Duocast --Oscillation in 27V

[playedincanada]

Greetings!

To be honest, I'm a bit stumped.

I've built this pedal many times and never come across this problem. No sound? Yes. The fix on that was reflowing the solder on a few places. Cold Solder joints seem to be the bane of most pedal builders existence and usually one of the first problems to come up.

So I'm familiar with that. I also, a long time ago broke a potentiometer because I tightened it a bit too much when putting it in the enclosure. Snapped it in half actually.

I encountered this problem a couple of weeks ago, but just got around to trying to troubleshoot it.

The problem is simply this --> The pedal works as it should, the gain trim pot, the low cut, volume for the lower gain side and the volume for the higher gain side. However, if I flick the switch to put the pedal into 27V as opposed to just 9V, the pedal begins to oscillate, very much like how a Devi-Ever Hyperion II might when it's "crazy" is turned up. It's a low octave/signal that bounces around. I mean, it's cool, but I want the pedal to operate as it should.

I have reflowed all of my soldering. Checked the cabling and whether the IC/Chargepump is put in the right way.

The only other forum post I've seen where someone had a similar problem, the builder said they turned down the gain using the trimpots until it stopped squealing. Mine's not squealing, it's doing crazy fuzz/octave stuff. I also remember seeing that someone posted that they had a faulty/broken switch so they switched it out and their problems also went away.

I desoldered the switch, thinking it might be because it's an "amazon special". However, I put in a nice Taiway switch and it didn't make a lick of a difference.

Has anyone encountered this problem as well and found the cause? Or does anyone know enough of the tracing of the circuit to know why it might be doing that and what I should be checking next? I even resoldered the legs on Transformer.

I could test another IC to see if the IC itself is faulty since I used a socket, so that will be my next step.

Thanks in advance.

 

[jwin615]

Just guessing that maybe it's the Germ PNP.
What did you use there?
Also, make sure all you caps are properly rated.

 

[Robert]

What's the wiring look like?

 

[playedincanada]

Just guessing that maybe it's the Germ PNP.
What did you use there?
Also, make sure all you caps are properly rated.

I used a Russian GT320A with hfe at around 70-75ish. It's a germanium PNP transistor, although the legs are a bit off, so you have to put the legs in a very odd, counterintuitive configuration. Maybe it has too much leakage, even though they aren't known for that?

As for the caps, the journey on learning all about that has been an interesting one from the first time I built the circuit. Needless to say, these days I have all my caps separated into separate bags where one bag is for 35V and above specifically.

I'll take a look at those, just in case the wrong one snuck in, but I have a feeling it wasn't any of that.

 

[playedincanada]

What's the wiring look like?

Are you speaking of the second footswitch? I'll check that too, but my brain wouldn't have thought that had anything to do with oscillating when in 27V mode since that footswitch moves things back and forth between the low gain and high gain settings, but I'll go over that as well, just to be sure.

 

[playedincanada]

Alright, I've checked the Caps. The only one that is NOT rated for 35V is C100- 47uf, which is rated for 25V.

I've also checked the wiring for the footswitch which switches it from low to high gain and the wiring seems to be correct on it. I'll reflow the solder on those wires though, just to be sure they're not the culprit.

 

[jwin615]

I think Robert was asking about the in our wiring more specifically...?
Try moving the io wires around and see if it changes the oscillation. May need shielded wire.