Page 156 - Troubleshooting Analog Circuits

12. Roundup of “Floobydust”

Loose Ends That Don’t Fit Elsewhere

In any serious troubleshooting situation, it is usually wise to plan what tests are most
likely to give you the answer quickly. rather than just charging off in random direc-
tions. Intermittents are the toughest, most frustrating kind of troubleshooting prob-
lem. And bench instruments augment an engineer’s senses and open the window of
perception to the circuits he or she is troubleshooting. “Floobydust” is an old expres-
sion around our lab that means potpourri, catch-all, or miscellaneous. In this chapter.
I’ll throw into the “floobydust” category a collection of philosophical items. such as
advice about planning your troubleshooting, and practical hints about computers and
instruments.

Troubleshooting Intermittent Problems

The car that refuses to malfunction when you take it to the shop, the circuit that re-
fuses to fail when you’re looking at it-does it really fail only at 2 am?-these are
the problems that often require the most extreme efforts to solve.
The following techniques apply to intermittent problems:
I. Look for correlation of the problem with something. Does it correlate with the time
of day? The line voltage? The phase of the moon? (Don’t laugh.)
2. Get extra observers to help see what else may correlate with the problem. This extra
help includes both more people to help you observe and more equipment to monitor
more channels of information.
3. Try to make something happen. Applying heat or cold may give you a clue. Adding
some vibration or mechanical shock could cause a marginal connection to open per-
manently, thus leading to identification of the problem and its solution. (Refer to the
notes on The Soul of a New Machine in Chapter 5).

4. Set up a storage scope or a similar data-acquisition system to trap and save the situa-
tion at the instant of the failure. Depending on the nature of the instrument, you may
be able to store the data before the event’s trigger or affer, or both. This may be espe-
cially useful in self-destructive cases.
5. Get one or more buddies to help you analyze the situation. Friends can help propose a
failure mode, a scenario, or a new test that may give a clue.
6. As the problem may be extremely difficult, use extreme measures to spot it. Beg or
borrow special equipment. Make duplicates of the circuit or equipment that is failing
in hopes of finding more examples of the failure. In some cases, you are justified in
slightly abusing the equipment in hopes of turning the intermittent problem into an
all-the-time problem, which is often easier to solve.

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