66 February 2016
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All Electronics Corp. ..................... 7
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Anaren ..........................Back Cover
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Cleveland Institute of Electronics.. 65
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All Electronics Corp. ..................... 7
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Ion Motion Control ...................... 33
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NKC Electronics .................. 23
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Halicrafter’s communications receiver
from at least the ‘50s. I’m planning
to replace the electrolytic capacitors
in the power supply with capacitors I
salvaged from a more recent TV set.
However, I’ve read that
electrolytic capacitors — once formed
at a certain voltage — can take
months, if not years, to reform at a
new voltage. Until then, the capacitor
value can be significantly off from
what’s on the label.
Can anyone shed some light?
#1 The thin aluminum oxide
dielectric (energy-storing) layer in
electrolytic capacitors is formed on
the specially-treated anode metal,
and the electrolyte contacts the outer
can (on early parts) or the cathode
foil (on later “dry” parts). When
radios have been unpowered for a
very long time, electrolytic capacitors
tend to lose their dielectric layer and
their voltage rating, but not usually
much capacitance. They may destroy
themselves and other parts when re-powered, unless slow-start techniques
are used to renew their dielectric.
When restarting long-idle sets, it’s
best to use a variable-voltage (variac)
transformer to increase voltage slowly
over several days. Note that variacs
are usually not isolated. An alternative
method is to power up with an
incandescent lamp socket in series
with the AC line, starting with a 60
watt bulb and increasing the wattage
gradually. Turn the set off occasionally
and check for excessive heating.
I have repaired many radios using
higher voltage TV capacitors, and
have never known one to take more
than a week, unless it was defective. If
the TV parts aren’t ancient, they may
still be sufficiently formed for your
new (lower) voltage, but for safety,
follow the above procedures.
There is a strong shock hazard
posed by AC/DC sets. Many early
versions of these sets had all B(-)
connections grounded to the chassis,
including one side of the switched
line cord, no matter which way it’s
plugged in. Missing or wrong size
screws or knobs or rotting rubber
mounting grommets could make
outer metal cabinets lethal. Some
otherwise well-built later radios,
such as the Hallicrafters SX- 41, had
a live chassis. Most later sets used
an isolated internal ground system
to minimize the hazard. It’s safest
to work on AC-DC sets using an
isolation transformer — especially if
you’ll be connecting any AC-powered
test gear. You can make your own
using back-to-back filament or power
transformers of appropriate power.
Good luck with your repair!
Stanley Pitman
via email
#2 Let me start with a rule of
thumb: Electrolytic caps drop some
value if formed to higher voltages
(within working range). This is
because of thickening of the barrier
layer, BUT this can vary. That’s why
most are rated plus or minus 20%.
They generally only need to be big
enough (power filter or bypass). Don’t
expect much value drift moving to
a lower voltage. Other concerns are
with internal resistance (can screw
with bypass performance in audio
stages, and heats cap at high duty
cycle).
Nick Vitinaros
Grand Blanc, MI
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66 February 2016