Marshall AVT150H

[macheted01]

Hey there everyone,
New to the forum here. We have a Marshall AVT150H head. From what I can tell from other forums here, they all seem to have the similar problem with the TDA7293 chips. We have just replaced these and still having some problems. We are looking for a schematic on this amp. If anyone could help it would be greatly appreciated. Thanks in advance.

[vintagekiki]

Hi mach
Today is your day. You're lucky man. Here is schematics for Marshall AVT150 & 275

[Enzo]

But you don't mention what the problems ARE.

If the amp still makes no sound, check the mute/standby circuit. Look on page 5, bottom center, T104,C217 and a zener. If one of those shorts out - as sometimes happens when the 7293s short out internally - it leaves the amp in a muted condition.

[macheted01]

Hey Thanks alot for the quick reply. It does get sound now, but, when trying to turn it up it peters out. Total loss of power. The guy that I have looking at knows more. I will find out exactly what he is talking about. He was in need of the schematic to trace some other angles down. Thanks again.

[Techead53]

Hi all, i am a new member here. I have a Marshall AVT150H. fault is with the B output module. If I disconnect CN2 on the module the amp will work ok on just the A module. When module B is connected the amp produces a very loud hum at the speaker as if dc is being applied to the output. I have looked at the schematic and see the two modules are used in a bridged configuration. If I run the B module from the A module connections it also works ok.So there is nothing wrong with the IC 7293 or the complete module.In fact I had previously replaced both A and B IC's as the main fuse was blowing. I think the fault may be with the two capacitors on the back PCB circuit, will go there next . Anyone have a view or seen this fault before? Regards. Hans.

[hamahakki]

New member here as of 5 minutes ago!!!

I have an AVT150H that is giving me troubles too. The overdrive 2 channel is experiencing major power loss, and sound much quieter than the other channels. It seems to be getting worse with time too. Anyone have any ideas? I've had an amp tech look at it and he couldn't find anything wrong with it.

J

[Enzo]

Hi tech and hama, welcome to the forum. Please take this advice, each of you please start a new thread for your own amp. I suggest in the guitar amp repair section. Adding onto the end of this thread here will result in three diffferent amps with three different problems all trying to be discussed at the same time. No one can keep that straight.

Tec, if you have two good modules, and the one plugged into the B cable results in hum, then see what is on that cable. Is one of the power rails missing there? The same power supply serves both modules, so I am not sure how a filter cap coul dbe the problem.

Hama, look at the output pins of the op amps in the overdrive channel on the front board. If any show DC offset of several volts, replace it.

[Techead53]

Hi Enzo, many thanks for the reply. As new not sure how to navigate this site yet to make a new thread.

The AVT150H fault I think you have misunderstood me. Both amp modules work as a single amp if connected to the connection for the A amp one at a time. I clear all power and reset connections before powering up as I know how sensitive the TDA7293 can be.Neither amp module will work on the B amp connection. It was not the cap's in the supply rails I was going to look at but the C191 and C216 , both 22uf @ 50v in the bootstrap circuit.

However, I have just looked very closely at the circuit and can now see my problem.
First and main problem, ME , I should put my glasses on!!!!! I have mixed up my stock of TDA7293 and TDA7294. I see from the chip data that TDA7294 does not use pin 11 and 12 these are NC no connects. You can use TDA7294 in place of a TDA7293 in some application but only if the boot-loader or bootstrap are not being used. Also if you want to use TDA7294 in bridge I see from the data sheet you must not use a load of less than 8 ohms.

Fitted two TDA7293 and all is well. So now to fix the reason for the amps going, both small fans have failed. Does any one make a better fan for these? These ones look cheap and I suspect they have sleeve bearings and not race.

On a good note, it made me look deeper into the circuits of this amp and has increased my understanding of it.

Best regards. Techead53 ( Hans )

[Enzo]

SOme fans are 24v and some are 12v, I forget which models use which. But the 12v fans are common CPU fans as used in your computer. They are not expensive and can be obtained from many computer parts sources.

[Showey]

I replaced the TDA chips as well as this amp just kept blowing fuses.
No longer blows the fuse but has a LOUD humm / buzz thru the speakers when switched on and is non responsive to the volume or any other control. May be DC voltage?(havn't checked yet). This amp came to me in a non-working state and I have no history on it. I can see someone else has been trying to fix it in the past and the traces on the power amp boards are questionable. So where do I start to resolve this loud buzzing noise?
Thanks in advance.

[Enzo]

Get the speaker load OFF of it NOW. get out your voltmeter and FIND OUT if there is DC on the speaker leads.

Then check the power rails. Are both positive and negative rails present up on the power modules? Are they about even? Does either one have a ton of ripple on it?

I don;t remember, is the whole heatsink floating, or are there individual insulating mica washers under the TDA chips? In either case, are the tabs atop either TDA grounded? SHouldn;t be.

And are the preamp rails, the +/-15vDC up and clean? And is the preamp out OK? or the FX send?

[Showey]

Well, Last night just experimenting I probably did something really dumb. I unplugged the module to the right and powered it up and nothing, no change,
and no sound (not even the buzz) So, I discharged the caps, plugged the right side back in and unplugged the left side. This time, Buzz BANG!! LOL
the right side chip exploded... So kiddies!! This is what NOT to do!
Really was spectacular though!
Ok. I'll get two more chips, I'll make DAMN sure the little washers are in
place to the heat sink. ( I'm pretty sure they were) and do some further testing.
I was really just trying to shotgun it and get it working again.
Sheesh, what a pain in the......