Bazz Fuss cutout problems. GRRRRRRRRRRRRRRR

[superferrite]

Sorry to post for such an easy circuit, but I built three BFs for my friends.  Two have started to sputter and cut out, one does not.
The variations on the Classic Darlington Bazz Fuss are as follows:  Both in and out caps 10uf.  Diode is an LED.  The tranny is an MPS A13 on the two that don't work anymore, and the one that does has two 2n3904s or something similar.
The one that works has a trim pot for the voltage bias.  The others have a fixed 10K resistor.  I think the problem is there, RIGHT?!?
I don't have any more trim pots, but I guess I could socket the resistors and swap up?

Anyone see an inherent flaw in my reasoning at 12 midnight after a long hot day?

[~arph]

dead batteries?

[foxfire]

 it could be a mis biased tranny i.e. the 10K resistor needs to be adjusted. also double check that all your components are in correctly. it sounds like everything in there but the resistors is directional. double check the pinout of the mpsa13's also. rylan

[superferrite]

The resistor is directional?  Don't understand.  The caps are electrolytic and therefore directional but I think I've got it right.

I'll try cramming a trimpot in there if I can!

[foxfire]

everything but the resistors... rylan

[kvb]

what are you using for gain control? If you are using just a pot and no bypass cap then this could be the source of the problem. try just connecting the emmiter to ground and see if that works.

[superferrite]

Okay, so it works on bypass, and fuzzes when stomped BUT the suzz starts to fade and fizz out after about 5 seconds.
While wiggling things around I found out that if I touch the circuitboard traces (using vero for this one), the fuzz comes back strong but fades out again soon!
So: ground problem?
Also, I tried using the "led trick" with a 3904 and a  330K resistor but the LED doesn't light.  Save that one for a different thread!

[R.G.]

The bias is changing. You probably have an open or short somewhere you don't see that is letting the bias run down.

Things that go bad after a few seconds, up to a few minutes are a capacitor charging up/down when it shouldn't. Things that go bad over a minute to longer than that are usually thermal - something heats up. There is an overlap. Sometimes something will heat up and cause a path to a capacitor to open/close, so you get both.

Check the obvious: Did you get the right PCB layout? Print correctly? Solder correctly? Wire correctly? Orient parts correctly?

And the non-obvious: is there a crack in the conductors? Break in a wire? Break in a ... component lead? Broken resistor body?

[superferrite]

I'm going to rebuild it on another piece of stripboard.   
The thing is, I think the same problem as before the rebuild is happening with all new components except the jacks, switch and pot.  I'll check those for leakage and shorts although it doesn't seem like they could be at fault.

Thanks again.