Marshall AVT-50 volume drop-out

[Jemster]

Hi,

Just found this board as I was looking for AVT-50 schematics - fantastic resource and I'm hoping someone here will take pity on me

My AVT-50 combo has developed a fault and I'm wondering where to start looking... Sometimes I'll turn the amp on and it'll work fine but more and more frequently it's starting to drop out the volume on both clean and gain channels. By dropping out, I don't mean a total cut-out, more like a reduction to, say, 1/10th volume. There's no noticeable dodgy pots, turning up the volume while it's dropped out does have an effect and to some degree I can compensate but then it'll suddenly cut back in at full power.

At first I thought it was the entire amp but last night had a CD player plugged into the CD input and its volume remained constant while my guitar kept dropping out. After an hour or so of erratic behaviour the amp seemed to settle down so I'm thinking perhaps its heat related, although an hour seems a long time to settle down. The only thing I've tried is a different tube as I'd replaced the original with an Electro Harmonix and kept the old one as a spare. Both tubes show the same issue so (probably) not that...

I've also read about the R58 weakness but from the schematics it looks like that would affect the CD volume as well as the guitar so I'm guessing it's not that one

Any pointers on what I could look at / test for would be appreciated - it's still too intermittent a problem (IMHO) to pay a repair shop as I'm guessing it would take them too many hours to locate the issue and be a costly fix for something that's hopefully not major.

[acorkos]

is there a preamp tube in your amp? might need replacing

clean the effects loop jacks....spray cleaner and insert/remove/reinsert a plug several times.

is it your input jack? does your signal drop out if you jiggle your guitar cord plugged into the input jack?

[Jemster]

I put a tube in last month and couldnt remember if the problem started before that (I only acquired the amp a few months back) so I put the old one back in and the problem remained so I don't think it's that.

It's not the input jack or cord, no amount of jiggling / plugging in & out makes any difference.

However, I've just tried the effects loop jacks (although I don't use the loop usually) and at the moment the problem hasn't repeated yet.

If such a simple thing has done the trick, thanks very much I'll report back in a day or two.

[CharlieP]

I put a tube in last month and couldnt remember if the problem started before that (I only acquired the amp a few months back) so I put the old one back in and the problem remained so I don't think it's that.

It's not the input jack or cord, no amount of jiggling / plugging in & out makes any difference.

However, I've just tried the effects loop jacks (although I don't use the loop usually) and at the moment the problem hasn't repeated yet.

If such a simple thing has done the trick, thanks very much I'll report back in a day or two.

I feel certain that if you spray contact cleaner into your EFX loop jacks and guitar input jacks and do as one above has said, work a plug in and out and repeat cleaning, problem will be solved. When an EFX loop is not used it builds up a coating on the switching contacts. This is common. You already affected it and it leads to a cleaning need.

[Jemster]

Thanks for your help guys, amazing that something so simple caused me such grief. I didn't even have to hunt out contact cleaner, a few quick insertions & removals of the effect loop jacks has sorted the problem out. If it comes back, I know what to do

Cheers!