philco: Strange problem with the power transformer.

I've just restored a Philco 60B and put it to a long time working test.
After some time of normal operation, it suddenly stopped and presented a frying noise coming from the transformer.
I turned it off and made an inspection looking for a possible short on the B + circuits.
Everything was as it should be.
Removing the 80 tube, the noise stops.
Disconnecting the B+ power line to the rest of the circuit and puting the 80 tube in it's place the frying noise come back.
I measured all AC voltages of the transformer without the 80, and all are ok.
All the Ohmic resistances of the windings are ok too.
I also measured the insulation between the windings and did not notice anything abnormal.
Thus I came to the conclusion that there is a sparking between the 350 and 4.8 windings. The sparking occurs only when the 80 is in place.

Bad news for me ... The power transformer must be redone.

Cheers,

Meyer

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You could try a separate isolated supply, maybe even 3 x D Cells if you have nothing else.
Also if you have an capacitor high voltage leakage tester you can test for leakage between the windings.
Assuming you don't have a known good type 80 tube to swap. As you say, it seems like a transformer issue. I once "fixed" a transformer that was intermittent by immersion in very hot polyurethane yacht varnish.

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Hi Michael,
I have a tube tester, and the 80 is fine. In spite of this I put another one (NOS) that I have and nothing has changed. I'm going to pull the transformer out of the chassis and take it apart to see if there's anything that I can do without rewind it.

Thanks for you interest

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Before rewinding the transformer, I would look at the filter capacitors.  Electrolytic capacitors have always been a source of trouble after a few years and I suspect that may be the problem here.

When you disconnect the B+ from the rectifier output, do you also disconnect the filter capacitors?  The frying noise may be coming from the capacitor.  With your hand, see if the capacitor is warm.  In general, if it's more than a few years old, replace it.

Bob

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Hello,

obviously the heater windings of the 80 are elevated to several hundred volts positive if you plug-in the 80. Without rectifier the level of the 4.8 V windings is more or less free.

If there is an insulation failiure to the chassis or the heater windings for the other tubes you may have sparks.

Also a failiure of the insulation paper between 4.8 windings and the 350 windings may cause sparks because one end of the 350 winding is always several hundred volts negative to the heater if the 80 is pluged in.

In German transformers these windings are on the top and easily accessible and can be repaired without rewinding of the whole transformer.

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