Reply-To: Discussion of AM Radio <[email protected]>
How do you intend to handle the resistor while doing the discharging? You
are
still going to need a shorting stick. It is much better to take the slight
chance of discharging the >cap the
direct way than messing around with some Rube Goldberg resistor
arrangement. If
the Great Electron Gods had intended for capacitors to be discharged
through a
resistor (other than a bleeder) before working on a circuit, them he would
have made discharge sticks with built in resistors. I have never seen one,
have
you?
It is very easy to build one. Use something like a 500-1000 ohm 20-50 watt
wirewound resistor. Rigidly attach it to an insulated rod, on the same end
that you use to touch the HV. Better still, use a large enough resistor and
small enough rod that you can pass the rod through the resistor, and clamp
it down. Make sure everything is rigid enough that the resistor isn't
dangling.
To be safe, short out the capacitor directly after first using the resistor,
just in case Murphy strikes and the discharging resistor opened up just as
you discharged. This can most easily be done with a second, more normal
shorting stick. I suppose one could get fancy and build a special shorting
stick with a resistor and a direct shorting tip, in a mechanical arrangement
that would resemble a Wouff-Hong, but to me it's easier to just have the
shorting resistor on hand along with the regular stick.
I don't use a resistor normally, after observing the expected drop-down of
the plate voltage when the transmitter is shut off. Since the likelyhood
that the capacitor is still charged is already remote, I am willing take the
risk of blowing the capacitor or shrapnel in my face. But I NEVER KNOWINGLY
DIRECTLY SHORT A FULLY CHARGED HV CAPACITOR. There's a lot of energy in
that discharge, which can in itself can be dangerous, as well as ruin the
capacitor. The resistor will feel warm to the touch after the discharge,
even though it may be of very short duration.
I once destroyed a 25 mfd 4000 volt capacitor by shorting it with a
screwdriver. I opened the case and discovered that the flexible wire lead
going from the guts of the capacitor, no larger than maybe #16, made from
tinned, bare, twisted copper wire, had opened just like a fuse. I
resoldered the wire and attempted to put the capacitor back together, but it
still didn't work. The wire was probably vaporised at the other end too,
and I didn't care to dig into the PCB saturated guts of the cap, so I just
took the whole thing to the county's next "household hazardous waste
disposal day," and was relieved when they took it without asking me what it
was.
Transmitting caps are not that hard to find, not that expensive. You can
find them at nearly every hamfest. At the Manassas, VA hamfest yesterday,
there
were 2 large piles of them at one vendor in the flea market and he would
almost
pay you to take them home.
People said the same thing about VT4-C/211's, 2A3's, 6B4G's, 845's, and UTC
LS-series audio transformers just a few short years ago.
Don k4kyv
