[SOLVED] YuSynth ARP VCF not filtering

mrmarbury · Joined: Jul 31, 2011 Posts: 5 Location: Germany

Hi,

I've just finished my first yusynth module -> the ARP VCF. But even though I checked a dozen times that everything is correct I have the following issues:

The sound volume is fairly low (like half of what's "normal" for all my other modules)
The frequency knob does not filter but it's raising and lowering the volume
The resonance knob does the same as the freq knob and it's "hissing" when turning it from 8 to 10. I checked with another pot but it's the same there as well. It doen't look like there is a short somewhere.

All other jacks/knobs are working as expected The filtering is the (only Rolling Eyes

) problem.

Some more info: I've built the 2N3906 version and I have it connected to the +/-12v of my Doepfer rack

Has anybody had the same issue and can give me some advice on this?
Do I need to change some parts to have it work with +/-12v?

Thx so much ... Smile

Cheers,
Stefan

mrmarbury · Joined: Jul 31, 2011 Posts: 5 Location: Germany

One dumb question because I'm not an electrical engineer Wink

Which part is doing the actual filtering? Maybe it is broken and I can check that by measuring some voltages?

-Stefan

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

Verify that the 2N3906 are correctly oriented on the PCB. Which PCB are you using ? Did you etch it yourself ?

Check also that your LM3900 is correctly oriented, try another one (the old one could be faulty).

Do you have an oscilloscope ?

mrmarbury · Joined: Jul 31, 2011 Posts: 5 Location: Germany

Hi,

yup I've created the board myself. It is not mirrored or anything Wink

I more like ready made PCBs but the USA is far away so I have to help myself Sad

Regarding the LM3900N I will get a new one tomorrow and check. As well as for the trani's. I want to measure them as well.
The indentation of the LM on the board is left (in direction of the 100n capacitor) and the indentation of the TL is down (in direction of the 47k trim pot).

Yes I've got an Oscilloscope and turning the freq/resonance pots only reduces the amplitude but the waveform is not changing in the way it should regarding harmonics and resonance. Or what do you want me to check check?

I will do some more checks after changing the LM.

Cheers,
Stefan

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

OK check that thread : http://electro-music.com/forum/topic-25093.html , there you will find some examples of what you must find with an oscilloscope and voltages at key points of the filtering circuit.

mrmarbury · Joined: Jul 31, 2011 Posts: 5 Location: Germany

Thx for the link. I already clicked on it earlier but since it looked like a link revolving around the 2SA798 only I skipped ^^

Will check tonight. Hopefully I'll find some time. There's definitely too much other stuff going on at the moment. ^^

Cheers,
Stefan

mrmarbury · Joined: Jul 31, 2011 Posts: 5 Location: Germany

ITS WORKING!!! Very Happy

The error was not the LM. I checked all tranis again and in the box of 2N3906 I've got was a 2N3904 that is not even working ^^ ... replaced it and measured all other tranis ... and voila Very Happy

Have a bit of tuning to do but even now it sounds gr8! thumbs up Mr YuSynth Wink

Thx alot,
Stefan

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

Great !