Weird problem with T Escobedo circuit

[nordine]

Hi guys,

this one is driving me mad, hope someone can think of something...

its the Q&D VCF:



from Tim's website
( http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html )

the problem i'm having is the following:

circuit works perfectly on everything, until... suddenly stops working ...this may sound like lots of causes, but i've: double checked soldering work, checked mechanical causes -found nothing- checked IC, covered everything i could think off

the weird part comes when, to "fix" it, i just have to touch the IN with my fingertip, or the Vr point... then comes alive ... i don't have to press it, i just barely contact any of both parts and the circuit starts running

any guesses?

[John Lyons]

Something to do with the biasing and or loosing bias I guess.
Maybe the Vref connection is off somehow...?
Try dedicated biasing between ground and +9v with a couple 1Megs.

John

[Mark Hammer]

What op-amp are you using?

[foxfire]

not to side track this but, what does CV connect to?

[nordine]

thanks for the replies,

John, i suspected too some kind of biasing trouble... resoldered everything, and the trouble seems to have vanished.. ..the whole circuit, is a mish-mash of that Filter circuit plus a sample rate reducer, plus an LFO, so things were TIGHT inside there

Mark, i used a tl072, why?

foxfire, CV is the IN for Voltage sources, such as LFO's, Envelope Generated Voltages, or Expression pedals

[Mark Hammer]

Mark, i used a tl072, why?

So that people who built it successfully and used that opamp could say so, or so that people who built it successfully with a different opamp but not with an TL072 could say so.

[foxfire]

for what it's worth i breadboarded one tonight with a 741 and it seems to work fine. thanks Nordine for the reply. thats what i was assuming it was for but, wanted to double check for good measure.