Marshall JTM60 Output mystery??

[Nooner]

Looking for help.........I've got a JTM60 that has a weird problem. I had it serviced, new tubes, bias , Transformer, output PCB. When I got it back it would work for a while then start to output low almost no sound then come back and then go out. I took it back to him and after having for 3 months says he can't figure it out. I took it apart and cleaned it up and checked the caps and looked for cold solder joints. When the chassis is out of the cab it worked fine. When I put it back in the problem came back. I did this a few times with the same result. For kicks I put a jumper cable across the series effect loop and it seems to work. My question is what do look for now or should I just keep the jumper in place. I'd like to solve it so I can sell it! Any ideas welcome. Thanks.

[Amp Kat]

I had these do that but it was not the effects loop but that is a symptom of corroded loop jacks. The ones I found were the heaters dropping out on the preamp tubes from cold solder joints. If that fixed it then good enough and you have two options. clean the jacks with a deburring tool and resolder the joints or replace the jacks.The latter would be the way to go.

[Nooner]

Thanks. I'm going to monitor the amp for a couple of days and if the jumper doesn't cure it which cold solder joints should I look for. I have a schematic and have taken this apart a few times so if you could point me in a direction I would be grateful. I'm still wondering why it doesn't cut out with the chassis out of the cab but starts as soon as I re-install it.Also why jumping the series loop solves it. Maybe a PCB flexing or........? I hate PCB and heat/stress. I just want get this fixed so I can sell with a good mind set and get a JCM 900 head or something heavier duty,

[clone4259]

same problem

Hi i had the same problem so used the jumper also and i put a cheap PC case fan under the power tubes i did try to power it from 1 of the amp pts(tapped into a 7.3v lead ) but it never worked so used a wall wart, up till now its been fine
i lov this amps sound clean/dirty but from what iv read on the net about it they can be a pain in the ass so might have to sell it

[Rick Erickson]

which cold solder joints should I look for.

All of them!

[6267]

Make sure the PCB isn't shorting out to chassis. Having one leg of the phase inverter grounded would produce those symptoms. It would also work when removed in that case.

[Nooner]

Good idea concerning the PCB grounding. Thanks.

[Nooner]

which cold solder joints should I look for.

All of them!

Funny.................not.