I tore it down today. First I ran the shunt all the way down and back up again,
then I wiggled every heavy wire. All looked good. So I plugged it in and no more
bad buzz. Didn't try running a bead yet, but the main symptom has gone. Rules
out a shorted diode. If it starts again when I'm actually running a bead, I'll
figure it's a thermal related issue with a transformer winding. Thanks for all
the tips, excellent help!

GWE

Randy Zimmerman wrote:

I had the pleasure of trying to keep such a machine going in a school many
years ago. I ordered new guides and parts for the crank screw assembly to
firm up the movable core. It improbved it a bit but the machine had run so
much rod at rates well over the duty cycle that I was very happy to see it
replaced with a 300 amp dialarc.
Randy
"RoyJ" wrote in message
nk.net...

If it has the many turn crank on the front, check to see if the magnetic
shunt has broken away from the crank. If it does, the shunt will pull all
the way into the core, sets the welder for the lowest possible amperage,
and buzzes like the devil.

Grant Erwin wrote:

I picked up an old AC/DC buzzbox the other day. It's branded Airco and
has a red case, but it's a Miller Thunderbolt AC/DC. It's probably from
1980 or so. I started testing it and it was welding fine but suddenly the
box started buzzing much louder than before and it won't weld worth beans
any more. I haven't taken it apart yet. I figure something could be
shorted to ground (although the 30A mains fuse didn't blow) or whatever
holds the transformer laminations together might have cracked.

Anyone got any bright ideas what to look for? I'd like to salvage this
one.

Grant Erwin
Kirkland, Washington