AVT150 (again, sorry)

[ajb]

Hi folks,

I have a Marshall AVT 150 which quit on me when I turned it on to soundcheck before our last gig. By luck I packed my effects pedal and managed to DI it. I had been running the amp in the morning that day so I know it was working as recently as 7 hours earlier.

It was raining heavily (well it was Glasgow) on the way into the venue and I was worried some water might have got in the vent at the top of the amp but this might be nonsense as I had it covered.

I've read through the posts and answers here but I'm kinda stuck with some of the terminology as I have no education in electronics and have only stumbled through repairs in a DIY basis for years.

The amp powers up, i.e. the power switch lights up and the fans are running. Therefore the fuse on the main board is good and power is flowing through at least part of the amp.

There is no sound from the speaker, no sound from the headphone socket when plugged in and no sound from the other speaker socket at the back.

The valve doesn't light up (I think it should glow?) and my mate gave me a replacement to try in case it was this but still no luck.

I've read that the faults might be some resistors (used as fuses), the TDA7293 chips or the mute circuit being shorted. The resistors I checked seem ok (tested with a voltmeter and showing resistance) but the chips and the mute cicruit I have absolutely no idea how to test.

I was going to order new chips and replace them but I've read stuff about using a variac (no idea what that is), something about releaseing teh voltage from some capcitors (not sure how to do that) etc to ensure I don't blow the new chips.

So .... short version is: can anyone give me an idiot's guide to what to do in order to safely replace the chips?

Cheers,

alan

[Jazz P Bass]

AVT150

If the tube does not "light up" and you have no signal from the headphone out, I would suspect the secondary power supply before the 7293 output IC's.
The secondary PS supplies the +15Vdc & the -15Vdc to operate the opamps.
It is also used for the tubes heater supply.

[ajb]

AVT150 (again, sorry)

Cool man. How would I go about testing to see if that's where the fault is?

By chance I just got the amp bit out to look for this and have found that the valve does have a small orange glow. Am I right to be going back to the chips then?

[ajb]

Sorry folks, again - can anyone tell me what I need to do in replacing the chips? Can i just solder them in and power it up?

Cheers,

alan

[52 Bill]

Before you start replacing the output chips, first be certain that the problem is not located somewhere else on the amp. In my experience with that amp, the chips always fail by shorting out. When this happens the fuse will blow and the amp will not power up. It is possible that your amp is the exception that proves the rule, but I would start by looking elsewhere first.

Have you tried plugging your guitar into the FX return jack? Try it, do you hear the signal?

Have you tried giving the amp a good thump with your fist? Does it cause any noise or sound from the speaker?

[ajb]

Hey, thanks for the reply.

One of my bandmates suggested checking the FX return socket at the time. I just tried it there again but no luck. I tried giving it a thump and hear nothing.

Something just clicked with me to try the headphone socket and I'm getting an output from it! It's not deafening but I've never used phones on it before so maybe that's just what you get?

It sounds to me like it might be the actual speaker or maybe solder dried somewhere on the circuit that goes to the internal (and external, I tried that socket) outputs?

Can't see anything else that looks like a fuse tho