'77 JMP with Frank Levi mod no sound, hot cap

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Hi guys. I have a JMP I bought early '90's. Seller said it was modded by Frank Levi and actually supplied his contact info. I didn't know who Frank Levi was at the time, but the amp sounded great and I bought it. I've replaced tubes and filter caps over the years. I also added the no loss FX loop, which did not change the tone of the amp what so ever. Recently I fired it up and there was no sound. Only one tube was lit up, so I quickly shut it down. I thought I had lost a 6550, so ordered a new pair. Pulled the chassis from the cab and installed new 6550's. Still one tube did not light up. I could smell "that smell". You know, the "something is cooking" smell. I shut it down. I found one of the mustard caps to be very hot. It's the one marked with red tape in the pic.
So, I need some help. Could the problem be the cap? Or, is the overheated cap a result of some other issue?
Also, does this look like the work of Frank Levi? It seems very messy and hacked to me.
Thanks in advance, and let me know about the mod as well. I have a good mind to grab a schematic and put at back to original spec...However, it has the best tone of any amp I've ever owned. Marshall '77 2204.JPG Marshall '77 2204.JPG
 

JP2036

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You might have the B+ feed to the metro loop in the wrong spot on the main PCB but im not sure.
How long have you been running the amp after the loop install ?
There always a chance the backside of the loop PCB could have grounded to one of the preamp sockets.
Thats why my metro loop board was installed with the wave solder side facing away from the tube sockets.
Frank was an amp wizard and did good work.
Hope you can get it fixed.
 
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I’ve had the loop in it for a year or so. Everything was fine until recently. I was driving it a little harder than usual with a two notes torpedo captor. But had been doing that for a while as well.
 

Kim Lucky Day

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Welcome to the forum, AWESOME user name! Sounds like something I'd call my overexuberant dog lol

Have an experienced tech take a look at it. I had a local pro put a Zero Loss in my JMP 2104 and it works awesome, hopefully you can get the issue corrected and get back to enjoying that beast!
 
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I don’t believe the fx loop has anything to do with the problem. I was running it hard and this particular cap overheated. I’m wondering if it the caps fault or if there is an issue somewhere else? Other than the loop. Any thoughts?
 

mixn4him

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That cap is the input to V3 so maybe check that tube and try a replacement tube. And yes your power tap is wrong for the FX loop but should be fine...
 

JP2036

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That cap is the input to V3 so maybe check that tube and try a replacement tube. And yes your power tap is wrong for the FX loop but should be fine...
I thought it might be. Seeing how his circuit is heavily modified who knows whats going on. Find a Engineer and put it on the bench.
The amp is likely fixable but not over an internet forum and Frank had died.
There is a B+ resistor from the loop kit that im not seeing. You may have fried something.
 
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neikeel

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Looking at what you have I see no issue with your loop power supply, you can take it from anywhere in the B+ line and you have used the 47k dropper to get the feed at the right level so nothing I can see wrong.
Mustard caps very rarely fail on their own (having said that I had to pull a leaky one from a 45/100 the other day!). That cap is the feed from the tone stack (ie preamp out) and into the phase inverter you will see from the schematic that it is hooked up to the grid on one half (other end of the 1M is part of the phase inverter tail to the presence circuit).
To me the amp does not look that radically modded:
Added bypass cap on V1 second stage
Adjusted value coupling cap on V1 (by piggybacking cap over the existing one)
some disguised malarkey around the mixer resistor and bypass caps, absent interstage treble peaker on V1 and swapped PI output couplers (probably removed 22nF and swapped for 100nF).
I would do as suggested above, remove the tubes and check the voltages on the tubes vs a known 2204 chart
WDFo_j_yR0i0RXZeh8BwrvepN_sg9wl5FuXTqxqNOgpfgthOWGeLhqk7_feYeKA6wMGKS_hdAQtCJontcRZG8abuPOo

Do you have a spare known 12AX7 to sub in once you have done this?
 

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