Failed primary winding on Power Tranny?

[hasserl]

A friend brought his Pignose G40V amp over to me to check out for him as it is completely dead. I checked it over, checked the usual easy stuff like fuses and voltages, and I've found the primary side of the power trans is open, no continuity thru it. The secondary side seems fine. The failure happened while he was playing, just after he had finished a song. He heard a small noise and looked at the amp to see the power light fade out. I see no evidence of the wires shorting out, there is a nice grommet in the chassis where the wires pass thru. The primary circuit is fuse protected, the fuse is fine. Any ideas on possible causes? I'd like to eliminate any before replacing the trans and having it fail a second time. It seems if there was a tube failure or something it would effect the secondary side, not the primary, correct?

TIA
Hasse

[Enzo]

OK, the tranny is dead so we are in the what have you got to lose condition. Or ORANGE on the alert scale.

The primary is open, first thing I suspect is a thermal fuse inside the tranny.

REmove the tranny, remove the end bells to expose the wiring. First check to see if the winding wire has not broken off from the exterior wires. If you have to cut through some tape or interior paper layers, do it.

WInding wire will be copper, and usually have the brown enamel coating, or sometimes green. Ever notice how diodes and things often have shiny bright silver colored leads? Easy to tell apart from enamelled magnet wire.

A lot of trannys have little terminals inside where the wires are bonded together. If you see the two primary wires from the outsie going right to two enamelled wires, you are probably sunk. But if you see one outside primary wire go to enamelled wire, but the other going to a bright silver wire, that bright silver wire is the lead of a thermal fuse. It will be tucked in between windings, but you can pull it out by the leads. Chances are somewhere nearby will be a bright shiny wire bonded to an enamel wire with no external connection. This is the other end of the thermal fuse.

If you find a thermal fuse, it can be replaced. But for testing, just short across it. It is in series with the primary. That way you can power up the system and see if other problems exist, or if the transformer itself has a problem which caused it to overheat in the first place.

If you short across the 6v winding secondary, it is more likely to damage the primary with its thin wires than it is the heavy secondary wires. Besides, the transformer transforms power, so it doesn't much matter why it got hot, it just did. Now if you can save the tranny enough to explore the rest of the amp you can find out if there was a cause.

Even if this tranny has no thermal fuse, remember the technique. Someday you will be very happy to save $100.

[R ski]

Way to save

Its a bitch when the wires are to short or they break
from the coil

great efffort sometimes pay off

[Ray Ivers]

Enzo, Hasse...

I had this happen once on some hi-fi component I was working on, and I got lucky - the PT had a dual-voltage tapped primary, so I switched over to the other side of the primary winding (120V-240V), and the lights came back on. Maybe this is a 240V unit, though.

Ray

[hasserl]

Thanks for the good suggestions, unfortunately neither of them did the trick. There is no internal fusible link or second set of primary windings. The external wires connect directly to the enameled winding wire, and there is no continuity between them.

I did check for continutity between any of the secondary wires and the two primary wires thinking that there may have been a short circuit the burned the winding leaving it open, but there is not. Both the HV and the filament windings have good continuity thru them, with some measurable but small resistance on the HV side.

[Enzo]

Then it is a bad tranny, sorry.