Marshall Avt150H

[Dennyps4]

Hello Everyone.

I'm a noob amp repair man, and just trying to track down the issue with fuse problem, and the av150h. I took the plunge, and bought a busted one off fleabay, and am trying to prepare. Guy stated that it blows fuses, but does power up, I then asked if he heard any sound, and advised that he had no speaker hooked up? Duh, anyway, it's on its way. I'm at Mouser and see that there are 2 models of the tda7293, one ending in V and one ending in hs. Is the hs any more durable, better? My game plan is to completely disassemble the head and clean the crap out it, disconnect all effects, reverb, (ribbon cables I suspect) etc.... and bench with just the main board, new fuse, new tube. Hooked to a cab. Am I on the right track? My goal is to hone my circuit skills, to make a little extra cash, since I've been out of work for over a year after back surgery. This stuff will keep me from going crazy, I think. Sorry for the long winded post, and thanks in advance.

Dan

[acorkos]

if you do need a tda7293, get the V version...V is for vertical, HS is for horizontal mount. your amp needs the vertical version....and besides, the HS version is not stocked at mouser

TDA7293 Data Sheet

also check the rectifier bridge (rectifier diodes?...not sure which you have)...another common reason for fuses blowing

[52 Bill]

Welcome to the place. Sorry to hear about your health problems.

I'm at Mouser and see that there are 2 models of the tda7293, one ending in V and one ending in hs.

The difference is in how the leads are formed for mounting V=Vertical H=Horizontal.

My game plan is to completely disassemble the head and clean the crap out it, disconnect all effects, reverb, (ribbon cables I suspect) etc.... and bench with just the main board, new fuse, new tube. Hooked to a cab.

First off, when you get this amp, you should repair it so that it works before you do anything else. That way you will not be creating possible unintentional problems with the amp. If in fact the output chip is DOA, then fix that and then see what else it needs.

My first suggestion would be to build yourself a light bulb limiter, as this will come in handy for this and any future amps that you will work on.

[Enzo]

Right, the V and H versions of the IC refer ONLY to the shape of its legs.

The AVT150 has two YDA7293s on little power modules. If one is blown ALWAYS replace them both.

And what Bill said. DOn;t go tearing it apart adn dismantling a bunch of stuff, that is more likely to mask the problems than to help. Fix it first, then worry about detailing the thing. The tube is likley to be just fine.

The solid state amp does not need a speaker load to be operated.

[Dennyps4]

It's coming Today

Ok, I get it, again, sorry for the noob factor. I'm competent all day with pc's and os'es, but amps elude me, Thanks for being kind. Why is this particular amp so fragile? I have seen all kinds of issues, from researching, with the power section, it seems. I am totally new to properly reading schematics, but I do have all of them, and notice that the power module seems awful wimpy for a 4 channel amp, not that I know what I'm really looking at. Lol Any opinions on how to beef it up a little, by more grounds, better resistors? Is there a commonality to why this thing blows fuses, or could it be anything? Seems Marshall really dropped the ball in the engineering of this amp. I was foolish in buying this, but will be a good learning experience in what not to do, building an amp, I think. Thanks again guys.

[Dennyps4]

Also will do as instructed. I will build the bulb tester tonight, given the fact that it is a 100w amp, I should use at least a 100w bulb, or higher, correct?

[52 Bill]

A working 100 watt output amp will not draw that much current at idle, so the wattage of the lamp doesn't really matter for testing. Use either a 60 or a 100 watt bulb.

The bulb is used to limit the current going into the amp under test. If the amp is working ok, then it will not produce a large current draw and the bulb will barely light up.

If there is a fault in the amp like a shorted output device that causes it to draw a lot of current, the bulb will light up brightly. Because the bulb lights up, it takes all of the power thereby reducing the current going into the amp. By reducing the current into the amp, there is less chance that there will be additional damage done to the circuitry while diagnosing and repairing problems.

[Dennyps4]

Thanks Bill, I appreciate all of you, who volunteer your time and knowledge, to help us wannabe techs, for nothing. When I finally get the thing I'm sure I'll have many more questions. At least I know where to start now. If there's anything I can do for you, let me know. Thanks again.

[Enzo]

I don't know that the amp is fragile. They have sold a bazillion of them, and a certain percentage fail. Keep in mind there is a difference between this part fails all the time and IF the amp fails, this part is the likely failure.

And also keep in mind that the number of preamp channels has nothing to do with the power amp circuits.

[Dennyps4]

Well, I got the amp, and looks like someone else had a go with her. Check out what I get to clean up. I think I know where to start now. Lol Can I use some arctic silver on these? Or, is a there thermal barrier that is supposed to go here. There are also a couple pins that aren't even soldered to the pcb. Pretty ugly. It may, or may not, be easier than I thought, as long as the tranny is still alive. What's the big box for? Lol

[Dennyps4]

Not even going to think about plugging this thing in yet.

[Dennyps4]

Also every screw on this is stripped. Looks like something I tried to fix when I was 15.

[Dennyps4]

I saw in a another post. If I unplug the header to the tda chip(s), and the unit powers fine. The rest of circuit should be fine, maybe? Aside from the mute possibility. I'm going to order the chips regardless since the amount of paste on them, fried them, instantly, I'm sure. Thanks Again.

[Ronsonic]

I saw in a another post. If I unplug the header to the tda chip(s), and the unit powers fine. The rest of circuit should be fine, maybe? Aside from the mute possibility. I'm going to order the chips regardless since the amount of paste on them, fried them, instantly, I'm sure. Thanks Again.

Paste does not hurt components. What we see here is needless, wasteful and causes me to distrust the previous tech, but the paste ain't the problem.

Your idea of disconnecting the power PCBs and attempting a power up makes sense. Basic divide and conquer strategy. Get set up with a lamp in series with the outlet you plug this into for testing.

Here's my suspiciousness. The outputs have obviously been replaced, but it ain't working. Is this the second time the outputs have blown OR is it not the outputs and having reached the limits of his troubleshooting skills sold the amp.

If you hear even a peep out of the fans, any undue whirring, buzzing, tapping noises, ANYTHING except a gentle rush of air, replace them.

The TDA7293 has blown up in lots of amps. On manufacturer called its use of them "the scourge." Never plug in or unplug while powered up.

[Enzo]

The little three wire connector on each module is the power connection. When you go to reconnect them after replacing the ICs, DO NOT plug the connectors on until you verify there is no charge left in the power supplies.

If the amp is silent, check the standby circuit. And make sure the headphones jack is not cutting out your speakers.

That heat grease job is messy but has nothing to do with any electrical failures, unless they left out the insulating mica washer.