Debugging, transistors

[Adamo]

I'm debugging a recent build I did. It's a ggg foxx tone machine and I'm certain that I have some cold solders which I'm starting to remedy but I have one basic question. When going through the circuit with an audio probe, if I have a signal passing through a 2N3904 at its base, should I also get signal passing through the collector and the emitter. I get a nice loud signal when I touch the B but get absolutely nothing at the C and E.

[R.G.]

I'm debugging a recent build I did. It's a ggg foxx tone machine and I'm certain that I have some cold solders which I'm starting to remedy but I have one basic question. When going through the circuit with an audio probe, if I have a signal passing through a 2N3904 at its base, should I also get signal passing through the collector and the emitter. I get a nice loud signal when I touch the B but get absolutely nothing at the C and E.

Not the right way to formulate the question.

Whether you should get signal at emitter or collector (or both!) if there's a signal on the base depends on how the rest of the circuit has held the base, emitter, and collector to certain bias voltages. If, for instance, the emitter is grounded, no, there will be on signal on the emitter in any situation.

Likewise, if the collector is open, there's not likely to be any signal on the emitter, and probably not on the collector either.

For an NPN, the base MUST be about 0.5to 0.7V more positive than the emitter. The collector MUST be more positive than the base, ideally a few volts in typical pedal practice, or there won't be any reasonable signal on the collector. If these are not true, it can't be amplifying.

This is the line of reasoning that led to the debugging procedure in "Debugging: what to do when it doesn't work". If the DC bias points are not right, it *can't* work.

[GibsonGM]

So, basically, we'd need to know what (DC) voltages you have on each transistor pin first!    IF you have the .5 to .7 at the base R.G. mentions, and decent voltage at collector, then we might assume the transistor then should be working....if you measure the pins of the transistor and find something "off" there, you may in fact have just discovered where your problem is (with the resistors around it that bias it for proper operation)