Page 27 - Troubleshooting Analog Circuits

2. Choosing the Right

Equipment

As discussed in Chapter 1, the most important thing you need for effective trouble-
shooting is your wits. In addition to those, however, you’ll normally want to have
some equipment. This chapter itemizes the equipment that is necessary for most
general troubleshooting tasks; some you can buy off the shelf, and some you can
build yourself.
Before you begin your troubleshooting task, you should know that the equipment
you use has a direct bearing on the time and effort you must spend to get the job
done. Also know that the equipment you need to do a good job depends on the kind
of circuit or product you are working on. For example, a DVM may be unnecessary
for troubleshooting some problems in digital logic. And, the availability and accessi-
bility of equipment may present certain obstacles. If you only have a mediocre oscil-
loscope and your company can’t go out and buy or rent or borrow a fancy full-fea-
tured scope, then you will have to make do.
If you lack a piece of equipment, be aware that you are going into battle with inad-
equate tools; certain clues may take you much longer than necessary to spot. In many
cases when you spent too much time finding one small problem, the time was wasted
simply because you were foolish or were unaware of a particular troubleshooting
technique; but, in other cases, the time was wasted because of the lack of a particular
piece of equipment. It’s important for you to recognize this lastmentioned situation.
Learning when you’re wasting time because you lack the proper equipment is part of
your education as a troubleshooter.
In addition to the proper tools, you also need to have a full understanding of how
both your circuit and your equipment are supposed to work. I’m sure you’ve seen
engineers or technicians work for many fruitless hours on a problem and then, when
they finally find the solution, say, “Oh, I didn’t know it was supposed to work that
way.” You can avoid this scenario by using equipment that you are comfortable and
familiar with.
The following equipment is essential for most analog-circuit troubleshooting tasks.
This list can serve as a guide to both those setting up a lab and those who just want to
make sure that they have everything they need-that they aren’t missing any tricks.

1. A dualrtrace oscilloscope. It’s best to have one with a sensitivity of 1 or 2 mV/cm
and a bandwidth of at least 100 MHz. Even when you are working with slow op
amps, a wide-bandwidth scope is important because some transistors in “slow” appli-
cations can oscillate in the range of 80 or 160 MHz, and you should be able to see
these little screams. Of course, when working with fast circuits, you may need to
commandeer the lab’s fastest scope to look for glitches. Sometimes a peak-to-peak
automatic triggering mode is helpful and time-saving. Be sure you know how all the
controls work, so you don’t waste much time with setup and false-triggering prob-
lems.
2. Two or three scope probes. They should be in good condition and have suitable

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