Blues jr PT HV dead

[nosaj]

I've got a Blues Jr pr 295 model that the HV is dead. I subbed a transformer in for just the HV and everything else works and sounds good too.
My question is what normally kills the HV on a PT, is there something else that may be lurking that I've not uncovered yet or can they just die due to a manufacturing defect.

Thanks,
nosaj
link to service manual
https://www.google.com/url?sa=t&rct=...13943665,d.eWE

[52 Bill]

It could be a manufacturing defect, or one too many shorted power tubes, or ???

I work for an authorized service center and in all of the years I've been working on them, I've only seen maybe 3 dead power transformers in any of the Blues Jr.s, Blues Devilles and Blues Deluxes.

[The Dude]

I've not seen one either. Is the winding open?

[nosaj]

I've not seen one either. Is the winding open?

No it is not only delivering about 72 volts disconnected from the circuit, when it should be nearer to 250v.
Definitely the transformer as cobbling in a test transformer makes the amp work again. Traces look good by power tubes will check the screens.

nosaj

[Mick Bailey]

Low voltage can be due to a shorted turn or high resistance. A shorted turn will warm up the transformer, high-resistance won't.

[nosaj]

Low voltage can be due to a shorted turn or high resistance. A shorted turn will warm up the transformer, high-resistance won't.

But either way its toast. What is high resistance?

nosaj

[Mick Bailey]

High resistance is where there's effectively a high-value resistance in series with the winding, so instead of it reading a few tens of ohms it can read hundreds of K Ohms. It can occur for a number of reasons. Usually you can get a voltage reading (sometimes the full voltage) with the transformer disconnected, but that will collapse under load. Sometimes the solder joint where the winding joins to the connection leads is defective - I've had two non-Fender transformers recently where this has been the case and the outer paper wrap can be carefully removed to expose the joint and re-solder it.

The winding can also burn through like a fuse and leave a conductive carbon deposit that will give a voltage reading. I've seen this at the very end of a winding where there's just been a sooty patch sandwiched between two layers of paper. Again, the voltage collapses under load.

[J M Fahey]

^^^^^^^^ that.
The carbon patch or maybe metal vaporized from a fusing wire and deposited over nearby insulating paper will really be, for all effects, a crude carbon film or metal film resistor

Resistive value? .... it's anybody's guess , but the layer may be atoms thick ... enough for a 1M input resistance multimeter to show *something* , definitely not for any kind of real load.
It might even light a weak Neon bulb ... not much else.

Another possibility, just for the record, might be a very corroded wire, with no metal left, but some greenish copper salts , passing a microscopic current if wet or humid, although I would expect that in an old amp (or radio, etc.) stored for ages in a damp basement or near the sea, not in an amp which was in use but a short time ago.

[Mick Bailey]

I rescued a 60s Japanese stereo amp that had that very problem. The PT and both SE OTs were affected, but fortunately there was enough space to unwrap the paper outer insulation and see the problem. Only the fly leads were corroded - the windings were fine, so just needed some new leads and wrapping back up with some new paper and some hot wax. That set made a nice little Hi-Fi amp that's still in regular use about 6 years later. Oddly, though the ironwork was rust-free, but the outer paper wraps weren't waxed - maybe there was some acidity in the paper.