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Post subject: Let me pick your brains ... amp query
Posted: Tue Jan 22, 2013 8:40 am
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Hey guys,

My friend has a Marshall JTM30 (112 combo) with an odd problem. He has already taken it to two authorized amp techs and nobody has been able to get to the root of it because they can't seem to get it to replicate the issue. I figure the collective amp wisdom here is prolly a better bet than his two techs put together so I'm just wondering if anybody has any thoughts.

The issue is the amp will warm up and play just fine for about a half hour or so and then it will fade out rather quickly and go dead silent. It doesn't pop or make any odd noises but the volume drops to nothing, ramping down quickly in about a second or two. The techs have apparently swapped out all the tubes, cleaned all the pots and checked for cracked or incomplete solder connections. They've let it heat up and subjected it to vibrations. It won't fade out for them but when he gets it back it lasts about a half hour to an hour and dies. He lives in town and so his power comes off the grid like everybody else. It will do it for him at home and when he plays out but he can't get it to do it for the tech. (Murphy's Law, I guess)

I'm hoping somebody here has a suggestion to point him in a direction for his tech to look for the issue. I have given him the contact info for "MY" amp tech but my guy is always swamped and he always has a backlog that takes 3 or 4 months before he can get you in. I was hoping if somebody had any ideas we might be able to get to the root of it sooner.

Fingers crossed.

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Post subject: Re: Let me pick your brains ... amp query
Posted: Tue Jan 22, 2013 9:22 am
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It is almost certainly a poor connection, maybe in the signal chain, maybe even the standby switch. Solder joints are the usual suspects, but also have a close look at all of the plug in connectors (such as "COM 1"). When the amp fades out, that is the best time to open it up and have a look around with a meter to see if all of the supply voltages are present at the tubes before turning it off. This might mean having the chassis out and on the bench, bottom up, while playing, waiting for the amp to fade out. :idea:

I have to point out that some techs will not completely disassemble the amp for a close solder joint inspection, especially if the fault doesn't occur while at their shop. :wink:

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Post subject: Re: Let me pick your brains ... amp query
Posted: Tue Jan 22, 2013 11:01 am
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Thanks, Shimmi.

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Post subject: Re: Let me pick your brains ... amp query
Posted: Tue Jan 22, 2013 11:49 am
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It'll 99.9% be a dodgy solder joint on a JTM - the way it's designed, there's nowhere for the heat from the tubes to go other than into the circuit board. Pretty much all of them go that way at some point (both of mine did!), most techs are well aware of the issue and advise fitting a small fan which solves the problem completely once whatever has been damaged is repaired.

Heat is a huge reliability problem on a lot of modern PCB based tube amps, whoever designs these things really doesn't think ahead very far. :shock:

EDIT - just remembered, have him check the effects loop jacks, as stupid as that sounds - they go dodgy on a regular basis on the JTM and cause similar problems. Pop a 1/4" patch cable direct from send to return and see if that changes anything. Wouldn't be the first time I've known a tech to overlook the completely obvious and go hunting for problems which don't exist. :lol:


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Post subject: Re: Let me pick your brains ... amp query
Posted: Tue Jan 22, 2013 5:49 pm
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Had a nearly identical problem with a Blackstar amp I own. I blamed the tubes and changed them all...still a problem. Took it to the warranty repair station and it was a bad solder joint. Took the tech a while to find it, because at first it wouldn't do it.

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Post subject: Re: Let me pick your brains ... amp query
Posted: Thu Jan 24, 2013 5:38 am
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It's probably the same thing that happened to my JTM60212. (the symptom you explain is identical to what mine was doing)

a failed capacitor I believe. I took it in for servicing, had him re-tube it (said the tubes were fine) it was a small $2.50 part that was giving out on mine.

Just have him take it in to a repair tech that knows wtf he's doing, and it will get repaired.

Once I got it back with the new tubes, it sounded better than ever.


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