Blues Jr PT smoked

[Randall]

1998 Type PR295, guy says he switched it on and smoke came out. I find evidence of arcing from chassis and secondary of PT. Primary is open, filament winding measures about 1.5 ohms to chassis ground. The fuse did not blow. I checked PS diodes in circuit, and filter caps out of circuit, as well as pulled C32 .1uF across the secondary, all look good. I ordered a Heyboer made PT from Mojotone, but I'd like to know why this one blew up? Is there more I should check? Or did this one just spontaneously combust?

https://photos.app.goo.gl/0xZSsFo3u6Joa2uq2

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[The Dude]

It could just be the picture, but if I zoom in, it looks like there may have been a solder blob left in there. Maybe careless soldering?

[Leo_Gnardo]

Where did you see evidence of arcing between PT secondary and chassis? Whups, I see it now. Hard to say it's actually from a transformer-to-chassis arc. One of the windings may have failed and splattered smoke & debris onto the chassis metal.

Check the foil side of the PCB. It might turn out there's no problem, but one of the most spectacularly arced boards I ever saw was on a Blues Junior. The problem zone was between OT secondaries and other traces much too close to those hi voltage points. What was Fender thinkin' when they laid out that board? Which version - I can't remember exactly. There's a pair of anti-flyback rectifiers on the PCB, plate windings to ground. You'll want to double check those. And I've seen a couple cases where the first or second stage filter caps have inexplicably gone bad in Juniors too. You'll want to dial it up slow on a variac once you've checked everything out, while monitoring AC current draw. As if I need to tell you that...

Sometimes PT's go bad for no apparent reason, I've had that happen too. Most recently on a Weber kit tweed champ.

[Randall]

The foil sides of both boards look fine, no evidence of any failure. The flyback diodes test good, as does CR6 in the filament circuit. Turns out, no arcing to the chassis after I cleaned up the foul smelling tar mess, the chassis looks unfazed. So, I am leaning towards a spontaneous failure. Fire in the hole!

[mozz]

I bought a old Heathkit HV power supply so i can test things like that. Check outputs for shorts. Maybe take a resistance reading from B+ to ground?

[cjenrick]

probably an offshore transformer, coils are wound 10 at a time , they pull the sticks and take them to the band saw guy, 100 watt light bulb per 100 workers in China, band saw guy cuts coils apart but gets off center, hi pot margins (clearance between coil and steel) are reduced, HV spike comes down power line and the fireworks have begun.

[Enzo]

It is done by machine and is automated. They are not made by guys wearing shower clogs and a pointed straw hat.