Marshall 6100 30th Anniversary trouble

[mtlbasslad]

Hello all, I'm back with the toughest problem I've had so far...
Friend who thinks i know what I'm doing brought me his Marshall 6100 - blowing fuses (I think there are 7) and making explosion/smoke...
This is no simple amp - 3 channels MIDI switching etc - there are 5 separate boards under the hood

Rickey told me he saw a black spot near where the smoke came from (he now knows how to check fuses) on the power supply board - Ha! methinks this will be easy, then I opened it up - arrgh there are eleventy-seven chips & ribbon cables everywhere. I feel like water rising to neck level

OK so R401 is fried, but wait - R401 seems to be 3 1/4 watt resistors ganged together in parallel - value obliterated by explosion/smoke - schematic says "10R 1/2W" ... I see the Brits have a slightly different symbol/language - is this 10 ohms?

I stare at the board(s) for some time accompanied by my best friend Stella Artois, looking for the 3 bridge rectifiers indicated on schematic... I see nothing that looks like a big black square... try a shot of Polar Ice (vodka) to see if that improves my vision. WTF? there are 3 little things marked DB1 DB2 DB401 that look like kernels of corn...??? these little things can handle the current for a 100 watt tube amp? Colour me confused...

I can see that someone has been in here before me, there are French sharpie (felt pen) markings here and here - 'NOIR, JAUNE' etc

Also, I see an additional 5.6K resistor on pin 5 of the output tube closest to the PT - OK I see them on the schematic but not on the board except this one ??

With my new Fluke 115 meter (congratulate me for spending $250) I measured those big square (very technical term) 5 watt 470 ohm (X4) & 1 4 watt 2.2K resistors surrounding the power tubes just because Enzo or another of you wizards would ask me about that (I think)

So friend Rickey has been dispatched to the nearest parts store an hour away (we got 15cm/6 inches of snow today) & the fun will continue tomorrow...

He has a show soon & really does not want to use his backup amp - a Line6

I am way over my head here, help!

P.S. Checked power tubes (Ruby 6L6) on my old Knight tester, they seem OK

Waiting on your wisdom, remember I'm an amateur with a good meter

Lorne & Joanie

[Enzo]

A schematic you can post or link?

10R instead of 10 ohms is not Brit, it is world wide. Not everyone uses it, but it gets more and more popular.

10R - 10 ohms
4R7 - 4.7 ohms
3K9 - 3900 ohms

No decimal point to miss.

The little 3/8" rectifiers are rated at an amp, just like a 1N4007.

[mtlbasslad]

Hi Enzo, thanks for the explanation - so 10R is 10 ohms

Not clever enough to post a link, but the schematic I'm looking at is from Dr Tube...

[mtlbasslad]

update - drtube.com/library/schematics

[Jazz P Bass]

Here is the power in schematic.

6100_65_04.zip

R401 is on the CT of the secondary 6 volt transformer.

[g1]

You said 6L6 so here's the 5881 version of the schematic. Power amp schematic is version 10 so there may be differences to what you have.
Which fuses are blowing?

[mtlbasslad]

Thanks Jazz P, I had the schematic, just not savvy enough to link to it...

[mtlbasslad]

Thanks G1, that's the same schematic I've been working from. As far as I know the 5881 is just a ruggedized 6l6. This amp was 5881 from the factory.
All the boards have different numbers - PS says 6100-65-00. Fuse was the 5 amp

[The Dude]

Do you have a light bulb limiter or other current limiting device?

[mtlbasslad]

Todays fun

So Rickey drove 2 hrs to pick up some 10 ohm 1/2 watt resistors (R401) to replace whatever was there before. The old 3 stacked 1/4 watt arrangement with 1 blackend measured 150 ohms - kinda far from the 10 spec.

Solderded in the new resistor & powered up with the light bulb limiter I built last year on the advice of you wise people
Everything OK on standby, bulb went bright for a few secs as caps charged up, then went dim = good
Flipped off standby, one of the 6L6's that tested weak but ok put on a light show !!
Replaced said 6L6 with a spare, repeated the process - no smoke, no light show, YAY
Yes, we had a speaker connected, but no git in the shop, big hurry so 'bye & thanks..

Back at home with a git & the proper cab connected... no smoke, everything lights up, but no sound except pop & crunch switch noises...
I'm thinking the famous grungy jack thing, but who knows...

Updates to come...

[Justin Thomas]

FYI: Don't pay attention to the vintage 5881 specs for the "5881s" in this amp. The 5881WXTs that came in this amp are comparable to a 6L6GC or even tougher, and nobody's entirely sure just how much abuse they can really take. That said, any "reissue" "Tung-Sol" 5881s won't put up with the same punishment as the ones that came in your amp will. Just be aware. I know, it's confusing how they reuse old tube designations, but it's just a fact of life in our world...

Justin

[The Dude]

If you had a shorted/arcing power tube, it very likely also took out HT fuse(s), which would account for no sound. Check that it's not open.

[mtlbasslad]

So, just before bringing the amp back to me Rickey decided on one more try - and all is golden!
I suspect the dreaded 'switching jack' (as in the sticky) but who knows...
I recommended a can of contact cleaner as his next purchase - but he's a guitarist, so faint hope for that...

Thanks to all who responded, no doubt I'll be back with more problems soon as my undeserved reputation locally as a 'fix it' guy grows LOL

[mtlbasslad]

Well, back sooner than I thought...

The amp worked great for a couple of hours - then sound faded away - 2 power tubes were redplating, so shut down.
The amp is 20 miles away, & I'm talking on the phone to a guitarist, not tech-minded...
Seems he has 2 6l6's & 2 5881's in there, never a good idea...

I suggested he get a new set of 5881's and we will go from there.

Here's my problem - no voltages on the schematic, so what am I looking for? Somewhere in the 400 - 500V range, but it would be nice to know something a little closer, eh?
My understanding of redplating is wrong biasing, but there is only one bias adjustment for all 4 tubes... so perhaps a resistor somewhere around those tubes?

Over my head again!

Waiting for wisdom,
Thanks, Lorne

[The Dude]

I'd start by pulling the power tubes and measuring bias voltage on all sockets. Monitor it for a while and make sure it stays put, particularly the red plating tubes. There could be a number of things wrong- leaky coupling cap from PI, connection problem, bad tube(s), etc. Let us know what you measure for bias voltage on the sockets as a step one.