Blowing Fuse on Marshall 5213 (100W, MOSFET Reverb Twin Combo) Amp

[JackD]

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.

[phatt]

I think I follow you,
Ok,, if you have AC secondary And then re fit Caps delivering DC rails ,,,
Now Bang when you reconnect the power amp.

Sorry sounds like it's terminal.

You could insert a pair of say 33ohm 10/ or 20 watt Wire wound power resistors before the power amp rails.
This will limit the current and may give you some time to check out what part of the poweramp circuit is overheating,, there is usually something getting very hot.
Check TR3 and TR6 these are the predrive and usually work harder than the output devices.
If it stays working long enough then see how much DC is on the speaker terminal.
it should normally only be within a few mV like 30 /100mV if it is many times higher ,,then it will just go bang again real soon.

Sadly SState power Amps like this are hard to fix at home,, you could replace ALL active devices but these things often have design flaws and the problem may well resurface again.
If you can source a poweramp module designed to run on the 50/50 rails then that is another option.  look at ESP site ,, a Mosfet power module ultra simple and probably a lot better than yours. http://sound.westhost.com/project101.htm

I have often found design flaws in some big brand name gear,,, yeah should not happen but sadly they don't care.
Sorry I can't be of more help.
Phil,,, Nambour Australia.

[JackD]

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

[J M Fahey]

Dear Jack.
By what you describe, you originally had shorted output transistors.
Now, although you replaced them, the fuse keeps blowing, fast!.
Probably  the original dead MosFets took some other components with them, which keep the failure alive.
First of all, please scan and post here the schematic (try to make it as readable as possible, specially small text)
2) Build a series lamp fixture, basically an extension power cord where the "hot" wire is cut and a socket and an an incandescent lamp (60/100W) are inserted in series, so that current has to go through it.
You plug it in the wall mains socket and the suspect amplifier on the extension socket.
If all is well, the lamp will blink and then glow barely orange; if the amp is shorted, it will glow brightly, limiting current but not blowing like a fuse would, then you can measure *something* on the amp, and start troubleshooting.
Don´t touch the preamp, reverb, etc. , this is a power board problem.
Keep in touch.
J M Fahey

[J M Fahey]

Hi. I just found the schematic you posted.
In decreasing order of probability:
1)Output transistors shorted/burnt
2)Output transistors improperly mounted (bad mica, etc.) Measure the insulation from any of their pins or body to ground.
3)Ooooops!, maybe you miswired them , check the coloured cables, pinout, etc.
4)Waaay too much bias (which would turn them hard on), measure the voltage across R14.
5)Bad, shorted power supply capacitors (not very likely but not impossible)
6)Bad Tr3 or Tr6
7)While testing, remove short protection , Tr4 and Tr5 or just remove D1 and D2. Anyway the series lamp is now your protector.
8)Don´t plug the amp straight into the regular power outlet until it´s finally fully fixed, the series lamp will let you play up to 20 or 30 W which is plenty for testing.
9)The amp, plugged into the series lamp but without output transistors *should* power "almost normally" and let you measure, mainly supply and bias voltages and D.C. voltage on the output jack.
You have already checked the power transformer and power diodes, so far so good.
Pray, it helps.
Good luck.
J M Fahey.

[JackD]

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

[J M Fahey]

Basically, yes, but don´t do everything at the same time.
I suggested *different* options to be explored one by one.
Of course the series lamp will be used in every one of them, until eventually you succeed.
Now I´m closing shop , very tired, but I´ll try to write for you some kind of "road map" to guide you in your testing.
I see a very high probability of success.
Bye.
J M Fahey.