NPN behaviour Question....

[motion]

Hi all,

I'm just debugging a Gus' Overdrive I put together this afternoon and was wondering about the behaviour of NPN transistors like the 2n3904 used there.

For reference, here's the schem:



Pretty simple stuff.

In my case, I've basically made a wire: the audio at the output is the same as at the input.

According to Wiki:

"A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal."

From that I'm inclined to believe that there is a problem with the Emitter on the bottom - i.e. signal isn't getting there, so there is nothing to change the output. Hence input = output.

Does that sound right? On one hand I'd like to get my overdrive working, but I'd also like to make sure I understand what's going on.

Matthew

[R.G.]

In my case, I've basically made a wire: the audio at the output is the same as at the input.
...
From that I'm inclined to believe that there is a problem with the Emitter on the bottom - i.e. signal isn't getting there, so there is nothing to change the output. Hence input = output.

You can solve many of these problems by measuring the DC voltages on the active device as laid out in "Debugging: What to do when it doesn't work". The DC voltages tell you if the device even has a chance to be working properly. Only when the DC conditions are satisfied can you hope to make the AC stuff work.

I ran a simulation of the circuit, and it gives between 10 and 14DB of gain from about 20Hz to 1kHz, gradually rising, then falls off above 1K. One thing that tells us is that your actual circuit does not match the schematic; otherwise, it would act very similarly to the simulation. I don't trust simulations above the real world, but for simple circuits like this one, they're usually pretty good.

Does that sound right? On one hand I'd like to get my overdrive working, but I'd also like to make sure I understand what's going on.

That's a good attitude. Do the necessary poking around, measuring things, checking pinout and components, and find out what doesn't match the schematic. By the way, if it's on a plugboard, those contacts get intermittent with just a little use.

[motion]

I just did some poking around, after having taken out the 2n3904, and realised it shows the same behaviour with the tranny out as it does with it in. I guess we can exclude that from the list of possible causes, then.  

I would hazard a guess that nothing should pass through that circuit with the tranny out, meaning I must have a bridge somewhere.

Also, thanks for the quick reply. I'll see what my voltages are like tomorrow.

[petemoore]

  In my case, I've basically made a wire: *the audio at the output is the same as at the input.
   Are you describing a wire you added or is this a ''the way it's workin' now'' circuit-result observation ?
  I would hazard a guess that nothing should pass through that circuit with the tranny out, meaning I must have a bridge somewhere.
   When the physical board has the same connections as the schematic it has to work, otherwise it has to not work.
  A DMM w/trasistor Hfe checker, can test Si transistor gain = a ballpark reading means transistor goodness.
   Every node must match schematic. Count the # of connections shown, test that these exact connections exist physically on the board.
  Note polarity markings [any component marking where one side looks different than the other, including the B/E diode direction/use transistor data sheet to verify/check transistor orientation].
   Likely unnecassary, but 'dogmatic' testing...measuring every connection, resistor value, and cross check from each node using DMM beep-mode tests to verify there are no cross-traces [probe a node, test all other nodes].
  The capacitors are 'leap of faith' [fortunately they're quite consistant performers]..and block DC [allowing DC bias], passing AC [allowing signal path].
   The voltage measurements tell whether it can be operational [and guide in bug finding], between these readings and the audio probe results cap-goodness is almost invariably verified.
   Polarized capacitors are exception after 1 reverse polarization they can be considered junk to be pulled and thrown out as an unknown variable eliminated, allow the other debugging/elimination processes to proceed.