Page 16 - Troubleshooting Analog Circuits

Experts Have No Monopoly on Good Advice 3

Now, it’s true that some scopes and some DVMs are more suitable for trouble-
shooting than others (and I will discuss the differences in the next chapter), but, to a
large extent, you have to depend on your wits.
Your wits: Ah, very handy to use, your wits-but, then what? One of my favorite
quotes from Jiri Dostal’s book says that troubleshooting should resemble fencing
more closely than it resembles wrestling. When your troubleshooting efforts seem
like wrestling in the mud with an implacable opponent (or component), then you are
probably not using the right approach. Do you have the right tools, and are you using
them correctly? I’ll discuss that in the next chapter. Do you know how a failed com-
ponent will affect your circuit, and do you know what the most likely failure modes
are? I’ll deal with components in subsequent chapters. Ah, but do you know how to
think about Trouble? That is this chapter’s main lesson.
Even things that can’t go wrong, do. One of the first things you might do is make a
list of all the things that could be causing the problem. This idea can be good up to a
point. I am an aficionado of stones about steam engines, and here is a story from the
book Muster Builders of Steam (Ref. 4). A class of new 3-cylinder 4-6-0 (four small
pilot wheels in front of the drive wheels, six drive wheels, no little trailing wheels)
steam engines had just been designed by British designer W. A. Stanier, and they
were “. . . perfect stinkers. They simply would not steam.” So the engines’ designers
made a list of all the things that could go wrong and a list of all the things that could
not be at fault; they set the second list aside.
The designers specified changes to be made to each new engine in hopes of
solving the problem: “Teething troubles bring modifications, and each engine can
carry a different set of modifications.” The manufacturing managers “shuddered as
these modified drawings seemed to pour in from Derby (Ed: site of the design fa-
cility-the Drawing Office), continually upsetting progress in the works.” (Lots of
fun for the manufacturing guys, eh?) In the end, the problem took a long time to find
because it was on the list of “things that couldn’t go wrong.”
Allow me to quote the deliciously horrifying words from the text: “Teething trou-
bles always present these two difficulties: that many of the clues are very subjective
and that the ‘confidence trick’ applies. By the latter I mean when a certain factor is
exonerated as trouble-free based on a sound premise, and everyone therefore looks
elsewhere for the trouble: whereas in fact, the premise is not sound and the exoner-
ated factor is guilty. In Stanier’s case this factor was low super-heat. So convinced
was he that a low degree of super-heat was adequate that the important change to
increased superheater area was delayed far longer than necessary. There were some
very sound men in the Experimental Section of the Derby Loco Drawing Office at
that time, but they were young and their voice was only dimly heard. Some of their
quit@ painstaking superheater test results were disbelieved.” But, of course, nothing
like that ever happened to anybody you know-right?

Experts Have No Monopoly on Good Advice
Another thing you can do is ask advice only of “experts.” After all, only an expert
knows how to solve a difficult problem-right? Wrong! Sometimes, a major reason
you can’t find your problem is because you are too close to it-you are blinded by
your familiarity. You may get excellent results by simply consulting one or two of
your colleagues who are not as familiar with your design: they may make a good
guess at a solution to your problem. Often a technician can make a wise (or lucky)
guess as easily as can a savvy engineer. When that happens, be sure to remember
who saved your neck. Some people are not just “lucky”-they may have a real knack