Attempting to resurrect a dead Marshall

I mentioned on my introductory post that I've got a JCM2000 TSL602 which is making a whole lotta nothing soundwise. I got it for free, so I'm not complaining, but I'd like to get it going again.

I'd take it to an authorised service place, but there's about a thousand miles of ocean twixt me and they, and I'm not sure I trust any local technicians here in Bermuda to touch it.

The problem is very strange. Essentially, there's no output from this thing at all, including noise from the speaker at any volume level. This would indicate perhaps a blown OT, but when taking output from the effects send, which is post gain/pre power amp, there is still nothing to hear. I've tried the DI XLR output and if I crank up the gain on my recording gear into which it's plugged there is some small amount of residual noise, but no signal.

If anyone's encountered this with a similar Marshall unit and has some insight, I'd appreciate any input.

Also, is it just me, or are Marshalls particularly flaky? Every Marshall I've seen which was bought new has deveolped some major operational problems within a year.

Cheers

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Hello EG. One of the possible suspects you've correctly identified is the OPT, but there are a few others. Before taking a multimeter out, I'd like you to know that all-valve amps have certain places that are nasty to touch, in a "death" sort of way. First off, do a visual inspection (and bear in mind I'm not familiar with the newer Marshalls).

The speakers in the combo will be connected in parallel and it's unlikely that both have blown, so (as long as the wiring from the amp to the 1st driver and back from the 2nd looks decently connected) they're probably not the source.

While unplugged, remove the back panel so that you can see the guts as best as. If possible, undo the chassis from the cabinet so that you can have a look at the underside, where all the non-valve, non-transformer gubbins are. Avoid touching things; wear insulating gloves if nervous; keep your weak (left?) hand in your back pocket, etc. All of these things are good precautions, and the filter capacitors (big metal cylinders) are the enemy to avoid. If you don't know how to discharge them using a 100k resistor, at least make sure that the amp's been unplugged and stays unplugged for a day.

So, if you can get a look here, visually scan the filter capacitors for bulging, bubbling or white powder around the seams. Unlikely cause, but worth a look. Most of the electronics will be on a PCB but have a look for any obviously burnt resistors or scorch marks. OK, we want to take a look at the upside of the chassis now. Make sure the chassis's screwed in place (if you did take it out) but keep the back panel off. Connect a guitar (you know the guitar and lead works, right?) into the input and plug the amp into the mains. Oh, a really obvious Q; the Marshall is set up for 120V, 60Hz I hope?

Anyhoo, with fairly low lighting, switch things on (to standby; I'm assuming this combo has a mains switch and a standby switch). Have a look at the bottom ends of every single valve. The heater filaments should be glowing orange in both the power valves (the two big ones, EL34s) and the smaller ones (ECC83s or 12AX7s). If they all do, then the PT is fine and the valves are likely to be fine. If none do, then you have a prob with the PT or something in between (more later). If one or more doesn't, then you have a valve/socket problem.

It's very late here, so for now I'll just direct you to this very useful page:
http://www.geofex.com/ampdbug/ampdebug.htm
but please come back here with any updates and I'll see if I can help further.

Ah, hi 1bass! Thanks for replying.

I'll give you a more detailed rundown on the sitch here.

Again the problem is that there is no output from any orifice on this thing, such that neither the pre nor power sections can be isolated as the culprit.

I've noted that this and some of the newer (and older? I dunno) Marshalls have muting circuitry as part of their Neutrik 1/4" connectors, so that if you don't have anything plugged into the input the whole system is in mute mode. I'm heavily leaning towards the possibility that it's somehow not unmuting when I plug in. The fact that there is utter silence from the speakers indicates to me that, assuming the OPTs are good, it's just a matter of finding out why the system is in mute mode permanently.

You can find the schematic for my amp at:

http://www.schematicheaven.com/marshallamps/jcm2000_tsl60_60w.pdf

I've not done any live testing yet, but I am somewhat experienced working with hifi tube amps in the past and the safety issues involved. I'm a total wimp when it comes to pain and electrocution so I certainly won't be taking any chances when it comes to measuring voltages, etc. lol.

If you can maybe give some insight on how to maybe test the muting circuit and/or fix it I'd be most grateful. I'd just as soon give this unit to an official repair shop, but there ain't none for about a thousand miles, and even then the cost of shipping would make it prohibitively expensive to do.

Cheers

Hello EG,

Crikey! Schems give me a headache at the best of times but that one made my eyes all funny. Lots of things I'm not used to (older Marshalls don't have any mute feature). Your theory is a possible, but there are still other things to consider and the valves are always a top suspect. The little ECC83s will give you zip output if one's failed and, because both EL34s work together in push-pull, one of those failing will knock your output, too.

Even if the mute is stuck in position, the valves will be fed power to keep them running. Do you have all of the filaments glowing when switched on?

Sorry if those schems made your head swim - they just about made my head assplode lol.

I do believe there is trouble with the muting feature. I first replaced all the 12AX7s with known working ones, but still no joy.

I don't have replacement output tubes handy, but I believe it is possibly not them due to an apparently healthy glow from their guts as well as the fact that I took the output of the FX send circuit to an external amplifier and still no joy. I've tried injecting a signal back into the FX return and still nuffin.

I read where the mute works electronically (it's not a matter of purely physical switching), but I can't find the post which stated this and how it worked.

As I said, it's the complete lack of output anywhere on this thing which has me thinking it's in a permanently muted state. Another thing: I tested the caps which seem to relate to the muting in situ and one of them tests fine, but the other doesn't give me a reading at all. I am going to try testing that guy out of circuit to see if it's just that and go from there.

Thanks again for your help here. I don't want to be seen as a noob who just barges onto a forum and demands help then leaves. As I learn more hopefully I'll be able to help other noobs as well ;-)

Cheers

"assplode" - I'll have to remember that :lol: . Yes, I think you've covered the valves. The AX7s would be the likeliest culprits if you're not getting anything from pre-power outputs like the FX loop and DI out. Seeing as you've checked them, and the powers are glowing, I think you're right to tick them on the list.

This muting thing is completely out of my zone as none of my amps have the feature. If you do ask around on the PP forum, you could try the "Marshall" category, but make sure you put a thread in "Tubes transformers and components". The latter cat has more of the solder monkeys and may get you a useful response sooner.

Thanks again. I'll start whispering sweet nothings into the ears of the folks at the Plexi forum and see if I can get some insight.

BTW, assplosions generally are what happens the morning after the night before of multiple pints followed by the inevitable consumption of otherwise inedible greasy nosh at a place even the health department wouldn't step foot in whilst sober. :shock:

Cheers

elbowgeek wrote:

BTW, assplosions generally are what happens the morning after the night before of multiple pints followed by the inevitable consumption of otherwise inedible greasy nosh at a place even the health department wouldn't step foot in whilst sober. :shock:

Cheers

Finally, something in this thread I understand :D Just thought I'd say Hey and welcome (unless I already did that on the other thread?...) new brains are always good to get on the board.

EDIT: 'something' not 'somthing' damn wino-fingers!

I once had a "helmet explosion" involving many beers, lots of curry, sleep, riding a motorcycle the next day, and a belch... Enough of that, though.

There is one thing to try if you haven't already, EG. The mute facility isn't so much designed for when no instrument is plugged in. It's for silent recording and, when activated, kills the power section and the output goes to the XLR "emulator equipped" socket at the back. If a permanent mute is the cause of your problem, then you can confirm this by rustling up a XLR-jack lead and plugging into a guitar amp or using a straight XLR-XLR if you have a PA head handy.

If you get a perfectly OK sound coming out of the PA/other amp, then you know for sure that the mute is the problem and you just have to find out how to fix it. If there's no output from the XLR, then it ain't the mute. This is a much better test than feeding off from either of the FX loops.

Hey Tim, thanks for the welcome!

To be honest I wish I did understand much of what I've written lol. Well, I do wish I'd apprenticed to one of those great amp tech guru types in my youth and studied audio electronics (both pro audio and hifi) as I'm much more an analog type of person, even though I'm a computer consultant by trade. Heck, I listen to most of my music on LP or open reel tape *grin*.

Anyhoo, I'll be seein' ya round de boards, b'y.

Cheers

Try posting in this group...a lot of smart folks hang aroud
http://music-electronics-forum.com/forumdisplay.php?s=ec6b3724ad0c3ef783485396c4422d2f&f=22

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