Fig. 2 is a case in point. The part in the upper left of the photo is a
carbon composition 2000 ohm resistor. It shows no sign of heating or
cracking. I found this one in an FM exciter causing trouble. It measured
in at about 20,000 ohms ó some 10 times its rated value. The resistor
below it is of a carbon film design, which made an excellent
replacement. Carbon film resistors are much more reliable but have some
inductance, which could be a problem in a very high frequency RF
The diode in the center of the photo came from a 1960s vintage Collins 5
kW AM transmitter. Itís job was to steer DC logic to turn the
transmitter on. The diode opened and disabled the start function. We
normally think of diodes as failing shorted. Not in this case, it went
open! Diodes are easy to check using the diode function on most
multimeters. They should show about 0.6 volts drop in one direction and
open in the other.
The transistor in the right side is a 2N2222A. I found this one going
open in a Continental 802 series FM exciters. It is used in the RF mute
circuit. Without it working properly, I couldnít get the exciterís RF to
turn off. It took component level troubleshooting to find all
three of these. Learning and using that skill is still important in
todayís plugíníplay throw-away world. What is the sense of trashing a
$5,000 FM exciter when a $2 part might make it run again?
When you find one aluminum electrolytic capacitor has failed,
the best approach is to replace all electrolytic
capacitors in that unit. Itís called the ďshotgunĒ method. Iíve done
that many times while servicing equipment on the bench. The reasoning is
that other electrolytic capacitors, of the same age, will fail soon too.
I wasnít going to send a piece of equipment back into the field and then
have an unhappy customer complain of yet another failure.
Electrolytic capacitors are something like batteries. They are charged
with energy and then they release it, as planned in a circuit design. A
good example is capacitors in a power supply. If they are preceded by a
full-wave rectifier system, the capacitors will charge and then
discharge at a 120 Hz rate to keep the DC level constant. They do this
while circuits in the equipment are drawing current from them. That is a
hard job and they get tired after 10 years or so of continuous use. They
lose their capacity to do the work and are often referred to as ďdried
out.Ē In an analog piece of gear, the first symptom is usually hum or no
bass in the audio. As I said, save time and trouble by replacing them
all. Youíll be glad you did.
Have spare capacitors on hand. In my business I stocked almost all
values so they were available immediately when a piece of equipment came
in for service. The downside is that the stock will go bad with time. It
must be rotated to make sure all capacitors are fresh. The good
side is that electrolytic capacitors are inexpensive. You might purchase
a 470 mfd/50 VDC capacitor for just $0.68 from Digi-Key or Mouser. I
usually ordered at least 10 at a time.
Let me warn you that not all electrolytic capacitors are the same. They
are rated for current handling, temperature and reliability. If you
donít feel comfortable making buying decisions, let a more experienced
technician do the work. Or order directly from the manufacturer of the
equipment you are repairing. They will know exactly what is needed, and
will charge accordingly. Usually you wonít go wrong when replacing
capacitors, but some circuits require capacitors that are heavy duty or
have low ESR (Equivalent Series Resistance) found in tantalum or more
exotic capacitors. You donít want to mix those. You are safe when
using a 50 volt capacitor in a lower voltage circuit where only say 25
volts will be across it. Donít do it the other way around. Capacitor
voltage ratings should not be exceeded.