Anyone know much about marshall AVT amps?

[El_Diablo572]

The first one is pretty simple. I recently changed the valve in my amp, to do this i had to remove the chassis and un plug the reverb tank from the board. Like an idiot, i didnt make a note of which way around it should go and there are no obvious markings to tell me either. It's a four pin plug with two RCA's which go to the reverb tank's send and return, any way to find out which pin is which on the board?

[El_Diablo572]

Obviously, i could just switch the RCA's around, there are only 2 possible way they can go, but i dont know if this will damage my amp if i get it wrong..?

[BuGG]

Are the plugs red and white by any chance?   If so, White is typically Send (Input to Reverb tank)...

You won't likely damage anything by hooking it up backwards though...

[El_Diablo572]

Eh, better to ask a silly question and look a bit dull than to stay quiet and look an absolute fool.

Thats that one solved. Next one is a bit more complicated...


The overdrive channel is fine, no problems, except maybe a bit of hiss which i think is more to do with my pre amp valve, however...

Theres something wrong with the clean channel on my amp. If i use it with the footswitch plugged in, the clean channel only works intermittently, and the volume seems to go up and down of its own accord, from nothing at all, to blow-your-windows-out loud, with no warning, no matter where the volume control is set. Sometimes it just cuts out altogether and the only cure is to switch it off and back on.

Also sometimes when on the clean channel it seems as though the clean and dirty channels are bleeding together somehow, and the LED indicator for the clean channel starts to flicker a bit. It almost sounds as though someone is manually turning the gain up and down.

Much googling shows that this seems to be a pretty common problem with the marshall AVT series, but i'm yet to find any solutions, so, can anybody here help me diagnose this and possibly find a solution to what might be wrong here?

[BuGG]

First of all, ensure that the footswitch itself isn't causing the problem.   Is this the original/correct footswitch for the amp? Is there a faulty/intermittent switch in the footswitch?   Is the cable damage or the plug dirty/corroded?


Next check the footswitch jack on the amp.   Does wiggling the plug cause the problem to appear?     Are the solder connections broken?    Ensure that the contacts of the footswitch jack aren't dirty/corroded.


This might not be the exact schematic for your amp model, but should give you a general idea of the channel switching scheme.    

IC3 and IC4 are switching dual op-amps, this controls your signal switching.  The clean signal goes into Input A (pin 2 & 3).    The overdrive signal goes into Input B (pin 6 & 7).   The voltage at Pin 1 (SW) controls which of these two input signals arrive at the Output (pin 5) and continues on to the power amp.


Looking at the channel switching section in the lower lefthand corner:

When the Clean channel is selected by SW1a or the external footswitch the Base of TR2 is pulled low, causing the Collector of TR2 to go High.   This pulls Pin 1 of IC3 High via the SWA line (activating the Clean channel of IC3 & IC4), and at the same time turns on TR1 to light the Clean LED.


The switching circuit is common to both the channel switch on the amp and the footswitch, so if your problem only occurs when the footswitch is plugged in I would suspect the problem involves the jack or the footswitch itself.

[El_Diablo572]

The footswitch is fine, i've had that checked and tested twice by "professionals" who were stumped when the switch wasn't causing the problem.

I've been using it like this for so long (always on overdrive) that i honestly cant remember how the clean channel behaves without the footswitch, i'll do some experimentation at the weekend and report my findings.

Thanks for the help man, i know where to start looking now, hopefully i can save my amp. it's a nice amp and its a shame it isn't as it should be.

[El_Diablo572]

Okay, finally got round to playing with this.

Seems that the amp is fine when switched between channels using the amp face, so the problem only comes in when the footswitch is in use, and like i said, only when using the clean channel and during switching. Does this narrow things down a little? Where should i be looking for the problem?

[El_Diablo572]

Done some more "testing" (pissing about), and i've found that the volume pot on the clean channel is really scratchy, could this be a part of the issue or is this just another problem that needs fixing?