Another Marshall MG100HDFX question...

[mccdam28]

Hi all,

So, I have used a MG100HDFX for sometime now and have never had any problems with it until last week! Turned up to band practice, plugged her in,was working fine. Few minutes later no sound and ALOT of smoke! She is now dead to the world.

Any ideas what this could be? I have been reading the forum and looking at the schematics, could it be an output transformer issue (does it even have an output transformer?) or an IC which is gone?

Any help, tips, pointers would be hugely appreciated to get her back up and running!

Regards,
Damon

[drewl]

Yikes.

Unplug amp, pull chassis out and look for burnt components or marks on the pc board.
There is a multi pin IC amp mounted on the back wall of the chassis, that's probably bad, and may have taken the power supply resistor feeds to it also.

[soundguruman]

It's usually a zeener diode that blows in the power supply, shutting off the "standby" voltage.
But also could be the output module IC amp. These modules are very hard to get, there is a shortage, which has driven the price to triple.
All in all, these amps seem to have a lot of problems.

[mccdam28]

Intresting. I might strip it down and see what has blown.

Cheers all, will keep you updated.

Regards,
D

[drewl]

It's usually a zeener diode that blows in the power supply, shutting off the "standby" voltage.
But also could be the output module IC amp. These modules are very hard to get, there is a shortage, which has driven the price to triple.
All in all, these amps seem to have a lot of problems.

The TDA7293's?

I usually buy a few at a time when I need them, always seemed to be available.....I just checked...plenty around for about $6 each.

[soundguruman]

Check the diodes that are located next to the voltage regulator heatsink. That one tends to blow.

[Jazz P Bass]

If the output IC is the culprit, what you will usually find is one or both of the voltage pins (13 & 15) are shorted to the output pin. (14)
Or the voltage pins may be shorted together
Pull the cable connector that goes to the TDA7293 board & check with a meter.
As already posted, the power supply must be checked out.
And please remember that the power supply must be "drained" before you reinsert the TDA board connector.

[soundguruman]

It's D-17, 1N4148, is the zeener that blows, now that I'm looking at the schematic.

[J M Fahey]

Ahem, what voltage would that 1N4148 Zener be?
Can´t find it in my Zener Data table.
Thanks.

[Enzo]

yeah well... If he got smoke and now the thing is dead - like blown fuse - I'd be willing to wager my lunch money the TDA7293 is what failed.

[Jazz P Bass]

It's D-17, 1N4148, is the zeener that blows, now that I'm looking at the schematic.

I believe the standby zener is D14.
6.8V/ .5 watt.
IN5235 would do.

[soundguruman]

Whoops sorry, too much something, I miss-read. It's D-14, it says 6.8 volt sorry.
My bad. Must have been cross eyed and stoned or something.

[mccdam28]

the IC strapped to the fan as a nice big hole in it and a look for dead. Just put some on order so hopefully will be able to repair it at the weekend. A lot of talk about "check the power supply", what should I be checking? Feeds to the IC are correct etc?

p.s. thanks for all the help, really enjoying this forum, lots of helpful people about!

Regards,
D

[Enzo]

Here is the deal. The most common trouble with this series of amps is blown power amp ICs, the TDA7293s. They often are visibly burnt, as in your case.

WHen the 7293 shorts internally, in SOME cases the power rail can get sent through the mute pin down to the mute circuit. When that happens it usually burns out the zener, or shorts the transistor and once in a while the cap. The cap, zener and transistor are in parallel. They are bottom center on the power supply page of the schematic.

SO if you replace the TDA7293, if the amp then works, you are done. If the amp still makes no sound with a new 7293, then look to that little circuit to see if the parts have shorted. If the mute pin - actually I think it is the "standby" pin in this case - is at ground, the amp is silenced. It needs to have several volts there to turn on the sound.

[soundguruman]

The +- rails can be tested after you unplug the output IC. The IC comes on a board pre mounted from Marshall. You just plug the board in and screw it to the heatsink after applying heat sink compound....
It is worth it to buy the whole output board instead of trying to change the IC. The last one I bought was $33, and a year ago it was $12! The board is hard to get and the prices have gone way up. You will find that the heat dissipation of the IC makes it hard to solder / unsolder from the two sided circuit board. Save yourself the headache and buy that little output board from marshall.
The heat sink grease can be arctic silver which improves the heat transfer, but don't get the silver on the circuit connections. I think you will get about 30% more dissipation with the silver grease.
The heatsink is energized. That's why it's attached to a plastic fan mount. Don't let the heatsink touch the chassis when you are testing it.