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2720 noisy VCA problem

Posted: Wed Jan 29, 2014 8:30 pm
by patrickvf
I finally got around replacing some modules in my working 2720 with those from the other one (the modules with the brown jacks). While the hum is mostly gone, and the signal is nice and strong (through the BPF at least), the VCA has some serious issues. There is a lot of noise and when patched to one of the LFOs it makes a kind of "thump-pa-thump" sound with nothing routed through it, and even then it seems to let only pass a signal at high CV levels and it's still distorted. While I understand that the cause could be failing components and bad solder joints, I'd first like to hear some opinions/suggestions before I start doctoring around on it. One thing that I noticed is that if I push capacitor C2 towards the op-amp, the noise increases and wise versa. Also, the noise level drops significantly (but doesn't disappear) when I physically touch the capacitor. There are also some very old style transistors on this thing (not the can ones but ones that look like cut in half. Any suggestions for a specific cause and quick fix of this problem? Which of the components would be most likely at fault?

Re: 2720 noisy VCA problem

Posted: Thu Jan 30, 2014 1:05 pm
by PAiA-Scott
Go through the procedure for testing and setting the trim, but also note the explanation about summing input control so there isn't too much which could cause distortion.

For the transistors, this topic covers the pin-outs for the various types we have included with the kits, matching them with the E(mitter), B(ase) and C(ollector) pcb points according to the silk-screened outline graphics.

Look for joints that didn't flow or may have bridged, verify all parts are installed as specified and confirm the +9 and +18 DC power supply voltages.

Re: 2720 noisy VCA problem

Posted: Thu Jan 30, 2014 4:08 pm
by Billb3
I posted an updated schematic a few posts down which might help a bit