Page 186 - Troubleshooting Analog Circuits

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know about it. (Go ahead. Say, “OK, Pease, you can’t keep on driving a 1968 VW
forever.” You can say that, but you are wrong. I can buy enough 1968 VWs to last
me for another 50 years. There are a lot of 1968 VWs in very good shape, here in
California. . . .)
Back to Electronic Circuits. . .

Just as many cars are designed to be repaired by swapping out a large modular seg-
ment and swapping in a replacement, so many electrical circuits and systems are
designed with swap-in cards, which are “non-field-repairable.” Even circuits which
are pretty easy to repair are said, by habit, to be nonrepairable. In fact, the advent of
“throw-away” modules has been debated. Personally I don’t approve of it, not a
darned bit. A couple weeks ago, this little Compaq portable computer quit-the one
I’m using for word-processing. When I tried to read its technical literature and find
out its advice on how to repair the computer, it told me to use various software in-
quiries to find out the problem. What a completely useless notion-the CRT and all
other functions were dead, so I couldn’tpossibly use its diagnostic software.
Fortunately I have a couple technicians who cannot imagine the meaning of “cannot
be repaired.” Paul climbed in and found a shorted rectifier, replaced it, and I was
back on the air in a couple days. If I’d had to take it to a repair shop, I hate to think of
the $ and days just to get a $2.00 rectifier replaced. I’m sure the power-supply card
would cost $90, not to mention the labor.
When I was wandering through Kathmandu last year, I saw workers repairing
things that would not be worth the effort in the USA. But Nepalis do not have enough
money to join in the “throw-away’’ society, so they make the effort to repair things.
Cars, tires, stoves, tools-any equipment that could possibly be repaired, usually is.
(And if it can’t be repaired, it often gets recycled.) I support that approach, and I
myself am usually willing to put in a lot more hours of effort to troubleshoot some-
thing, than the cost of replacing it would justify. Why? Because, sometimes I learn
something.
Once I had an old 1970 VW which I retired because it had 249,850 miles on it and
because it was leaking oil badly, from a cracked block, or so I thought. When I actu-
ally started to dismantle the engine, I found, not a cracked block, but that the bolts
that fasten down the oil cooler had come completely loose. And why had they come
loose? Because there were no lock-washers on the nuts. So in the future I made sure
that anybody working on my engine would use lock-washers, to minimize the chance
of nuts coming loose. It was an educational experience, and well worth the effort.
So, let’s presume that we may actually do some troubleshooting and repair, rather
than just chuck the circuit in a wastebasket. I was talking with a guy the other day,
and he said, “Bob, be sure to spell out the difference between Lab Troubleshooting
and Production Troubleshooting.” I don’t think I know what that difference is. In
either case, it can be very important, and a small amount of time and money can have
great returns. Of course, on other occasions, you can put in a lot of hours and get
virtually nowhere. . . .
As with any other system, troubleshooting is an art which can be developed with
practice. You have to learn the failure modes, the patterns of abuse, the procedures
for replacing bad parts, the documentation, and all the other things we have
discussed. But how about a Modus Operandi? Let’s look at the following table for a
simple op-amp inverter: