Peavey 5150 - What's toasted?

[miketbass]

I have a Peavey 5150 I am looking at for a friend. It blew it's 5A mains fuse as well as the internal 2A fuse that appears to be tied to the plate and screen supply from the power tranny. With the "red" wire that feeds this fuse and power supply disconnected I can get the amp to power up just fine. With it connected, even singularly, the amp will pop it's fuse. All tests are with all tubes out. I also noted that the "yellow" connector that feeds the heater supply has some scorching at the connection, and while the 10A fuse is intact this is likely a sign of something pulling heavy current.

Question is, where to go from here? I am concerned that the power transformer may be toast. But of course there may be something further down that is the culprit. All the filter caps look fine and nothing is obviously toasted. Any thought to get me in the right direction?
Here's the schematic ~
5150evh.pdf

[Chuck H]

The problem is likely the PT. But it may have happened because of a failure later in the circuit. Possibly a power tube short.

Are you saying that with no tubes it's still popping the 5A mains fuse AND the 2A HV fuse?

[miketbass]

At this point with no tubes in, just the 5A mains is going.

[Chuck H]

Just to be sure, since it may just be that the 2A fuse isn't blowing as fast as the 5A, check the rectifier circuit, that cap that bridges the PT primary and of course look for any burned or charred areas on the board. But it sounds like a shorted PT primary.

[g1]

Could you clarify a little more, you said it powers up with everything except the red wire connected? Schematic shows fast blow for the 2A fuse, you are using fast blow? (maybe a slow blow here would result in the 5A blowing instead of the 2A ?) After checking the areas Chuck suggested, maybe try disconnecting the OT primary.
As far as the heater supply goes, I worked on a combo that had the two 100R open (R200,R201 on power tube board), as well as a burnt R69 (4K7 by heater fuse). The 10A heater fuse had not blown. Looking at the power tube sockets, I could see marks from the guide pin that showed someone had installed the power tubes incorrectly. So you may want to check those resistors, in case that has to do with the overheating at the yellow wire.
The power tube board was riveted to the chassis (for the combo, not sure about others), so rivet removal and replacing with new rivets is required. Not sure if there is a rivet removal tool/method that doesn't require drilling or grinding, as I think this is a bad idea with the potential of metal filings/dust in high voltage areas around the power tube sockets, but I could be nitpicking .

[Chuck H]

I think this is a bad idea with the potential of metal filings/dust in high voltage areas around the power tube sockets, but I could be nitpicking .

No. That's pretty dumb. But I don't think they expect many of the amps with a failure on that board to be repaired.

[J M Fahey]

From what I understand (the description is not that clear) the 5A fuse blows *only* with the "red wire" connected to the HV fuse, but not when disconnected.
That clears the PT as a suspect.
I´d suspect the power rectifiers, main filter caps, etc. , or shorts/carbon/arcing at tube sockets ot carbonized areas in the PCB.

[Chuck H]

Good catch. I read too fast over that detail in the first post (happens all the time).

+1 on this diagnosis

[miketbass]

Alright guys, thanks for the input. Ill check the rectifiers out. J M Fahey nailed it, When I connect the red HV wire to the board the amp will blow fuses. With the OT primary disconnected the fuse still pops. So it sounds like both transformers are safe.
The filter caps do not show any obvious signs of damage, is there any easy way to test them without removing them from the board? Oh, and I've started using a variac to power up and monitor current with an ammeter. Saves a lot of $$ in fuses.

Anyways, I'm home for the day and I will dig back into this amp. I will report back with my findings. Thanks guys!

[Chuck H]

When I connect the red HV wire to the board the amp will blow fuses. With the OT primary disconnected the fuse still pops.

With the "red" wire that feeds this fuse and power supply disconnected I can get the amp to power up just fine. With it connected, even singularly, the amp will pop it's fuse

Which is it???

If the fuse pops with the OT primary CT removed it may still be your PT. If the fuse doesn't pop with the OT primary CT removed your PT is probably fine.

[miketbass]

Alright, I tracked it down to one of the 1N4007 diodes in the rectifier circuit. Replaced it and the amp powers up just fine. Interesting failure though, what would cause this to fail?? Many thanks to everybody for your insight and help. Thanks guys!

[Chuck H]

Not sure about why they fail if they're properly rated diodes. But it seems like I've read about this failure on those amps a few times now. Glad it wasn't the PT.

[Enzo]

They failed because parts fail. Most parts in most amops are properly rated, yet they sometimes fail anyway. Parts fail at random, parts fail for cause. A bad power tube could have directly caused the rectifier to fail, or a failed power tube at an earlier time could have easily stressed the rectifiers, and one just took a while to fail.