Page 61 - Troubleshooting Analog Circuits

48 4. Getting Down to the Component Level: Capacitor Problems

For either of these techniques to work, it is helpful to have a large stock of assorted
capacitors. In our lab, we have several cartons of used-but not too badly beaten
up-components left over from old experiments: One is a box of small mica and
ceramic capacitors, one holds various electrolytic capacitors, and one is a tray of
assorted wound-film capacitors. These boxes are extremely valuable because if I need
an odd type or an odd value, I can usually fish in one of those cartons and find some-
thing close. Or I can find some capacitors that give the right value if I parallel two or
three of them. I can use these capacitors per the add-it-on or the substitution method
to find out what my unhappy circuit is trying to tell me. In addition, I keep a couple
of Teflon capacitors in my file cabinet for when I need a super-good capacitor.
A technique that nobody talks about (but is as old as the hills) is a favorite trick of
mine. Sometimes it drives my technicians wild, but then they learn the trick and find
it awfully useful. Let’s say I want to compare a Mylar capacitor with a ceramic ca-
pacitor in a small, precision circuit. The technician starts to remove the Mylar capac-
itor and install the ceramic one. Wrong! Instead, remove one lead of the first capac-
itor and lift it up slightly. Then tack solder one end of the second capacitor to the
circuit. At this point, neither capacitor is actually in the circuit-both capacitors are
just waving in the breezes.
After the solder thoroughly cools down, I can use the springiness of the leads to let
me “touch in” one of the capacitors or the other or both, as needed. It only takes a sec-
ond to go from one mode to the other. (Of course, I’m assuming there’s not enough
voltage to “bite” my finger. If there is, I’ll just push the end down with a popsicle
stick or a bare piece of glass-epoxy material. . . .) If I actually desoldered and resol-
dered the capacitors and allowed enough time for these temperature-sensitive compo-
nents to cool off, I’d probably forget what the difference between them looks like. So,
this technique can save a lot of time, and greatly facilitates A-B comparisons-it lets
me use my eyeball to evaluate the nuances of small performance changes.
Of course, if I have two or three of these spring-loaded options at one time and
they begin to get wobbly, it may be time to tack-solder down the ones that I am not
actively pursuing. In general, though, this technique is extremely valuable, and I’ve
never seen it mentioned in any book. Use it with my compliments. It works with
diodes, resistors, and transistors, too. Just make sure that solder flux doesn’t prevent
the spring-loaded component lead from contacting the conductor. And make sure that
your finger doesn’t add a lot of capacitance, impedance, or noise into the circuit. If
you do have this problem, push on the component with the tip of a fingernail instead
of a finger. A fingernail adds less than 1/2 pF.

But Is This Really Troubleshooting?

When I passed the first draft of this chapter around to a few friends, one guy asked,
“Why are you telling us all these things about weird capacitors? What does that have
to do with troubleshooting?” I gave him the same answer I give you here: If you had
a mediocre coupling capacitor and you didn’t realize that it could keep on “leaking”
for many seconds or minutes longer than a good coupling capacitor would, you
wouldn’t look for problems traceable to that capacitor. I cannot foresee every problem
you will have in a circuit, but I can point out that similar-looking components can
have startlingly different characteristics. You can’t learn about these characteristics
from looking in books, or even in data sheets. So, if you get in trouble, I’m trying to
suggest clues to look for, to help you get out. Conversely, if you study these precau-
tions and think about what can happen, you may be able to avoid getting into trouble
in the first place. That’s even better than being able to get out.