5150 blowing fuses

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Thejosh0666
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5150 blowing fuses

Post by Thejosh0666 » Wed Jan 26, 2022 11:02 am

Hi there all, I’ve been reading on here a ton and got to a point with my 5150 signature head where I’m stumped. Working on an 5150 amp I received for a trade not working. First thing I did when I got it was pull out the chassis and look for any of the greatest hits for burned up components. No burned ribbons for heater circuits, no blown fuses, so I ordered a set of matched Sovtek power tubes and Pre amp tubes that were recommended by the the Tube store for this peavey head. Installed tubes. Took it to the jam pad fired it up and it sounded amazing! Lead channel sounds good and rhythm channel sounds good.

Played it a couple days in a row for a half hour or so making sure all was good. The third day I turned it on and it was sounding fine so I was dialing in the rhythm channel and all the lights went out and lost power. Blown fuse. Everything I’ve read, heard, and know point to the power tubes or an issue with the power tube grid screen resistors, socket connection solders, or loose tube socket terminals. Took the power tube board out. Someone had already been in there because it was screwed in not riveted. Looked at the board and it looks good. Didn’t find any cold solder joints, the grid screen resistors looked like they had been replaced and all read 100ohms. No loose sockets for the tube spikes. Ok. Put it back together and installed a new outside 5amp fuse and the 2amp fuse on the preamp board. Turned it on, blew the 2amp. Replaced it, took the power tubes out, turned it on, blew the 5amp fuse again. Ok. Replaced the 5 amp fuse, took out the pre amp tubes, and turned it on and blew the 2amp fuse. Looking for some insight where the next steps are in testing the power circuit as it blows fuses with no tubes in it with the proper fuses installed. I feel like I’m in the diode/capacitor area now but would love some feedback. The only mod to the amp is a detachable power cord and that all seems to be in good working order. Not familiar with testing diodes or capacitors so any help would be much appreciated. Another clue is it will blow that 2amp fuse by just turning the on switch on(red light)Haven’t touched the standby switch yet

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Enzo
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Re: 5150 blowing fuses

Post by Enzo » Thu Jan 27, 2022 12:32 am

I tell you what, I was an authorized Peavey repair shop for about 30 years before I retired, worked on many 5150 amps. Removing the socket board is something I rarely had to do. And then only to replace a socket or something.

Look at your schematic, the original screen resistors are 100 ohm.

Keep the tubes out until it no longer blows fuses. Does it blow a fuse with ONLY the main power switch on? Or do both power and standby have to be on to blow the fuse?

Isolate the problem. Unplug the output transformer from the power tube board (red-brown-blue wires) NOTE WHICH WAY THE PLUG GOES ON.

Does it still blow fuses with that plug off?

DIodes are easy to check for shorts. Just measure resistance end to end. If it is zero ohms or close to it, looks like it is shorted.

johnny be good
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Re: 5150 blowing fuses

Post by johnny be good » Thu Jan 27, 2022 5:55 am

Hey Enzo I though that a diode should show zero resistance one direction and nothing in the other direction? I am a novice so I may be wrong. Thanks for all the help with my Valveking doing same thing I am waiting on new diodes for mine.

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JamesPaul
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Re: 5150 blowing fuses

Post by JamesPaul » Thu Jan 27, 2022 8:25 am

It depends on the diode, but measuring in forward biased will have some resistance. Say 1k ohm to 10M ohm as real devices are never ideal. Measuring in reverse biased should be infinite resistance.

Here is a article from the Fluke site on testing diodes, which may be helpful.

https://www.fluke.com/en-us/learn/blog/ ... est-diodes
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
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dak
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Re: 5150 blowing fuses

Post by dak » Thu Jan 27, 2022 12:09 pm

johnny be good wrote:
Thu Jan 27, 2022 5:55 am
Hey Enzo I though that a diode should show zero resistance one direction and nothing in the other direction?
Nope. If your multimeter has a diode symbol as a range (often the same setting that will beep for connectivity), that is the one you should use for checking the diode and it will display the voltage across the diode which should be somewhere in the 0.6V/0.7V range (in the conducting direction, of course). If your multimeter does not have such a symbol, the "resistance" display will be the test voltage divided by the test current. Since the diode will have a voltage drop before it starts conducting, the test voltage will end up being this voltage drop, and the current is whatever limit the multimeter uses. That's sort of arbitrary and will likely even differ on the same meter between various different resistance measuring ranges.

There are not many failure modes with diodes: it's pretty much a short in both directions, or open in both directions. While of course a diode can also be damaged where it would be likely to fail eventually, it usually works right until it fails completely. That's different, say, with power transistors which lose their gain with damage and where one final damage mode still has working BE and BC diodes but with the common current paths burnt out, giving no gain at all. So when looking just at the diodes of such a power transistor damaged in secondary breakdown, it may appear fine but since it does not actually amplify current, it will eat through its driver transistors that have to do all the work.

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Re: 5150 blowing fuses

Post by johnny be good » Thu Jan 27, 2022 5:01 pm

Thanks for the info I was not doubting Enzo's statement I was trying to understand what I was looking at wrong.

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Enzo
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Re: 5150 blowing fuses

Post by Enzo » Thu Jan 27, 2022 5:30 pm

When I am blowing fuses, I am not concerned with nuance. I can use resistance or the diode test. ALL I want is to see if we are shorted. On diode test that is zero volts. On resistance that is zero ohms - or close to those readings. A shorted diode is going to read shorted both directions.

I agree, shorts or opens is pretty much the failure mode options. You might see an occasional marginal performer, but usually it is one of those or it is good.

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JamesPaul
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Re: 5150 blowing fuses

Post by JamesPaul » Thu Jan 27, 2022 7:21 pm

johnny be good wrote:
Thu Jan 27, 2022 5:01 pm
Thanks for the info I was not doubting Enzo's statement I was trying to understand what I was looking at wrong.
Yeah, Enzo knows more about this stuff than the rest of us combined. No disrespect his way. A lot of us are still learning from him. He'll truly retire someday and we'll miss him like an arm or a leg. I am a degreed Electrical Engineer (BSEE), plus a Technician (AASE), but I still have to fight my way thru a lot of my issues. Enzo recognizes a lot of these issues off the top of his head.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
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Enzo
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Re: 5150 blowing fuses

Post by Enzo » Thu Jan 27, 2022 7:41 pm

Oh the only thing off the top of my head was hair. Now all of it is off. I think it was a 5150 that blew the last of my hair off my head...

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JamesPaul
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Re: 5150 blowing fuses

Post by JamesPaul » Thu Jan 27, 2022 8:12 pm

Genius plus modesty/humility are rare qualities. That indicates we have someone truly interested in helping us. Not sure where we'd be without you Enzo. I'm a second generation Buckeye, but the next time I make a MSU game, drinks are on me Brother!
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

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Enzo
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Re: 5150 blowing fuses

Post by Enzo » Fri Jan 28, 2022 4:02 am

The old folks home in which I dwell sits next door to a local brewery

https://www.badbrewing.com/

And OSU? Afraid I'll have to root against you...

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JamesPaul
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Re: 5150 blowing fuses

Post by JamesPaul » Fri Jan 28, 2022 6:40 am

No offense taken and the beers will still be on me.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

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dak
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Re: 5150 blowing fuses

Post by dak » Fri Jan 28, 2022 9:18 am

Ah, but now they will be root beers.

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Enzo
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Re: 5150 blowing fuses

Post by Enzo » Fri Jan 28, 2022 2:13 pm

I can walk the few yards to the brewery. We also have an A&W, but that is almost a mile away.

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JamesPaul
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Re: 5150 blowing fuses

Post by JamesPaul » Fri Jan 28, 2022 11:28 pm

Great brews, next door it is. WTF knows if I'll make it to the next game. Should I show up early, I'll give a week's notice.

Meaning I'll probably show up early.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

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