Marshall MG250DFX - Smoke Generator!

[Andy1960]

Hi. I know this is an old post but did anyone ever find a definitive answer to this seemingly common problem?

[MarkusBass]

Hi. I know this is an old post but did anyone ever find a definitive answer to this seemingly common problem?

I don't remember the details but the problem (and solution of the problem) was described on this forum. Some problems were caused by users trying to fix the amp but forgetting to isolate the IC from the heatsink. But, as far as I remember, the main problem was that one of the pins of the IC wasn't connected on the PC board as suggested in the datasheet by the manufacturer of the integrated circuit. Maybe someone else remembers more details. You have to either search the forum, or look very carefully at the datasheet and compare it with the board you have. It seems to me that the original design was for an old version of the IC. So you have to find out a new datasheet for currently manufactured IC.

Mark

[Andy1960]

Thanks for that Mark, that throws a spanner in the works! Ive trawled all through for a definitive answer but found nothing so far. Id thought about the IC isolated from the heatsink but the heatsink is actually isolated from the chassis so shouldnt be a problem. The rogue pin is a new one on me, I hadnt read that before, Ill check the data sheet.

[MarkusBass]

Another problem that I remember (but it is not the case you have) was that some company in the States was selling replacement boards for the power amp but they made a mistake when designing the PC board. As a result all replacement boards were failing on first turn on of the amp. This case is also described on the forum.

Mark

[Andy1960]

Yes I read about that. As you say unlikely in my case as Im in the UK so even if they have been replaced its not going to be with US ones I wouldn't have thought. Everything runs fine with the boards unplugged and Im loathe to believe theres anything wrong with the electrolytics on those boards although that was suggested on one thread on here. They all measure accurately on a capacitance meter anyway.

[MarkusBass]

What about contacting Marshall directly:
Denbigh Road
Bletchley
Milton Keynes,
T: +44 (0) 1908 375411 ?

Mark

[Andy1960]

Are they helpful in that way? Some manufacturers dont like giving out repair info. I used to be a tv engineer years ago and every manufacturer had a tech dept who would help if you were in the trade. In the end they started charging for it. I'll give them a try

[MarkusBass]

I would give them a try. There is a big chance that they are "user-friendly".

Mark

[Jazz P Bass]

Somewhere around 2003 the TDA7293 Slave mode pin hookup was changed by the IC manufacturer.
It was now recommended that on the slave ic, Pin 2 & 3 (Slave +In/ -In) & pin 4 (Slave Ground) be tied to the V- rail.
On the MG250's, these pins are at a ground connection. (like the older datasheet indicates).

Whether or not this causes an issue was not really indicated.
There was a follow up post to this issue where the OP did make the changes & the amp did function properly.

At any rate, the key to working on these amps is that the power supply must be drained before working on the circuit.
And most importantly before the IC's are reconnected to the board.

Edit: the main post on the TDA change concerned a Marshall Mode 4.
It should also apply here.

Original post: http://music-electronics-forum.com/t3873-2/#post359980

http://music-electronics-forum.com/a...cb-modules.doc

[Andy1960]

Hi, Ok so are we saying that the chip itself has changed or just the recommended connections? I did read something similar from a guy wanting to use a 7293 in place of a 7294 I believe. Yes I know about draining the caps I've been careful with that.

[Jazz P Bass]

I do not know if there was an actual internal change to the IC.
I do know that the 2003 datasheet is different.
(the 2010 datasheet shows the same slave hookup as the 2003)

davman actually contacted a rep with this reply:
(http://music-electronics-forum.com/t3873-3/#post361875)
"I contacted a Field Applications Engineer from the chip manufacturer, who said it would be difficult to find the relevant info to explain the change - however, any TDA7293 purchased after the the latest (2003) datasheet should be wired to the application circuit recommended there.

In other words, the 2 Slave TDA7293's must now have +IN & -IN wired to -Vss. "


The 7293 & the 7294 are different chips.
I would not swap them.

[Andy1960]

the mg 250 though just has the one chip per output

[Jazz P Bass]

the mg 250 though just has the one chip per output

Oopsie.

Belay the last posts & file it under ("Parallel/ Slave Changes.)

In that case, there are no known issues with the IC.

Other than there inherent limitations which tend to be exceeded.

[Andy1960]

Just spoken to Marshall and they said the only problems they had with this chip was either because of not discharging the power supply before connecting (which i have) and the heatsink being shorted to the chassis (which it isnt)

[52 Bill]

Andy1960- You have asked about a definitive answer to the problem, but you have never stated exactly what your problem is. Are you blowing up new chips? Are you testing without a speaker load? Are both outputs dead? What have you checked so far?

Please help us to understand what your situation is and maybe you will come up with the definitive answer to these problems. I haven't had to fix a lot of these, but they don't seem to be any worse than the bigger multiple chip ones. Once you get it working, they seems to work just fine until somebody does something to make the output chip unhappy.