High Gain Oscillation

[Barracuda]

Obviously this is a common problem, and usually I'd be able to figure it out pretty quick. But I've had this problem in a couple of builds, although I think the one before this was due to untidy wiring. I have 100uf caps filtering the power supply, my concern is with the input/output, I think this is where the problem lies. Because even with no jack in the input I get high-pitched oscillation. There are caps from base-collector to filter out high frequencies, which makes me think somehow the output is getting fedback to the input. What confuses me most is that this would probably be more apparent on a breadboard, and when breadboarded I have no issues at all. This is also my first build to feature a AC transformer, but I don't think thats causing issues iether, because again, on breadboard it was fine.

Are there any tips or pieces of advice you could give me to try?

[Plexi]

I have that ghost feedback many times on many builds.
The problem beggans when I place the board/jacks/pots on the enclosure.
Take a moment and check all the hardware and the board. Sometimes a small shortcut causes the trouble.
Is good to use jacks with 'cut' at the input: so the circuit will close (input to ground) when you unplug the input jack.

[MrStab]

Is good to use jacks with 'cut' at the input: so the circuit will close (input to ground) when you unplug the input jack.

+1. does the oscillation change at all if you have a guitar plugged in, with the guitar volume pot turned down? what if you use a buffered pedal before this one?

other possible solutions:

• Reposition and shorten wires, keep an eye on what's coming into close proximity on the board itself
• Shield high-impedance wires, such as the input (RCA cable is good for this on the fly)
• Place a buffer close to the input (if you're willing to sacrifice True Bypass)
• Kill as much highs & RF as you can afford (via. a 33k/100p LPF on the input for RF, and small ceramic caps in feedback loops - maybe try 7KHz or so first)
• Get it in the enclosure and try it in a different area

[anotherjim]

Bypass switch wiring can make a huge difference. 2PDT or 3PDT?
Apart from costing more, 3PDT offers several advantages.
1 : Using the outer switches for input and output with the middle switch for the active LED greatly reduces capacitive coupling between in and out compared to 2PDT.
2: Easy to arrange grounding of the input to the circuit when bypassed.
3: 3rd pole gives you direct LED switching.
I would expect most people would assume point 3 is the main reason to use 3PDT.

Try to make sure as many of the pots/switches/jacks have metal bodywork grounded. Watch out for accidental insulation from the enclosure by paint.

If pots for high gain controls or LFO control are wired, twist the wires together as tight as you can as far as you can.

For high gain circuits, use screened cable for the jack/bypass/pcb links. If you have opted for 2PDT bypass, this is probably essential. The screening only needs grounding at one end & don't be tempted to use the screen for 0v connections. Thin gauge lapped screen cable is fine in the box. Old hi-fi rca phono hook up cables are a good source if you're thrifty or in a hurry.

[LiLFX]

Could be a lot of things. Would need to see a schematic, the PCB layout, and pics of the build. If it's good on the breadboard, but bad on the PCB, then it could be anything from wiring issues, soldering mistakes, or even coupling between stages from a poor layout.