In article ,
Ignoramus6985 wrote:

On 2008-08-13, BobH wrote:
dave wrote:
I just bought an old miller dialarc HF "whiteface" vintage. while
getting ready to test it, and cleaning it out, I noticed some of the
leads on the tiny capacitors in the main rectifier bridge are loose,
kind of like how quarter-inch round rod would rattle around inside a
piece of half-inch water pipe. they're "probably" bad, because of that
alone, I'd guess, but, umm, what exactly is the electrical function of
those four tiny things supposed to BE? like "what're they doing for me?"

here'a an extreme closeup of one of the 'loose leads' capacitors, next
to one of the four "big honkin" diodes in the bridge.
http://www.image-upload.net/files/53...or_closeup.jpg

here's a page from the owners manual, where they're descibed...
http://www.image-upload.net/files/5385/parts_blowup.jpg

and here's the parts schematic, for the same machine.
http://www.image-upload.net/files/53...0schematic.jpg

electrically, I'm a "real slow learner", so, thanks for your help, guys :-)

toolie
They probably do a couple of things. As Iggy mentioned, they will help
keep the HF out of the diodes. The other thing is that when diodes start
and stop conducting in a rectifier, they do so very abruptly, which
creates high frequency noise. This noise can cause interference problems
with nearby radio and TV sets (though nothing like the HF will) and it
can also shorten the life of the diodes. These caps will short that
noise to ground.

Bob, these are SCRs, for sure they stop conducting at zero crossing
only. As far as starting to conduct, are you positive that they do it
very abruptly? I thought that it was not the case, but maybe I am
wrong.

SCRs are like switches, and conduction starts abruptly upon triggering,
with a risetime in the microseconds. This abrupt rise can and does
cause interference to nearby radio receivers.

War story from the late 1960s, when I was in school: In the Geophysics
Department there were a number of furnaces used to heat samples of
powdered rock in platinum capsules. Accurate temperature control was
needed, and so one of the professors built SCR temperature controllers
for the furnaces. (SCRs were new then.) It was soon discovered that
all the furnaces were varying together, without regard for the
temperatures of the furnaces in question. Huh? What was happening was
that the RF noise pulse from the first SCR to fire was travelling over
the AC power lines to all the other SCRs and firing them prematurely.
The solution was to add a simple LC filter at the input of each
controller.

Joe Gwinn