Re: [AMC-List] stuff gets old

[Tom Jennings <tomj@xxxxxxx>]

On Mon, 11 Dec 2006, JOE FULTON wrote:

> Just curious Tom.    How could you tell that a
> particular transistor was bad?  

The technical service manual gives voltages throughout the
circuit for normal operation; 8.9V on this pin of that part,
etc. You can check that with a voltmeter. The problem is
interpreting what a wrong result means -- you kinda have to
know how a transistor works (and I do electronics as part of
my work). You put current in the base of a transistor, it makes
a larger current flow in the collector, etc.

But in this case you could also guess armed with a little
knowledge; it's a big transistor, bolted outside the case,
and it runs hot, normally. It drives the louspeaker, so if it's
utterly silent, if it's not wiring, it's likely that transistor.

(I also have an oscilloscope and was able to poke around the
circuit and see the actual signals, but it wasn't necessary in
this case.)

The tuning thing, that was mechanical! it just took a lot of
staring and working the tuner to see what was wrong.  Same way
I diagnose a lot of car problems, pull up a piece of carpet,
get comfortable, lay under the car and stare at it, yanking at
things to find the squeak :-)

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