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COTA not filtering
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author COTA not filtering
lasselakken
Hi
I just assembled a COTA filter, but it doesn't filter the sound - the frequency knob does not have any effect - just unfiltered sound passing through. The Feedback knob doesn't work right either - it works like a volume control; loud when fully counter clockwise and no sound when fully clockwise.

I checked the wiring, and that part seems to be right. I also swapped the lm3700's and the lf412's with no effect.

Anywhere special to look for faults? hmmm.....
Synthbuilder
It does sound like it could be a wiring fault. Especially that thing with the resonance control. You would expect the resonance to decrease volume when turned up but never to the extent of completely silencing it.

So I'd check the wiring again particularly the FBK_IN connection. Also check that Q3 and Q4 are correct.

Tony
lasselakken
I checked the wiring again and everything seems fine (triple checked around Feedback input). Q3 and Q4 are BC560 and BC550 as they should be.
The feedback control does work quite like a volume control, but it doesn't completely silence the sound when fully clockwise - just noticeably quieter than fully counter clockwise. The frequency knob doesn't affect the sound at all...

One thing confused me a bit, though. It says in the builders guide, that the NC lugs of all six input sockets should be connected to module ground. The six inputs are input 1/2/3, 1v/oct, CV1, and CV2, right?

By the way - I'm building the 2u full format version (forgot to mention that at first, but it might be obvious from the other information).
Synthbuilder
lasselakken wrote:
The six inputs are input 1/2/3, 1v/oct, CV1, and CV2, right?


Yes that's right.

I think perhaps your filter is working but the frequency control part may not be. That is, it's filtering but at a really high frequency. This would suggest a problem with either of those transistors, Q3 and Q4, or the circuitry around U1. If this is true I would expect the BP output to be very quiet and 'fizzy' sounding.

Check the voltage on pin 7 of U1 varies smoothly as you move the frequency control pot. If not, you have a fault around U1.

Tony
lasselakken
Your assumption seems to be correct - the BP out is indeed quiet and "fizzy" sounding.

The voltage on pin 7 of U1 varies smoothly when I tweak the frequency pot - from 4,6V (ccw) to -2V (cw). Is that what it is expected to do?

I guess the next thing would be to change Q3 and Q4, but I haven't got any at hand, so I will have to order them.
Synthbuilder
lasselakken wrote:
The voltage on pin 7 of U1 varies smoothly when I tweak the frequency pot - from 4,6V (ccw) to -2V (cw). Is that what it is expected to do?


That sounds about right. The actual value will also depend on the TUNE and V/OCT trimmers.

Maybe also look at the temp co resistor. Measure the resistance across R33. If it's around 1K then all is well. Check R32 is 33K and not something like 33R.

Tony
lasselakken
Synthbuilder wrote:
Maybe also look at the temp co resistor. Measure the resistance across R33. If it's around 1K then all is well. Check R32 is 33K and not something like 33R.

Resistance across R33 is very close to 1K and R32 is 33K as it should be... I guess I will have to wait for the BC550/560's arrive to check those.

Tony, your help is simply brilliant! I am so impressed by your patience and kindness.
lasselakken
It was Q3 (BC560)! I just got back from vacation and changed Q4 first... no luck. Then I changed Q3 and suddenly everything reacted as expected It's peanut butter jelly time!
Once again: thank you for superb designs and even better online help, Tony - simply awesome!
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