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VCO Controller problem with VCA Vibrato
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VCO Controller problem with VCA Vibrato
lasselakken
I just assembled a VCO Controller and all seems fine except the VCA Vibrato Depth-knob does affect the vibrato even though nothing is connected to the VCA CV IN socket. When I connect the modulation out from my MIDI-DAC, it doesn't affect the vibrato depth.
I thought it might be the wiring, but I can't se any mistakes (pin1 is the square pads on the PCB right?).
Anywhere special to search for errors?
Synthbuilder
This sounds like the internal VCA is not controlling the LFO signal. So the problem is either in the VCA itself or the VCA control circuit.

Pin 1 on U8 should be 0V - if not there is a problem around U8 pins 1, 2 and 3.

Check Q3 is a BC560.

Check around U8 pins 4, 5 and 6, R52, C18, Q3 and D2 for shorts.

Maybe try another LM13700.

Tony
lasselakken
It turned out the BC560 once again was the culprit (see this thread)
The one I had in, was a BC560 B - changed it to a BC560 C and everything seems fine.
Synthbuilder
I'm glad you got it fixed. However, a BC560B should work just as well as a BC560C in this application. It is likely in this case the BC560B had a fault or was a fake. Indeed, since the COTA build also needed a BC560 replacing, you may have inadvertently bought some fake BC560B transistors. There's a lot of dodgy parts around these days.

Tony
lasselakken
Thanks for clearing that out. I did buy the faulty BC560's in the same batch from one of the large online component-shops I regularly use. I guess I will give them a heads up, since they probably don't know, that they're selling faulty/fake ones.
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