Last
month’s column wound up with the engineer’s lament: no matter where you are, something’s always
trying to break on you! Like most sad
stories, that one’s completely true. The
good news is that there are predictable patterns to components’
malfeasance. This month, we catalog
their misdeeds:
We
all know that transistors fuse faster than the fuses that are there to protect
them. But fuses get tired, too, particularly
the slow-blow variety—from metal fatigue in the expansion during each initial
surge, supposedly—and finally blow when they shouldn’t. Thermal problems cause intermittent faults in
rectifier bridges, too.
Dust
particles are attracted to anything at a high voltage potential. They’ll stick and eventually provide a
conducting path, which will then carbonize and probably blow to pieces. This is partially counteracted because all
physical connections, meantime, are busy expanding and contracting as they’re
cycled, trying to work themselves loose.
Typically they’ll get hot and burn before they open up completely,
though.
Electrolytic
capacitors are always trying to either leak or dry out. If they dry out, they’ll intermittently go
open. If they leak, the corrosive
electrolyte will proceed to wreck any printed circuit board in the
vicinity. Printed circuit board
material, meanwhile, will gradually carbonize under the influence of heat.
The
heat generated by power carbon composition resistors actually causes the carbon
granules inside the resistor to regranulate over the years, causing the
resistance to drop over time. Of course,
in most circuits, this results in more current, and more heat, etc., etc.,
until the inevitable short. Usually
after that they’ll present a very high resistance—kind of like they’re trying
to reform for their previous current-hogging ways…
Transmitting
mica capacitors develop a series resistance that increases over time, often
variable with ambient temperature. The
resistive component makes them get hotter and hotter, until the inevitable
fire. The old style, with the white
ceramic coating, will sometimes contain themselves—the
newer black plastic ones will usually spray a flaming rubbery plastic all over
the place until the transmitter overloads and gives up. They smell bad, too.
Metal
oxide varistors will always fail by suddenly providing a dead short. Of course, generally they’ll blow up when
this happens, blowing off the leads and noisily restoring a nice open circuit that
means no harm to anyone. But then the
ex-varistor is no longer providing any protection, either.
Mylar
capacitors, or plate blockers, are prone to pinholes hidden under the plate
bypass element. If they were easy to
see, there wouldn’t be any challenge!
Hollow
doorknob capacitors heat up until the solder connections to their terminals
melt. The solid red
doorknobs get pinholes or carbon traces, sometimes internal, usually very hard
to see. Typically they’re very
intermittent, too. O joy!
Oil-filled
capacitors generally leak, or short out and explode. Tantalum capacitors prefer the dead
short. Disc ceramics go “leaky,”
developing a shunt resistance, or just absorb water vapour and start drifting
in value.
High-tension
wire will break down from heat and ozone, eventually carbonizing, typically
near one end. Carbon traces can travel
several inches at the end, however.
Neoprene insulation will rot from heat and ozone, and flake off,
exposing the copper beneath.
An
old trick of the high-power inductors in the plate supply is to maintain their
inductance, but short a point in the windings to the case. You can sometimes solve this by slipping last
year’s phone book under the case, insulating it from ground.
Ceramic
tower guy line insulators (“eggs”) will develop carbon traces, then start arcing, creating RF interference. The fiberglass ones will break down in the
sun, absorb water, and arc until they’re fully carbonized. Then they explode.
Old
circuit breakers, like old engineers, just get “tired” and start tripping over
nothing.