WANTED: Good Source of Electronics Theory

According to the datasheet for Q205, the Vbe is:

MINIMUM: -0.58 TYPICAL: -0.62 MAXIMUM: -0.68

These figures were arrived at using the following test conditions:

Vce = -6.0V Ic = -1.0mA

As I have said, I am a hobbyist. Not sure how to do "real world" testing for Vbe or why it's important. I do know how to test NPN and PNP transistors using the diode function of a multimeter, that's it.

That's a good starting point, and will likely identify 95% of any problems in an amp. Transistors and diodes, that is.

Other things to look for are blown emitter resistors, other burned resistors, and bulging capacitors. Problms relalting to drifting component values are pretty rare.

geoff

And from your own measurements there is only 0.50V Base-emitter, so the transistor is not correctly biased. Possibly a fault in a preceding stage, or even an O/C resistor or possibly a dry joint. Keep looking and measuring other voltages.

MrT.

Silly thought, but you *have* connected 205 the right way round when you replaced it.........

S.

One of the basic principles of troubleshooting -- which I think other posters have been trying to get across -- is to look at the DC voltages on a device to see what they tell you about the device's operation.

For example, in "normal" operation, a tube's plate voltage should be at least 100V higher than the cathode *, while the grid should be several volts lower. If this isn't so, something is wrong and you need to find out why.

I was recently troubleshooting an electronic crossover with about 10 volts DC output. The output stage used an op amp driving a current-boost device. I had no schematic. The parts were difficult to remove, as there was no easy way to remove and flip over the PC board. I didn't want to go through the hassle of removed a 14-pin op amp from the top of the board, if it wasn't defective.

A friend (a well-known audio designer -- yes, I'm name dropping) said "Look, the current-boost thingy has a gain of 1 for both AC and DC. The input and output should be the same, regardless of circuit design. What are you getting?"

"There's 1V on the input, -3V on the output."

"Then it _must_ be defective. Right?"

Right. I clipped the leads and sucked out the stubs. Then I shoved in the replacement and carefully applied solder.

It worked.

Voltage checks provide really good troubleshooting clues -- if you know how to interpret them.

• Obviously, the exact nominal voltages vary with the tube, the circuit, etc.

William:

That was very good advice. In a way, I'm trying to do the same thing, but from a "functional" standpoint as well.

In other words, I know what the expected voltages should be, but I can't help thinking that if I knew how this circuit actually amplified the signal, I would be in a better position to troubleshoot it, hence my original request for good sources for learning electronic theory. (Some of the sources that have been recommended so far seem to be excellent, by the way.) Where exactly does the signal go?, what should happen when it gets there?, etc. I believe that these are questions that I should be able to answer.

So far, what I have is a collection of components that all seem to test perfectly OK, but don't work, for some as-yet unknown reason.