Simulcast Oscillation Squeal

[farwest1]

EDIT: SOLVED! It was a faulty Russian Ge transistor. I swapped it out for an NTE 2N404 Ge transistor from Digi-Key, and it works great. When Small Bear reopens, I may try a few other transistors in the socket.

ORIGINAL POST:

I built a Simulcast that will be incredible.....if I can get it working. It has a terrible squealing oscillation problem.

Above 10 O'Clock on the Gain Cut knob, I get a brutal whine/squeal, like feedback. When I play it goes away, but comes back as soon as the signal fades. I've tried different power sources. The pedal is not in a chain. It's direct into my amp.

Here are my theories:
1. Faulty Ge transistor (Russian Ge P416B flat hat.) I am going to order a different Ge type, probably a Rangemaster from the 1970s.

2. Wiring. I adjusted the internal wires to no effect. I may try shielded wire for input/output.

3. Component Tightness. In order to fit the caps and resistors on this board I had to really wedge them in. They won't fit directly above their slots. None of the bare wires or solder joints are touching (I've checked them all), but the bodies of the caps/Rs are touching. Could this cause problems?

4. Wrong Resistor or Cap value. Maybe I installed a wrong resistor somewhere? Would that cause oscillation? I can check them, but I'm really bad at identifying resistors by striping.

5. Blobbed solder. One or two of my joints have "blobs" of solder on them, instead of smooth. Not sure if that could cause an issue. If nothing else works, I'll pull the board out and re-solder some joints.

I'd be so appreciative of any help or ideas! FWIW, several other people on this forum or Reddit have had the same oscillation issue with the Simulcast pcb. It's not clear how they resolved it. And sorry I keep posting about this pedal

 

[Robert]

I've seen a couple reports of oscillation with the Simulcast, but I don't believe it's a widespread issue. There have been far more successful build reports, and I believe most of the builds with issues have been resolved.

First, it's not the most likely culprit, but there have been several cases of oscillations recently (with other PCBs) caused by the charge pump IC. Based on your description, your issue sounds more like a signal feedback issue, but keep it in mind if all else fails.

The components physically touching should not be an issue. The only concern I would have there is the bent over leads of those 1/2W and 1W resistors that might be laying across traces underneath them.

Going by your other thread, am I correct that the oscillation issue didn't occur until you swapped the Ge transistor? Although this could be simply because there wasn't enough gain to cause an oscillation, I also would not rule this out. I've had many Fuzz Face builds squeal like a pig because of an off-spec transistor.

I looked over your resistor values, and unless I'm mistaken I didn't see any errors there.

 

[farwest1]

Thanks! Yes, all of the build reports are great and it seems like a fantastic PCB. I just read two reports of oscillation. One (on Reddit) was apparently fixed by replacing a faulty Ge transistor.

My earlier issue was because I had the pin out wrong on the Russian ge. So I got some signal but it was very thin. Once I got the pin out correct, the volume jumped but so did the oscillation. (Can’t figure out how to delete that post, since it’s resolved.)

By the way, you are awesome

 

[Ratimus]

Does the squealing go away if you put a jumper from pin 1 to pin 8 of the ic?

 

[farwest1]

I solved this one! As I suspected it was a bad Russian Ge Transformer. I got a batch of new ones from NTE and it now works great. It’s a huge sounding pedal, in a kind of old school dirt way. I don’t yet know how I’ll use it.

 

[Ratimus]

Sweet! Glad you got it working. I hope to build one myself pretty soon.