Marshall JCM 900 fried! Need help :)

[sibo]

Hey guys,

I had this JCM 900 Hi Gain Dual Reverb combo that I used frequently before. Eventually I put it in the garage where it stood unused for like 2 years and decided to sell it.
Buyer comes by, tries it and picks it up. Everything went fine.

Now I get an email where he claims that is was faulty and I quote:

At first there's a buzz, after that the clean channel increased in volume, then it started distorting. Finally it started smelling burned.

My question would be, what could be wrong and will it be expensive? I thought of the power tubes at first but then it wouldn't smell burned, would it?
Could it be the transformer that's malfunctioning? He didn't mention any fuse blowing but who knows..

Being the nice guy that I am, I told him to take it back and I'll repay him. I know it's not really what I have to do but morally it feels right. Maybe there's something I can do to fix it myself? I know some of you will suggest trying stuff and I will, as soon as I get hands on the amp.

Thanks in advance!
Simon

[Justin Thomas]

Bias cap went bad while in storage, owner turns it on, stresses bias, tubes go into runaway, cap burns up? Highly doubt it's the transformer... Maybe a bug crawled inside, too... Maybe the new buyer did something to it and isn't telling you... Ran it cranked into an attenuator, wrong speaker cabinet, changed tubes, could be anything. Wait til you get your hands on it first. If you CAN fix it, do so and see if the guy is still interested in the amp!

I don't sell or gig with an amp that I haven't turned on in the last month or two... Good luck with it!

Justin

[audiopete]

I'm going to go with power tube(s) shorted and burned up screen resistor(s). Of course it's all speculation until the chassis gets pulled and you see what burned. He can check the HT fuse (500mA) and see if it's blown. The mains fuse may not be blown.

[Pedro Vecino]

It is difficult to find a Dual reverb with damaged screen grid resistors (can happen, of course). I guess the 500mA fuse protection on the cathodes are related to it.
With these symptoms do not forget check the +- 15V line supply (zeners, resistors and electrolytics x2).

[sibo]

It is difficult to find a Dual reverb with damaged screen grid resistors (can happen, of course). I guess the 500mA fuse protection on the cathodes are related to it.
With these symptoms do not forget check the +- 15V line supply (zeners, resistors and electrolytics x2).

Great! How do I do that? Multimeter set to Ohm, then what?

S

Edit: I realized how dumb that question may seem.. What I'd like to know is rather where I'm supposed to measure and what results I should be expecting?

[Audiotexan]

I don't sell or gig with an amp that I haven't turned on in the last month or two... Good luck with it!
Justin

I don't sell an amp without the express statement that it's AS-IS and if you want to test it before you buy it please do! And if something happens after that, then it's on them.

There's too many idiots/thieves out there that will try to get over in one way or another for me NOT to do things this way in this day and age. YMMV.

Now selling out of a 'store front' OTOH, I'll warranty whatever it is for a minimum period of time. But with any tube electronics, there's always the potential unknown. But the reputation of the business is on the line in that case. And you just have to eat things sometimes in the name of good business.

Case in point, had an asshat buy an old late 60's SF Fender, swapped the chassis' out, then brought it back saying it was squealing.

Walking up and seeing the back first, the first clue was that the output tubes had been changed (had black plate RCAs when it left). The second was that the front panel was now bashed in at the top on the front. Upon getting the chassis pulled, now I see that shielded cables on the inputs had been cut/butchered...
So he says it worked fine in store when he demo'd it, and then when he got it home it "didn't work". Yeah. Whatever.

The owner 'ate it' and refunded the sale because "we couldn't prove that it left working". Burned my ass, and his because it was obvious that the clown had swapped chassis'...but had it been my shop, I'd have banned the clown from setting foot in the place again. We already learned he'd do it once. No need for that type of customer.

Anyways, sorry for the minor derail, but, too many instances of folks like that has somewhat jaded my approach as a whole. I wholly agree with your overall outlook of 'do the morally right thing' and I try to maintain that in general. But some of these fools make it damn hard sometimes.

[sibo]

OK so now I've got her back home. Just opened it up and shot some pictures of it internally.
Pictures can be found here!

I noticed there's voltage on that big badass capacitor next to the text "JCM52C". Can I just use a shielded wire to unload it? Should I unload anything else to work safely? Anything else that's common sense or good-to-know?

As you may see, there isn't any component that seems burnt so I figure I should start poking the multimeter around. Any place in particular I should be curious about? I don't think my multimeter can measure capacitance as it's one of the cheap ones, do I need that? If so I'll go buy one.

How can I know if tubes are shorted or malfunctioning in any other ways?

Anything else I should do now that it's opened anyway?

Thanks in advance.

[Pedro Vecino]

http://www.pedrovecino.com/john/dualrev.jpg

I leave a picture with the strange things I see:
The white plastic tube connected on the transformer not looks original. You should check where is connected and if exists any component within.
Missing the screws that secure the board to the chassis (probable symptom of manipulation)
In the blue square will see the six components of the line + - 15V. One of the electrolytics (right) looks swollen. You should find voltages of + and - 15 volt in the zener diodes. In standby, put the tips of the multimeter on each end to measure the voltage

This last can be related to the problem or not but I would not let aside.
Also check T500mA external fuses on the cathodes. If you find one open, it is likely that some power tube has a problem.
And check the status of the screen grid resistors (4 x 2K2 - 4 watt)

[Audiotexan]

Good eye Pedro, and it does look like C22's run a bit on the warm side...but I'd be a bit more concerned with C29 and C30, as those two are DEFINITELY in need of replacement (at least in my book).



Any time you see a cap's label shrink, it's an automatic "test me" sign, but when I see shrunk labels in a PSU, I usually replace them, as 'lytics are cheap and they've obviously been stressed.

[sibo]

Thanks both of u, I will replace all of them. Just out of curiousity - how can you know that C29 & C30 needs replacing? What do they do? Excuse these questions that may seem stupid but I'm novice.

Probable cause of them being too hot? Worn tubes?

S

[audiopete]

Check out R33 (2K2 4W) there - is that a small pinhole burnout on top? Test it with an ohmmeter. In fact test R31-34.

Also R8 and R9 look scorched. They are part of the LED warning on the power tube cathodes. Check the fuses that go with them too.

All signs point to fried power tube(s).

Those caps don't look bulged or bad to me. The label looks a little funny, but to me that looks like it came from the factory like that. Personally I wouldn't bother with caps until I fixed the burning issue first and then still had problems.

[sibo]

Check out R33 (2K2 4W) there - is that a small pinhole burnout on top? Test it with an ohmmeter. In fact test R31-34.

Also R8 and R9 look kinda scorched. They are part of the LED warning on the power tube cathodes. Check the fuses that go with them.

All signs point to fried power tube(s).

Nice! Unscrewing it again as we speak (had to reassemble cuz of my daughter running around x).
Any specific power tubes that would be good replacement? Is it important with matched sets? How do I adjust BIAS?

<3

[Audiotexan]

Just out of curiousity - how can you know that C29 & C30 needs replacing? What do they do? Excuse these questions that may seem stupid but I'm novice.

No question is stupid if it helps you learn/understand something! So don't worry, just ask!

They may actually be fine. However, when the label shrinks, it's usually due to one of a couple of reasons:
- Either it was designed with tolerances that were too close (meaning for example: using a 60v cap on a 55v line) which puts the component under considerably more strain or...

- It's just an old component that's endured hours and hours of stress under load.
Which is probably the case here. Knowing that the 900 series is what? Potentially up to 30 years old (if it was made in early 80s) So it's had a good life, but it's showing signs of heating (label shrinkage), which usually means the electrolyte is much closer to "shot", and not fully doing it's job any more.

It's just that the power supply section (or subsection as this is) is one of the most important aspects to any piece of equipment. If the power is not clean/good...then components will not operate at peak nor even properly (if things get really FAR out of whack).

C29 looks like 10uf/63v
C30 looks like 2.2uf/63v

Can't tell about C21/C22 due to the pic/angle... But, if you don't have a capacitor meter, if the cap is "out of round" (meaning it should be uniform width from top to bottom) then it needs replacement. If you run your fingers over the cap on the left (C21) and you feel the factory dimples, and then run your fingers over the cap on the right and don't feel the dimple, then that means it's expanded due to internal pressures, and should be replaced. Likewise, if you ever have a split (where the container has actually opened) it should be replaced.

Generally speaking, any bulges or deformations on a capacitor are bad. Same for any signs of leakage. Or past evidence of leakage. (kinda like a dried spot of soda on a desk...a little off-colored, almost brownish, sometimes white though).
Though don't confuse leaking/leaked dielectric with the "insulating material" that's used for stability like that between C31 and C32. That's there to simply keep components in place and/or limit movement. (That's usually silicon based, and in this case -black RTV/silicon)

[audiopete]

I like Mullard reissue EL34s in Marshalls and yes I would get a matched quad (you shouldn't have to pay extra for that). Ideally you would bias with bias probes to read current of the tubes. The schem shows the bias voltage to set at, and where to measure it, but I prefer to actually read the bias current, as different tubes will bias different at the same bias voltage. Also good to know plate voltage when setting bias current. Before I had bias probes I would use the output transformer shunt method, but since you are a novice, I don't recommend that. If this isn't your last amp you will work on, buy or build some bias probes - safe and easy. I have these ones - Dual BiasTester-MPD Probe from AMP-HEAD

[audiopete]

- It's just an old component that's endured hours and hours of stress under load.
Which is probably the case here. Knowing that the 900 series is what? Potentially up to 30 years old (if it was made in early 80s) So it's had a good life, but it's showing signs of heating (label shrinkage), which usually means the electrolyte is much closer to "shot", and not fully doing it's job any more.

JCM 900 series was 1990-1999 - http://www.drtube.com/library/schema...schemas#JCM900

That's why I don't suspect bad caps. Not saying they couldn't be, but this amp isn't that old and all those caps are in the low voltage section, so again I don't suspect them.