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Yusynth Minimoog VCF - no frequency control
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Yusynth Minimoog VCF - no frequency control
I have just built a Yusynth Minimoog VCF but for some reason I don't seem to have any frequency control - either via pot or CV. Changing the range trimmer has no effect either. It sounds like it is just stuck full open. Resonance appears to work okay and the sound is in the right ball park.

I can watch the voltage change at the base of the BC557 as I turn the frequency pot or as a result of modulation via a CV input but this seems to have no effect on the voltage at the emitter. Am I making an incorrect assumption that it should?

All resistors in the frequency control area appear to measure up as expected, caps are the right value and the polarized ones are orientated correctly. The BOM just lists BC557, however the PCB highlights that this should be a BC557C and I am using a B (and have already swapped it out with no change in case it was just a dud) as that is all I have. Could this be cause of my problems?

Any assistance appreciated.
I had a 2N3904C witch had a different pinout as the 2N3904 (C-less).
Maybe this is the case with the BC557 also?
Is your pot wired correctly?
I built one for my euro system, did not match or measure anything and it still works like a charm...
I think you will need the C variant of the BC557 as it has higher gain.
I am pretty sure the pot is wired correctly and the pin outs are right - although I can't seem to work out what make the Bs I have are and hence check a data sheet.

I have some Cs on order so I'll swap one in when they arrive.

The 557 I used in mine is the A variant and it works just fine... But now I wonder how much better it would be if I had I paid attention and got the C variants d'oh!
Mine is built by the specs.

It doesnt have the creamyness of the real thing but it is super warm.

I love it for filtering external audio (music, drumtracks). It has that warm French discoh-house/daft punk classic filter sound.

I matched my trannies to 1hfe accurate.
I swapped in a BC557C and it has had no effect. It sounds lovely as is, but without frequency control it is just teasing me!

Anywhere else I should be looking?
What about swapping the CA3046?
I had a similar issue with Freq control. It turned out to be a ground issue. I would give that a try. Ground everything to a solid groundplane.
Thanks for all the input. In the end I think I had 2 problems. Firstly my bodge to make a trimmer fit the range trimmer footprint seemed to be causing issues - it seemed to behave as expected when off the board so I'm not sure why this was. Replacing it with 2 x 5K resistors meant I saw some modulation at pin 12 on U1 so that is what I have at the moment. I can't seem to find the right trimmers to fit this footprint so if anyone has a part number I'd be interested to know.

The second issue was that one of the CA3046s I used doesn't seem to react to frequency control. Having tried a number of different CA3046s I discovered that when I use this particular CA3046 in U1 I cannot change the cutoff frequency. However with it in U2 and any other CA3046 in U1 the filter does react to frequency modulation and sounds fine. Or at least I don't notice a difference in the sound when compared with a set up using CA3046s that do respond to frequency control in both U1 and U2. That said I only have one board so can't side by side compare them. I am assuming the NPN at across pins 12, 13 and 14 isn't behaving but since it is unused when in U2 this is why it sounds fine with it in there.

Anyway it seems to be working I just need some trimmers with the correct footprint to enable me to calibrate it properly.

I have the same particular issue like you described.

So the problem were faulty CA3046?
Yes, swapping out the CA3046 in U1 fixed the problem for me.
robot909 wrote:
I had a similar issue with Freq control. It turned out to be a ground issue. I would give that a try. Ground everything to a solid groundplane.

hi. i'm having the same issue with the freq pot (the filter doesn't respond when i move it) can you describe how did you resolved it?
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