Re: WANTED: Good Source of Electronics Theory

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.