Page 59 - Troubleshooting Analog Circuits

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

as the main timer. Troubleshooting by phone-it’s always a wonderful challenge. I
ask him, “This COG-ceramic 0.01 FF capacitor . . . is it. . . as big as your little finger-
nail?” He says, “Oh, no, it’s a lot smaller than that.” I reply, “Well, that’s too small; it
can’t be a COG.” Problem solved. Actually, there are some small COG 0.01 IJ.F capac-
itors, but they are pretty uncommon unless you order them specially.
One observed failure mode for ceramic capacitors can arise when the capacitor’s
leads are attached to the dielectric with ordinary, low-temperature solder. When the
capacitor goes through a wave-solder machine, the lead may become disconnected
from the capacitor. If this problem occurs, you’ll have to switch to capacitors from a
manufacturer that uses high-temperature solder.

Don’t Forget Silvered Mica
Silvered-mica capacitors have many features similar to COG capacitors. They have
low ESR and a TC of 0 to +lo0 ppm/”C. They can also work at temperatures above
200 “C if assembled with high-temperature solder. Unfortunately, they have poor
soakage characteristics-unexpectedly bad dielectric absorption.
A major problem with silver-mica capacitors is their marking. The silver-mica
capacitors in old radios had completely inscrutable markings-six color dots. Some
of the new ones have such odd codes that even if the marking on the capacitor hasn’t
rubbed off, you can never be sure whether “1OCOO’ means 10,100, or 1000 pF. You
really need to use some kind of capacitance meter. Similarly, in the old days, some
ceramic capacitors were marked in an inscrutable way. I remember two little capaci-
tors both marked “15K.” One was a 15 pF capacitor with a “K” characteristic, and
the other was a 15,000 pF capacitor-yet they were both the same size and had the
same marking.
I must also mention that, in the past, you could buy a pretty good capacitor that had
never been tested for its capacitance. About 99% of the time, they were excellent,
reliable capacitors. But once in a while, some of the capacitors came through with a
capacitance value completely different from the marked value. One time I saw a

Figure 4.3. If you saw a capacitor that looked like this, you’d know the manufacturer hadn’t tested it
before shipping, right?? (Photo by Steve Alien.)