Marshall AVT150H Valvestate 2000 Power Amp Problem

[fenster94]

Greetings,

I have recently repaired a friends amp due to blown TDA 7293 Power Amp failure. Thanks to all who have posted info in regards to this. The posts in this forum allowed me to troubleshoot and fix the problem on two different occasions now.

To summarize the problem, the fuse was blown and both TDA 7293 chips were blown. Therefore no worky. The first time I fixed this amp it was the same problem, bought some new chips and re-fit them, all was well. It worked for about a year under heavy use and then the same thing happened. This time, bought new chips, changed them out, turned it on and then immediately blew the new chips.

After evaluating the Vs levels on the chip, solder connections on the power amp module, and other points of continuity, this is what I found.

The Vs levels were +- 49V so this was good and no other burnt components in the amp, so after closer inspection I found pin 12 on the TDA7293 (Bootstrap Loader) was not making connection with pin 8 on the power amp board connector, the pad was mangled from soldering/desoldering. I installed a jumper wire on the back to fix the problem and to eliminate future problems, I removed the socket pins from a typical IC socket I had, and soldered these into the holes of the power amp PCB. Now when these chips go bad again, I can simply plug a new one in rather than risking more damage to the PCB

After checking all connections and voltage levels again and then again, I installed two new TDA7293 chips into the sockets, reluctantly turned it on, and thankfully all was well again.

Does anyone have any input as to why the bootstrap loader pin being disconnected would cause a short that would smoke the chips upon power up? Thanks.

[poison]

I have 2 of these amps in a rehearsal room i operate. Both have run really well but just today had a burnout.

The amp smoked and cut off witrh the ever distinct smell.

What is this likely to be? I am new to amp repair but comitted to learning, so i can sort these thing out with out costly repair centers.p

[singa]

Hi,
It's probably the tda 7293 chip like the story above.Replace it and it will work like new but check all pin connections before and after soldering like
the pin8 is making contact with the relevant component elsewhere.Also if other component is shorted or pcb track broken before switching on again,it pays to check.Ps make sure you apply silicone /heatsink grease to the back of the 7293 chip for proper heat conduction, it comes in a tube like toothpaste.And don't overtighten the screw.Just tight enough will do.
Regards.

[B12a3d]

I had the same problems and have now replaced the chips twice. this time the amp turns on and fans come one and no blow up but also no sound...? any ideas what my problem is?

[jaycee]

Honestly, I think that the problem here is the TDA chips are simply being worked too hard with inadequate cooling. Guitar amps have to be very RUGGED as playing pushes them into clipping and distortion which greatly increases stress. I am not a guitar player myself but i would be weary of any solid state guitar amp (except perhaps small practice amps) that do not have oversized discrete output stages.

[B12a3d]

Well what am I to do than? The amp comes with the TDA and worked for over ten years... now the amp itself is working I'm just not getting sound from the speaker when I plug it in, not even a hiss or crackle.

[llwhtt]

If you have a schematic check the stby circuit that goes to pin 9 on the IC.

Craig

[jaycee]

if you have replaced the chips twice, there are probably more components damaged, or the PCB itself is now damaged. Especially so if the chips failed and scorched the PCB

[B12a3d]

I checked all of the points on the pcb coming from the pads/holes that the TDA fit in and all good. I don't think the amp would turn on if those parts were not working or it would fry them if that board was bad.

[amptech]

I would agree with jaycee.. there's more fault's with these unit's I repaired 15 or so and all with the same fault crappy o/p stage's broken front pot's and damage preamplifier stages etc...yawn...

to ensure the front end is ok remove the blown o/p stage and power up, next feed a signal in and use the line out on the rear and link this to a scope if handy or a small test amp. now turn up the marshall's pre-gain just a little followed by the master that done turn up your test amps volume to hear if you have clean level. if you have clean sound and tested all the channels for the same level's the front end is ok.

Now, the ht is high for the tda7293 and is close to it's limit no wonder it goes pop..bang and smoke.. oh did we forget them fans as they start groaning on start up! plus them under rated power cap's 'only a 1000uf '...up grade them to 3300uf or 4700uf 63vdc much better now on to the power stage...you could get replacement o/p power ic and board and end up with the same fault time and time again...here's a good fix that will last better than the 'old chip' stage..just fit a power stage that will work on the supply rails say +/-dc of 55v..60v and tap the signal from the main pre-amp and there you have it..
I've done is to all the one's I've repaired and there still in working order..
there's plenty of kit's out there that will do the job..I built a power stage fitted them to the amps. line 6 amps use these ic's at less supply rails and last better!