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buzzbox blues

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

Reply to
Grant Erwin
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Reply to
JR North

Look for a loose connection. I've fixed lots of welders that needed nothing more than a new lug or a connection tightened. You did not mention smoke escaping or internal arcing so look at your connections.

Pete

Reply to
Pete

It buzzes loudly regardless of whether any leads are plugged in at all.

If it had any capacitors I'd guess they shorted. But it doesn't. I'll pull the cover as soon as I can.

GWE

Reply to
Grant Erwin

My guess would be a shorted diode, assuming the diodes are always in the circuit. Second guess would be a shorted secondary winding in the transformer or a short to ground, either in the transformer or a lead shorted to the cabinet.

John

Reply to
JohnM

Sounds like it burned through some insulation and made some shorted turns. Those boxes are only good for a 40% duty cycle and overheating ruins the winding insulation. Probably need to chuck it or try DIY rewinding. Bugs

Reply to
Bugs

Pop the case and look on the right side for a main bus ribbon that has melted in half.

Gunner

Reply to
Gunner

Reply to
RoyJ

oooo..yah...that could be it!

Gunner

Reply to
Gunner

Reply to
Randy Zimmerman

Reply to
RoyJ

Is it a arc point to point that wore down and once jumped it makes plasma and dumps. Might be a matter of adjusting for the wear of he points so it does it normally.

Just a thought - might be it. Can't think of the 'real name' it is a gross form of a regulator.

Martin

Reply to
Martin H. Eastburn

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:

Reply to
Grant Erwin

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