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Bananalogue VCS Fall Problem
MUFF WIGGLER Forum Index -> Fractional Rack Modules  
Author Bananalogue VCS Fall Problem
Hermetech Mastering
Finally got some patch leads, so got my mini system up and running after about three years of collecting the various modules. All seems well except for my VCS. It's the Serge/Banalogue/CGS frac version. I know they are notoriously difficult to troubleshoot, and have a bit of a rep for being "difficult", but I am wondering if anyone might have any ideas.

The Rise is fine and works as expected, but when you twiddle the Fall knob, (or it's corresponding VC knob), it emits a seemingly random stream of voltages, which do sound very cool, but are definitely not doing what it is supposed to do. Could it be a dodgy chip that would be an easy replacement? Any other ideas? Should I post the same query in Synth DIY?
Hermetech Mastering wrote:
...(or it's corresponding VC knob)...

Is there anything plugged into the VC Fall Input when that knob does the funny things?
Hermetech Mastering
No, only if you pull it, haha. hihi

The knob doesn't do anything unless you have something plugged into the inputs, EXCEPT when you pull it for non-linear, then it does affect the sound. This is how my first VCS worked too. So as far as that goes, seems to be normal.

Also notice the frequency becoming quite unstable when you twiddle the Fall (or Fall VC pulled out), when at LFO speeds. At audio speeds it's obvious as pitch warbling.

The Rise works as expected.
Gotcha! I forgot that it has switch pots.

Assuming it is just a single CGS75 board behind the panel, or based off that schematic, there should be a single TL074. If it is socketed & you have access to a spare, try swapping it out. Hopefully one of the opamps in the chip is busted and that it's nothing more.
Hermetech Mastering
Cheers dude, I'll order another TL074. As far as I know they are all socketed.
It's quite possible this is down to component values and the circuit itself, the Serge VCVS topology is notorious for instability, unfortunately. It might end up needing someone to look at it and tweak it better.
Hermetech Mastering
Thanks dude. Yep, read Tim's article on it so I know it can finnickity. Fingers crossed the chip replacement will sort it. Do you know the best place in the EU to order such things slow_riot? If it doesn't fix it, I may have to send it to you. Miley Cyrus Seriously considering the new Wiard Controller BTW!
I would have guessed a dodgy pot. The quad opamp also serves the rise input, which you say is ok:

Is there a point at which it is stable/ doesn't emit noisy CV? For example, can you change the decay time by CV-ing the 'All CV' or 1V/O input? If so, that might rule out one of the trannies being blown.

I have reverse-plugged my Bananalogue VCS twice over the years; the only chip that needed changing was the LM3900 Norton Amp. I haven't found this circuit to be difficult and the only instability is a mismatch between the peak threshold one has to set on the trimmer to get the circuit to cycle. This is different for each LM3900 you use.

In short, I'd wager it's the potentiometer.
That's what I would have originally thought (a pot), but it sounds like 2 pots would have gone bad.
Hermetech Mastering
Thanks for all the ideas, will update thread when I have found a fix!
Hermetech Mastering
I currently have this up for sale for £50 if anyone is interested, still has the fault:
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