Hi J M,

Thanks for your inputs -- I like the series light bulb idea.

If I understand all that you suggested:
-- I'll remove TR3,4,5 and 6
-- Replace TR3 and TR6 with new parts leaving TR4 and 5 off board
-- Hookup the series light with the amp plugged into it
-- plug the light fixture into AC outlet and turn on the Amp
-- If the light is dimly lit, then all is OK up to the input of the MOSFETS
-- If this is the case the problem is either the MOSFETS and/or TR4 and TR5

At that point if I replace TR4 and 5 with new parts (power off), then repower with the light fixture hooked up and the bulb is dimly lit, the problem is fixed.  If the light bulg glows than the problem is with the MOSFETS.

Is that about right?

PS. I'm heading out for the weekend now and will be back on Sunday -- Thanks again for your help.

Regards,
Jack

Hi Phil,

I'm not sure if this is how I reply to your reply -- hopefully so.

I want to thank you for your detailed response to my post.

I decided to check into the availablility and cost of all the transisitors and IC's in the PA and both PreAmp's.  It turns out that I found all (or equiv.) in stock at DigiKey for $12 shipped to my house, so I'm going to give it one final effort.

Oh yeah -- I also received an email from Marshall today, basically referring me to one of their service centers.  I'm glad they responded -- My thanks to Mike from Guitar Product Support.

Regards,
Jack

See 3 attachments -

I have a Marshall 5213 MOSFET Amp (1986-88?) that I can not power up.  All knobs to min settings, speakers hooked up OK, no guitar or pedal inputs, keeps blowing the 1.5A fuse. (The fuse glows like a lightbulb!)

I bench checked both circuit boards (both sides) -no physical burns, all solder joints look OK, no discolored components.  Checked the reverb box (coils/components) -- OK.

I unsoldered the secondary (output) of the transformer, the fuse doesn't blow and I have both AC voltages available.  I lifted up the rectifier quad so that the output was off the PCB and checked the DC out and it is OK, fuse doesn't blow.

I removed the 2 MOSFETS, tried checking and I thought I found the problem.  I was able to find replacements on eBay, purchase and installed -- Blew the fuse AGAIN!

I was able to find the schematic.  This is a project AMP that I paid $45 for --with the MOSFETS, I'm into it for $70.  The 2 Celestion 12" speakers (and the cab) are in perfect shape and worth more than that, but I'd like to get this baby working as the specs and features are awesome.

Aside from taking this into a repair shop or parting it out, any ideas on what could be blowing the fuse?

If you are a repair shop in the San Francisco/Bay Area -- let's talk.

Marshall -- If you are out there, I sent you an email about a week ago on this and never heard back.