calibrating the negative semitones. Everything works and calibrates as usual except the negative semitones. I get -13V from each of the negative semitone settings on the switch.

I double checked all the part values and triple checked for a short somewhere amongst the trimmers that set the negative semitone voltages but I can not find one. I am gonna check the switches but I don't think that is the problem.

Anybody have any other ideas?

Thanks in advance. You guys are the best.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

I cn be a short-circuit or a broken track, as a matter of fact check that R8 (470) is correctly soldered (one side at the 0V rail) then check the voltage along the trimmer chain T16 to T20 (on the track connecting a trimmer to another).

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

Thanks for the quick help Yves.

Checked R8. Verified that one leg is connected to ground and the other is connected to T16.

Went through the trimmer chain and everything is connected properly on the board so there don't seem to be any broken tracks. Checked for any shorts between adjacent parts in the trimmer chain and couldn't find any.

Checked the voltage down the trimmer chain and there is -12.7V consistently at each trimmer. I started at R7 and went through the trimmers and ended at R8.

I turned the T20 five full rotations and it had no effect on the voltage after the trimmer (at T19). It was still -12.V.

hmm...

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

There is definitely a shortcut somewhere. Did you etch the PCB yourself or did you buy it from Bridechamber ?
Question : are you powering it with +15V/-15V ?
Can you check with an ohm-meter te resistance between the negative power rail (after the 10 ohm resistor) and the connecting points of the T16 to T20 trimmers ?

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

Sorry I haven't gotten back to you sooner. I wanted to separate myself from the project for a few days because I was getting a bit frustrated by it.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

I tried to measure the resistance at the junctions of...
R7 and T20
T20 and T19
T19 and T18
T18 and T17
T17 and T16
T16 and R8
and was unable to get a steady reading at any of the points. I got a couple measurements that started around 160K and linearly climbed up to about 440K after 30 seconds. I measured again and it first got 140K then climbed up to 190K in about 10 seconds and then it jumped up to 440K and then started to decline in resistance.

Bottom line: the resistance slowly increases or decreases over time with occasional "jumps". The resistance was never above 470k and never below 130K.

I have never observed resistance fluctuations like this and I am still totally stumped.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

It's because of C4, it would be better to unsolder this cap before doing the measurements. I should have told that before... sorry about that

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

I should have done that on my own. That makes total sense.

echoer · Joined: Dec 10, 2009 Posts: 40 Location: lawrence, ks

Well I kinda got some measurements together here. I am having a difficult time getting a consistent reading at these points. All the measurements would sort of "float" around the readings I have here. Flotation being +/-.5V or so.
R7 and T20 = 139.4K
T20 and T19 = 140.1K
T19 and T18 = 141.6K
T18 and T17 = 142.4K
T17 and T16 = 143.1K
T16 and R8 = 143.1K
I hope this helps.
Thanks!