Page 15 - Troubleshooting Analog Circuits

2 I. First Things First

repair 20,40, or 60% of these complex units. Switching-regulated power supplies can
also be quite complex. If you manufacture them in batches of 100, you shouldn’t be
surprised to find some batches with 12 pieces that require troubleshooting and other
batches that have 46 such pieces. The troubleshooting may, as you well know, be
tough with a new product whose bugs haven’t been worked out. But it can be even
tougher when the design is old and the parts it now uses aren’t quite like the ones you
used to be able to buy. Troubleshooting can be tougher still when there isn’t much
documentation describing how the product is supposed to work, and the designer
isn’t around any more. If there’s ever a time when troubleshooting isn’t needed, it’s
just a temporary miracle. You might try to duck your troubleshooting for a while.
You might pretend that you can avoid the issue.
And, what if you decide that troubleshooting isn’t necessary? You may find that
your first batch of products has only three or four failures, so you decide that you
don’t need to worry. The second batch has a 12% failure rate, and most of the rejects
have the same symptoms as those of the first batch. The next three batches have
failure rates of 23,49, and 76%, respectively. When you finally find the time to study
the problems, you will find that they would have been relatively easy to fix if only
you had started a couple of months earlier. That’s what Murphy’s Law can do to you
if you try to slough off your troubleshooting chores ... we have all seen it happen.
If you have a bunch of analog circuits that you have to troubleshoot, well, why
don’t you just look up the troubleshooting procedures in a book? The question is
excellent, and the answer is very simple: Until now, almost nothing has been written
about the troubleshooting of these circuits. The best previous write-up that I have
found is a couple pages in a book by Jiri Dostal (Ref. 1). He gives some basic proce-
dures for looking for trouble in a fairly straightforward little circuit: a voltage refer-
encehegulator. As far as Dostal goes, he does quite well. But, he only offers a few
pages of troubleshooting advice, and there is much to explain beyond what he has
written.
Another book that has several good pages about the philosophy of troubleshooting
is by John I. Smith (Ref. 2). Smith explains some of the foibles of wishing you had
designed a circuit correctly when you find that it doesn’t work “right.” Unfortunately,
it’s out of print. Analog Devices sells a Data Converter Handbook (Ref. 3), and it has
a few pages of good ideas and suggestions on what to look for when troubleshooting
data converter and analog circuits.
What’s missing, though, is general information. When I started writing about this
troubleshooting stuff, I realized there was a huge vacuum in this area. So I have filled
it up, and here we are.
You’ll probably use general-purpose test equipment. What equipment can you buy
for troubleshooting? I’ll cover that subject in considerable detail in the next chapter.
For now, let me observe that if you have several million dollars worth of circuits to
troubleshoot, you should consider buying a $100,000 tester. Of course, for that price
you only get a machine at the low end of the line. And, after you buy the machine,
you have to invest a lot of time in fixturing and software before it can help you. Yes,
you can buy a $90 tester that helps locate short circuits on a PC board but, in the
price range between $90 and $lOO,OOO, there isn’t a lot of specialized trouble-
shooting equipment available. If you want an oscilloscope, you have to buy a gen-
eral-purpose oscilloscope; if you want a DVM, it will be a general-purpose DVM.

1. I must say, I recently re-read Mr. Dostal’s book, and it is still just about the best technical book on
operational amplifiers. It’s more complete, more technical, but less intuitive than Tom Frederiksen’s
Intuitive IC Op Amps. Of course, for $1 13, it ought to be pretty good. It is getting a little old and dated, and
I hope he plans to update it with a new revision soon.