My Marshall Blew a Filter Cap!

[Paul Marossy]

Got a Marshall Series 9000 (9005) stereo power amp that I bought from a friend along with an ADA 2x12 half-stack speaker cab for $100. Anyhow, I don't think it had been fired up for several years and had been sitting in a HOT garage for years in what is probably 140-150 deg. F in the summer. So, after having the amp for about 2 weeks, I finally powered it up and checked it out. I was really enjoying the stereo effects that I had going thru thru amp when I heard this noise that sounds exactly like that PSSTTT... sound you hear from that Pixar Animation lamp "popping the ball". Then it was no sound, and smoke pouring out of the amp, and a terrible smell. Took it outside and pulled the bottom cover off to find that one of the LCR filter caps literally blew-up and squirted this oily stuff all over everything.  

So, here is my question: After I have replaced all of the filter caps, what should I expect? I'm not sure what else could have possibly been damaged. BTW, the fuse never blew, either...  :?

[R.G.]

Anyhow, I don't think it had been fired up for several years and had been sitting in a HOT garage for years in what is probably 140-150 deg. F in the summer.

That's an almost perfect set of conditions for accelerated aging of big electro caps.

So, after having the amp for about 2 weeks, I finally powered it up and checked it out. I was really enjoying the stereo effects that I had going thru thru amp when I heard this noise that sounds exactly like that PSSTTT... sound you hear from that Pixar Animation lamp "popping the ball". Then it was no sound, and smoke pouring out of the amp, and a terrible smell. Took it outside and pulled the bottom cover off to find that one of the LCR filter caps literally blew-up and squirted this oily stuff all over everything. Sad

The cap in question started leaking more and more and the heat from the leakage increased the leakage - classical thermal runaway. I had this happen once on an old tube amp.

So, here is my question: After I have replaced all of the filter caps, what should I expect? I'm not sure what else could have possibly been damaged.

It's possible that nothing else is damaged. But test that carefully by making youself up an AC lamp limiter. Go to a hardware store and get a lamp cord, a light bulb socket, and a duplex outlet and box. Wire this stuff up on a board so you can plug the cord into the wall and have the light bulb in *series* in the hot/line lead to the duplex outlet.  Do the AC wiring in a safe manner, no sense in electrocuting yourself now.

Put a 50W or 75W bulb in the socket, plug the limiter into the wall, and the amp into the limiter. The bulb should be quite bright, then taper off to a lesser glow if the amp's OK. The amp should make some kind of listenable sounds (although possibly quite low volume before distortion) if it's OK. If the bulb stays brightly lit, something is still sucking too much current in the amp and you have more work to do.

Be sure you clean all the oily gook out. It's conductive (obviously)  and will prevent the amp from working until you get it out. You may need to completely disassemble the amp and use brushes, spray bottles, etc to get all the boards clean. Do this. Then dry the board with compressed air.

BTW, the fuse never blew, either...

No surprise. The fuse is there to prevent fires, not to save components.

[Paul Marossy]

Thanks RG.  

Yeah, that cap was still kind of hot 20 minutes after this happened. I guess that means I am going to replace all of them.  
Fortunately, that oily goop was mostly on the bottom plate. One PCB got sprayed with it, but it shouldn't be too hard to clean it off. I got most of it off right after it happened. I just have to go back and detail it now...

EDIT: What kind of caps are those LCR caps anyways? Are they paper and oil or like big honkin' electrolytics?

[tungngruv]

This makes me wonder about my amp. I read somewhere that any tube amp (mine is a 79' Marshal MKll ) over 10 years old should probably have the filter caps looked at and probably replaced. I just hate the thought of some amps (like mine :lol: ) not being original. Any ideas on the truth in this?

[R.G.]

What kind of caps are those LCR caps anyways? Are they paper and oil or like big honkin' electrolytics?

They're almost certainly electrolytic. If there is a polarity mark on them, they are electro; paper and oil technology is non-polar.

I read somewhere that any tube amp (mine is a 79' Marshal MKll ) over 10 years old should probably have the filter caps looked at and probably replaced. I just hate the thought of some amps (like mine Laughing ) not being original. Any ideas on the truth in this?

It's entirely possible that I wrote what you read.

It's like this: electrolytic caps are a wear-out item. They last longest when operated within 20% of their rated voltage and kept cool. If they are left on a shelf, the maker usually won't warrantee them after five years. Sitting still lets the oxide un-form in place. Keeping voltage on them keeps any weak spots formed up. If they're run hot, their life is halved for each 10C increase in internal temperature.  Cap makers try to achieve things like ten years of power-on life, at some assumed number of hours per day. Eventually, even carefully maintained parts die. When? Can't tell for one unit, happens at random. Over a statistically significant sample, you can say things like "at 30K power-on-hours, 10% of devices surviving infant mortality will wear out". Obviously I can't speak about every manufacturer of caps, but this is representative of the thinking.

A lot depends on whether you want your amp to work or not. If it's OK for it to degrade in performance over a long period of time, then fail at any random moment, and you have some idea that "original" is valuable, keep it original, run it til it drops, then deal with the problem. If you have the idea that you'll sell it as "original" to someone else, then again, take your chances.

If you use your amp on stage and depend on it, then you probably ought  to ask yourself each time before going on stage "Do I feel lucky today?"

There's another school of thought here. Some people want their amps to *sound* original. Amps with old, leaky, high ESR capacitors do not sound like they did when they were new. Some people have been *amazed* how good their amp sounded when it got a cap job.

In the end, one or more electro caps will die, and it will then be original, but it'll be an original piece of sculpture, not an original amp until some fixin' gets done.

Can you detect my opinion here? 8-)

[Arno van der Heijden]

I just hate the thought of some amps (like mine :lol: ) not being original. Any ideas on the truth in this?

Mmmkay... do you ever change the batteries of your pedals?....  :roll:

[ninoman123]

Filter caps dont change the sound of an amp at all. Most people replace them no problem and I dont really think it will hurt the value of an amp. Its better to replace those than have them crap out and ruin other more important stuff. I would recommend it.

[Paul Marossy]

My 9005 was built in 1990 from what I can tell. I guess that's nearly 15 years ago, and probably a 1/3 or more of the amp's life has been spent in a really hot garage for 6 months of each year for probably 5-6 years. I never really thought about the filter caps at all. This is a good lesson to learn. I will be real cautious about any amp that has sat around for several years exposed to the environment before ever turning it on! But, on the other hand, my father-ni-law gave my an old Tektronix 453 scope that probably hadn't been turned on for 10 years and was stored in a non-conditioned shed in his backyard in Simi Valley for at least that long. It's got a bunch of filter caps in it - it fired right up and hasn't ever given me a problem. You just never can tell with filter caps!

[R.G.]

my father-ni-law gave my an old Tektronix 453 scope that probably hadn't been turned on for 10 years and was stored in a non-conditioned shed in his backyard in Simi Valley for at least that long. It's got a bunch of filter caps in it - it fired right up and hasn't ever given me a problem. You just never can tell with filter caps!

Something tells me that Tek bought the 105C longlife caps...  

I will be real cautious about any amp that has sat around for several years exposed to the environment before ever turning it on!

Before the tube revival caught on really big, I made a few bucks finding old unloved tube gear, reworking it into life, and reselling. The light bulb limiter is a life saver on any piece of old gear.

[aron]

Before the tube revival caught on really big, I made a few bucks finding old unloved tube gear, reworking it into life, and reselling. The light bulb limiter is a life saver on any piece of old gear.

Yep. I still have mine. It works great. Thanks!

[tungngruv]

Mmmkay... do you ever change the batteries of your pedals?....

Yep!! :lol:   But a pedal's battery related to a filter cap :?:

Seriously though, R.G.'s comments about if I feel lucky made up my mind. Cheap insurance to make sure it will sound good in another 26 years!!!

http://home.grics.net/~tungngruv/mywebs/rig.jpg

[vanhansen]

I'm sorry to see that this happened, Paul.  This is exactly why I refuse to leave my Marshall head in the garage or attic for any length of time at all other than when jamming in a garage.  If it sits, it sits in the house.  I cherish it too much I guess but it's my only amp that's loud enough for playing with a drummer.  I hope you can get her up and running again.

[Paul Marossy]

The light bulb limiter is a life saver on any piece of old gear.

Yes, I remember reading about that at GEO. I forgot about it until you mentioned it, though. I'm going to be building one probably tomorrow.

I'm sorry to see that this happened, Paul. This is exactly why I refuse to leave my Marshall head in the garage or attic for any length of time at all other than when jamming in a garage. If it sits, it sits in the house. I cherish it too much I guess but it's my only amp that's loud enough for playing with a drummer. I hope you can get her up and running again.

Yeah, I keep all my stuff inside. It can get up to 115-120 deg F outside and it's even hotter in the garage. My stuff gets to be in a nice and cool environment in the summer (and winter). Anyhow, four new filter caps are on order. I should hopefully have it up and running by next weekend.  

[putrefusion]

Be sure you clean all the oily gook out. It's conductive (obviously)  and will prevent the amp from working until you get it out. You may need to completely disassemble the amp and use brushes, spray bottles, etc to get all the boards clean. Do this. Then dry the board with compressed air.

BTW, the fuse never blew, either...

No surprise. The fuse is there to prevent fires, not to save components.

I was under the impression that the liquid, being the dialectric (dielectric?) is not conductive?  Thus thats how the capacitor works... The foil is conductive and seperated from the other poles foil by the dialectric.  Now I wonder...?

[R.G.]

I was under the impression that the liquid, being the dialectric (dielectric?) is not conductive? Thus thats how the capacitor works... The foil is conductive and seperated from the other poles foil by the dialectric. Now I wonder...?

You're confusing capacitor types. In oil/paper capacitors, the paper is the insulator, the oil fills any voids and makes it impossible to have surface leakage around edges. The real plates are the metal foils.

In electrolytic types, the insulation is the oxide film on one of the aluminum foils. The other "plate" is not the other non-oxidized foil, but the conductive electrolyte (... hey... electrolyte... electrolytic...) that's just outside the oxide layer. The non-oxidized "plate" just serves to contact the electrolyte.

The liquid in electrolytic caps is and must be conductive for the cap to work.

[Paul Marossy]

I never took apart an electolytic cap, so that's why I asked the question if they were paper & oil. It was like an oily substance, not some thick gooey stuff like I would have thought. But it may have been oily because the cap got really hot before it blew. I cleaned off that stuff immediately because I didn't want it to solidify all over everything...

[Paul Marossy]

Last night I changed out the caps and built my light bulb current limiter. Tonight I will test it and the amp out.




Here's the cap that blew. Notice that hole at the center.  ^_^

[Paul Marossy]

Just to wrap this one up, it lives again! The light bulb limiter worked great and it sounds really good. Apparently the only thing damaged was the filter cap. Maybe I was lucky?  

[vanhansen]

Wonderful.  Glad to hear it!  There's not many things I dislike more than a dead Marshall.

[Paul Marossy]

I'm not an expert here, but it seems like the leaded axial type caps are a lot less prone to blowing than the LCR type cap. The only thing keeping the electrolyte in the cap is that rubber lid at the terminals. No wonder the thing blew, the rubber probably has been weakened by the heat in storage. I also noticed that two of the other caps have a small bulge in the same area as that one that blew. I think these LCR type caps are the ones to be really cautious about.  :shock: