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The past couple of days my marshall avt20 will suddenly drop from normal volume to about half volume without any change in tone after playing for about 30 min. Where should I begin in looking for the problem?

With an AVT you might just have to take it to the doctor, who will probably rip out and replace whatever board has the failing part.

ok thanks any idea on cost and if continuing to play it will mess it up more?

Cost may be more than the amp is worth. Looking at ebay for market, most likely so - $100-125 seems to be the prevailing market.

I'd assume it's probably a failing cap or resistor that drifts out of spec once it gets hot.

Eventually it may fail, or it may not.

First step would probably be take the chassis out of the cabinet and blow out dust etc if it's present - it could be as simple as too much junk in the wrong spot built up over the years. It's not the most likely problem, but it'll be free to find out.

Also, does the problem occur with headphones or just the speaker/powered out?

I haven't used it with headphones. Last night I got the clean channel working which didn't seem to work originally and it seems to work perfectly like that. For the $80 I spent I'm not sure if the price of fixing it would be worthwhile relative to price paid

For price point etc, just blow out dust, use 'til it blows, if it blows, and then buy something else!

Would I open the top bottom or both on the back?

Not sure, I don't have that particular model (my last Marshall was a 2x12 Chorus model, I'm not a huge Marshall guy).

When in doubt, take photos of what you're doing with your cell phone camera (since everyone on earth has one except me) and backtrack based on that.

Also be careful, as there are capacitors in there which will be full of charge even when the amp is unplugged.
You can get quite a jolt from them, and to risk sounding like your mom, could be fatal if you were unlucky.

Just play it till it blows!

It seems to work on clean channel so I'll just leave it. I typically play with minimal amp gain anyway

Problem evaded!