AC128 Fuzz Face Kit Oscillation

[pringe]

Hi guys, I've successfully built a few pedals now and thought I'd try my hand at this classic circuit - I ordered a PNP AC128 kit with the mayer mods and have just put it together but it quite savagely oscillates when either the tone / vol / guitar vol is turned up too high, with no fuzz until just before it oscillates...I've checked the gains of the two AC128s following R.G.Keen's test circuit and have the following readings:

Q1: Gain 109
Q2: Gain 209

I know you can use a range of gains in the circuit but these are higher than the ranges I've seen written on the web - would this have something to do with it perhaps?

Here are the voltage readings:

Supply: 9.62V Battery

Q1:

C: 0.39v
B: 0.08v
E: 0.00v

Q2:

C: 4.8v
B: 0.39v
E: 0.33v

How do these look? I'd really appreciate any advice to point me in the right direction with this!

Thanks

Steve

[Electric Warrior]

Looking good. Did you rebias Q2? Its collector voltage is quite low. With Q1C sitting at 0.39V, I guess Q2C would read around 7V with stock values, maybe a tad lower.

That wouldn't cause your problem, though. It may be caused by the layout of the circuit or the wiring. I guess some transistor types may be more prone to oscillation than others as well. Usually it's more of a problem with silicon transistors.

You could try fixing it by putting a small capacitor between Q1's base and collector.

[Plexi]

Check for any shortcut from + to mass.
It's the first thing I think when I hear "oscillation"; that's what solved many oscillations in some pedals.

If the transistors are socked, try another PNP there...

[PRR]

Oscillation is often layout: output wiring close to input wiring.

[Plexi]

Oscillation is often layout: output wiring close to input wiring.

That's it!