5150 Power board problem

[Danglin' Fury]

I have an earlier post about the same 5150. I had some issues with resistors blowing. Fixed that, but this power board has me stumped. I turn it on without the power board plugged in and no worries. Immediately when I plug in the power board, I get a flame, yes a flame, shooting from the ribbon cable that attaches to the main board. I'm looking at the trace layout and the flame shoots from the power board side where J60 connects. I had to replace the trace connecting the standby resistor and now the space between the trace and the jumper has almost burned through the board. I checked all the resistors and they are good. I replaced all 3 caps on the board. Still flaming. A tech at Peavey said it could be the flyback diodes. Checked em and they're good. I replaced them anyway. Still flaming. It doesn't matter if the tubes are in or not. HEEEELLP!

[Enzo]

Sounds like your board is arcing. Once an arc path starts, it won;t stop.

The arc carbonizes whatever it is flowing over, which makes it a better conductor. If it were a tube socket we would replace it. Since this is a whole board, we have to fix it. Find the arcing area, and grind away any carbonized material, yes even if it leaves a hole. if that cuts through a trace, then install a wire to take the place of that trace. Just replacing a damaged trace with wire doesn't remove the arc.

J60 is one of the heater legs, and it appears that nothing either side of it carries high voltage normally, so it is p[ossible there is another arc or a short from high voltage to one of hte heater pins on a power tube socket.

Try this. Pull F1, the 2A high voltage supply fuse. This will remove the high voltage, now turn on the amp and see if the flames are gone. If so, then we have indeed some sort of high voltage escaping its normal bounds. if the flames persist, then we more likely have a high current but low voltage short in the heater circuit.

[Danglin' Fury]

Hey Enzo. I removed F1 and indeed there were no flames. Should I grind out the black carbonization as my next step? Thanks for your reply.

[Enzo]

Yes, carbon is conductive. You can scratch away the bulk of it with something sharp and pointy probably, but you do need to get it all, and I mean ALL, like cancer. I use my Dremel tool with various bits and wheels. I grind away the material until I no longer see the darkened stuff. See if you can determine exactly which two traces were arcing.

Have you ever seen a power tube socket or even a rectifier tube socket mounted on a pc board, and there were machined slots through the board between some of the socket pins? Those are air gaps to help prevent arcing.

Just be aware that before we are over, you may find high voltage shorted or arcing from somewhere else, and this stuff we see now is merely secondary damage. or you might be lucky.

[Danglin' Fury]

When I had the tubes in, I also had a problem with R69 blowing up. and on the power board R200 and R201. With that R69, could it be possible that C44 in the power supply is bad. Just a thought. I'm gonna clean the carbon off the power board and see what we see. No other problems in the amp though. Only when I plug in the power board. Thanks Enzo.

[Enzo]

Note that all those additional resistors all relate to the heater lines. Yes, I would replace C44 and also C200 on the power board.

All this could very well have been caused by a shorted power tube. or a power tube with the center post broken off and shoved in the socket pointing the wrong way.