"Chuck Harris" wrote in
message news
Count Floyd wrote:
I just received the radio, brought up the power, played
well for about
five minutes, then the hum started, a noise like sizzling
bacon, a faint burning odor, then I shut off power
immediately. Is it the electrolytic capacitors, the
rectifier, both? Any suggestions would help.
Thanks,
Bob Grimes
That is typical of an electrolytic capacitor that
desperately needed for
you to spend a couple minutes of effort reforming it.
This is the one case where electrolytic capacitors fail
shorted. The oxide
layer is too thin, and it arcs over, creating an
electrolyte bridge between
the plates of the capacitor. All of the current goes into
heating the
electrolyte, causing it to boil, and given time, spew
forth from any weak
spot in the capacitor's case.
In spite of what a lot of the guys will tell you, bringing
up the capacitors
on a variac doesn't help all that much. It's better than
doing nothing,
but will still kill capacitors that could have been
reformed. The reason
this is so, is when the capacitor is reforming, it
inevitably creates little
short circuits. The variac doesn't limit the current, so
when one of these
little shorts occur, it damages the aluminum foil.
A better way, with a tube radio, is to charge the
capacitor up to operating voltage
using a bench power supply with a series resistor (2K,
5W). Because the tubes
aren't lit, there is little else in the circuit that will
draw current while
you do this.
The capacitor, the rectifier tube, and possibly the
transformer are
the likely collateral damage.
-Chuck
Its easy enough to check the transformer. Remove the
rectifier and measure the AC voltage at the socket where the
plate pins connect. Actually, if the tranformer has shorted
it will probably make noise just having AC on it.
Electrolytic caps often have a strong ammonia-like odor when
the go short and often vent electrolyte. If yours have done
this clean the stuff off whatever its gotten on because it
can be corrosive.
--
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Richard Knoppow
Los Angeles, CA, USA