Overdrive strange sound issue ???

[HOTTUBES]

I have built several GrindCustoms Chimaera overdrive's with great success with no problems etc ...

I now have two of them that have this strange sound as a chord decays , it sounds like a sizzling distortion noise in behind the overdriven tone !!
The pedal fires up and works 100% , sounds great when pushing a dirty or clean amp , quiet running no loud noises , no hum , and quiet switching ...
This is a hard one to narrow down as i have replaced the charge pump chip , op amps , clipping diodes , various caps , pots and still no luck ?

I'm stumped ...



Thanks for any help on this  !!

[anchovie]

Try comparing pin voltages of a good one to a fizzy one - might be an iffy bias voltage somewhere.

[HOTTUBES]

But i have already replaced the op amps more than once ...
I have also swapped out the change pump ic as well.

[HOTTUBES]

I have checked the circuit boards for any solder bridges , i also reflowed every solder joint on the circuit board ...
I take my time loading the boards so i'm positive that the components types and values are correct .
I have  checked for the proper value and operation of all pots .
I removed all IC's and checked for continuity between the sockets and the circuit board solder pads , all check out good .
Verified proper voltage at circuit board and at Op amps .

[Quackzed]

these type issues are tough to track down, as there are alot of suspects... even the guitar or cables could be in the lineup... power source? or batteries to eliminate wall crap...
could try to audio probe the second opamp output... see if its before or after that point... then depending on results test before the first or 3rd.. etc... think like sherlock holmes...

when you have eliminated the impossible, whatever remains, however improbable, must be the truth... S.H.

[Transmogrifox]

The symptoms sound like an improper bias or really bad capacitor leakage current somewhere.

I would be particularly interested to know that op amp 1,2,3 and 5,6,7 are very nearly the same (like within 1V or so).   Pins 1 and 7 of each op amp will be way out there if bias is a problem.

You should see "Vcc" at 4.5V to ground, "-9V" at the obvious level, and "+18V" at the obvious value.   If any of those are very far from what is expected, then you will have something to look at.  I know you said you checked voltages, but I thought I would state the obvious since it is usually the obvious things that I overlook.

I see some opportunities for mischief around R17, R13&R14 since these are not DC bypassed.  At first glance it all looks ok, since it seems like everything is neatly DC referenced in one valid form or another to "Vcc".

What I fear is the opportunity for impedances not anticipated at a schematic level working together to make things unstable at some frequency that is not audible, but the instable condition makes it sound bad.  For example, what is the self-resonant frequency of one capacitor vs another?  Even though a TL072 has a GBP of <10 MHz doesn't mean it doesn't have any gain at 30 MHz -- a typical self-resonant-frequency of many high value electrolytics.  Probably a lot of mischief possible at less than 5 MHz too.

If this was actually the case, you would be able to see this with an oscilloscope if you have access to one.  If not, then try some other op amps (like even a BJT type -- something totally different) to see if you get different results.  

It sounds like you need to take a DMM and make sure all the steady-state DC bias stuff is nicely balanced between +18 and -9 (+4.5V from ground).

If all of that is good, I can't imagine anything else other than bad leaky capacitor or an unstable circuit.  

Unstable circuit will be a result of things that aren't readily visible -- even DC levels will appear good.  That seems a possibility when you say you are sure proper voltage at circuit board and op amps.

Sometimes you can see DC levels that seem reasonable, but slightly different than normal/nominal.  This can indicate an RF instability that averages out to something slightly more or less than normal.

After some of the "Sherlock Holmes" work, you need an oscilloscope and signal generator to see what's happening when you hit it with dynamic input signals and watch as signal levels decay.  A scope would be so nice in these kinds of instances.