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