Marshall Mode Four MF350 Problems

[ZOMB13]

I got a Marshall Mode Four MF 350 off craigslist for free (guy said it needed a new fuse and tubes) but it would not turn on and after further inspection, 2 ICs are blown (I already ordered replacements)

I also got some brand new Groove Tubes 12ax7 to replace the tubes that came with it and it needs a fuse which I can't find around town.

Does anyone know where I can get this type of fuse?
fuse is T8AL (8 amp slow blow) right?

I really want to try out this head.

http://img.photobucket.com/albums/v6...f350inside.jpg
http://img.photobucket.com/albums/v6...350inside1.jpg
http://img.photobucket.com/albums/v6...350inside2.jpg
http://img.photobucket.com/albums/v6...350inside3.jpg

[Enzo]

My standard advice for the Mode 4:

If any power module IC is bad, REPLACE ALL FOUR.

DO not connect or disconnect any power module with the amp turned on.

Always make sure the power supplies are discharged before connecting power modules. If the connector sparks, you may have just blown a new module.

ALWAYS read the load/jack instructions on the rear panel and obey them.

NEVER connect or disconnect a load while the amp is running.

Yes, you can fire up the amp and run it with all the power modules disconnected, so yo ucan test the rest of the amp. But do heed the above advice on connecting them back up.

If the power modules are new and it no longer blows fuses, but so sound comes out, then check the mute circuit for shorted component. That is on the power supply page and is a transistor, a zener, and a cap in parallel, making the STBY signal.

I find consistently that Mouser has the best prices on the TDA7293. Currently $3.42 each. Less in 10 lots.

[ZOMB13]

I have the 4 ICs coming in this week (hopefully)

I also got a new fuse for it.

I am not powering up anything until I replace all the ICs.

From there I'll see if it works by having all 4 power module ICs unplugged, then I'll turn off the amp, plug them in, then re-check.

[Enzo]

Make sure to discharge the power rails before plugging them in.

[ZOMB13]

Make sure to discharge the power rails before plugging them in.

How exactly do I do that?

[Enzo]

Check with a meter and see if it has discharged itself.

otherwise get a resistor - 1k maybe - and clip one end to ground and the other end use to discharge the supplies.

[ZOMB13]

Check with a meter and see if it has discharged itself.

otherwise get a resistor - 1k maybe - and clip one end to ground and the other end use to discharge the supplies.

Ok, got it.

[ZOMB13]

Problem

I de-soldered all the ICs, but one looks really bad, like the board blew up.



Is that going to be something to worry about?

[oc disorder]

Hi Zomb13
Check this post out
http://music-electronics-forum.com/t3538/

where Enzo says :-

The Marshall part number for the TDA7293V board for the MF350 (Mode 4) is KITS00004. List is $40, dealer cost $20. The IC itself is cheap from Mouser, but you will need the little boards. Maybe someone has some dead modules from prior repair they would part with. Usually the little parts are still OK. If a trace burns, repair it.

There are several versions of the little 7293 board depending upon amp model, so get the right one. Differences are like 15 pin cable on one, and on another 10 pin and 3 pin connectors.

To me it looks like it should clean up ok.

That copper track looks like it got rather hot !
It should clean up ok with a solder sucker and if in doubt fit a wire like the one in yellow on the attached photo on the underside of the board.
Isopropyl Alcohol and an old soft toothbrush should make it cleaner
so you can check all the tracks connect to the right donut holes.

[ZOMB13]

Thank you for the tip and idea, it does look scary, I'll try the wire method, hoping it does not fail, that should get the amp back in action.

I am also putting brand new tolex on the head, as it was ripped and scuffed.

Hi Zomb13
Check this post out
http://music-electronics-forum.com/t3538/

where Enzo says :-


To me it looks like it should clean up ok.

That copper track looks like it got rather hot !
It should clean up ok with a solder sucker and if in doubt fit a wire like the one in yellow on the attached photo on the underside of the board.
Isopropyl Alcohol and an old soft toothbrush should make it cleaner
so you can check all the tracks connect to the right donut holes.

[ZOMB13]

Update;
I plugged in my headphones into the emulated line out, plugged in my guitar, powered it on, fans started spinning played my guitar, no sound turned the amp's volume on every channel to 10, no sound turned up my guitar's volume and BAM

So now I just gotta re-solder the ICs and we should be good.

[HTH]

Update;
I plugged in my headphones into the emulated line out, plugged in my guitar, powered it on, fans started spinning played my guitar, no sound turned the amp's volume on every channel to 10, no sound turned up my guitar's volume and BAM

So now I just gotta re-solder the ICs and we should be good.

lmao, thats my favourite trick too and only second to not plugging in the guitar at all

[ZOMB13]

Update #2:

I soldered the new ICs and put everything back together, the amp turned on, but no sound came out, I tapped the heat sinks and sound cut in and out, I went back and re-checked the soldering on the ICs and one leg was not touching, so I re-soldered it and put everything back together, plugged in my guitar and cabinet (16 ohms, connected cable to input II) then the IC I re-checked sparked.

What went wrong?

[pablocarrillo]

Hi; i just have one mode four of a friend; and all four 7293v are burned; then i replace fuse; and replace all the IC,s

Preamp from input to send is ok; but no sound fron the poweramp.

Does anyone have the full schematic of this amp???


Thanks to all, and sorry about my bad english...

[J M Fahey]

Hi Pablo, I suggest you check this thread:
http://music-electronics-forum.com/t3873/
It's all there.
Good luck.
Nota: Cualquier problema insoluble que tengas con tu Inglés me avisas, ¿vale?