Page 60 - Troubleshooting Analog Circuits

First, Try Adding A Second 47

whole box of “capacitors” in which the two leads were still made of one loop of wire
that had not been snipped apart.
Obviously, the manufacturer wasn’t interested in testing and measuring these
capacitors before sending them out the door! So, if you are buying capacitors to a 1 %
AQL (Acceptance Quality Level) and not 0.1% or 0.01%, you should be aware that
some low-priced part5 have not even been sample tested.

Variable Capacitors May Have Finite Rotational Lives
Variable capacitors are usually made of low-K material with characteristics similar to
those of COG capacitors. Their electrical performance is excellent. The dielectric
doesn’t cause much trouble. but the metal sliding contacts or electrodes are. on some
models, very thin; after only a small number of rotations-hundreds or even
dozens-the metal may wear out and fail to connect to the capacitance.
In general, capacitors are very reliable components; and. if you don’t fry them with
heat or zap them mercilessly, the small-signal ones will last forever and the elec-
trolytic ones will last for many years. (Old oil-filled capacitors aren’t quite that reli-
able and have probably been replaced already-at least they should have been re-
placed.) The only way you can have an unreliable capacitor is to use a type that is
unsuitable for the task. And that’s the engineer’s fault, not the capacitor’s fault. Still.
some troubleshooting may be required; and if you recognize the clues that distinguish
different types of capacitors, you’ve taken a step in the right direction.

First, Try Adding a Second
What procedures are best for troubleshooting capacitors? I use two basic procedure>.
the first of which is the add-it-on approach. Most circuits are not hopelessly critical
about capacitor values, as long as the capacitors’ values are large enough. So, if there
is a 0.01 FF capacitor that I suspect of not doing its job, I just slap another 0.01 pF
capacitor across it. If the ripple or the capacitor’s effect changes by a factor of two.
the original capacitor was probably doing its job and something else must be causing
the problem. But if I observe little or no change or a change of a factor of three. five.
or ten, I suspect that capacitor’s value was not what it was supposed to be. THEN I
pull the capacitor out and measure it. Of course, the capacitor substitution boxes I
mentioned in the section on test equipment, part 8 of Chapter 2, can be valuable here:
they let me fool around with different values. But in critical circuits, the lead length
of the wires going to the substitution box can cause crosstalk, oscillation. or noise
pickup: so I may have to just “touch in” a single capacitor to a circuit.
Suppose, for example, that I have a polyester coupling capacitor that seems to be
adding a big, slow “long tail” to my circuit’s response. I don’t expect the
performance with the polyester capacitor to be perfect, but a tail like this one is
ridiculous! (Note: when a capacitor’s voltage is supposed to settle, but there is actu-
ally a ”long tail,” that is just another way of saying that the capacitor has poor dielec-
tric absorbtion or “soakage.” It’s the same thing with different aspects.) So. I lift up
one end of the polyester capacitor and install a polypropylene unit of the same value.
I expect the new capacitor’s characteristics to be a lot better than those of the old
capacitor. If the tail gets a lot smaller. either my plan to use polyester was not a good
one or this particular polyester capacitor is much worse than usual. It’s time to check.
But usually. I’d expect to find that the polypropylene capacitor doesn’r make the
circuit perform much better than the polyester capacitor did. and I’d conclude that
something else must be causing the problem.