Marshall Mode 4 (mf-350) IC's blowing up

[ILAC]

g-one, That spark was definitely a warning but at that point i kind of knew the IC's were fried considering how sensitive they are. Just had to confirm by powering it up. I missed the 4th of July fireworks this year so i figured what the heck.. JazzP, thanks for the input i will check that out before i power this thing up, hopefully all goes well if not i may just have the same mindset as Enzo and never touch one of these again. Thanks to all for your comments.

[Mick Bailey]

I've been running these amps up as per post #14 since I read that and it's a good means of assessing the health of the ICs before they fry. Switching these amps on at any time is a lottery-draw anyhow.

[gearhead63]

Great information guys! I haven't had the "pleasure" (or headache) of having one of these amps on my bench,
but I'm damned sure gonna keep this info for reference. Gotta love this forum!
THANKS!

[ILAC]

The metal plate on the back of the TDA ic's is connected to the minus power supply.
So, yeah, you should check that the power supply is good after being shorted to the chassis.
I would also check that there is no continuity between the ic heatsink & the chassis before powering up the unit.

The 'mute' pin is a slowly charging voltage that tells the TDA's to unmute.
I believe 3.5 Vdc is the unmute threshold.

If the ic's are new and the capacitors are discharged, you should be good to go.

Considering the preamp works just fine through headphones can i assume that the power supply is OK? Walked away from this one for a bit but going to start working on it again.

[Mick Bailey]

You can't assume the PSU is OK with just running through headphones. If your car is fine at tickover, does it mean it's fine climbing a hill with your foot down? Check it and make sure. Sometimes a supply can have one rail collapse under load.

[ILAC]

I hear ya.. Will test it. What a pain this amp is.

[davman]

Update Slave Modules to latest Datasheet - detailed instructions & pics!

Thanks JPB for highlighting this. It may well be an 'improvement'.
This Amp was designed around the 1999 datasheet application circuit, so does not incorporate the latest changes.

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.

I have done this mod on a recent repair (where the chips lasted 12 months after the last blowout) & all seemed well.

I will show pictures of the mod soon.

As promised, I have attached the detailed instructions for this worthwhile modification.

Note1. The mods apply to the two SLAVE modules only.
Note2. The 'longshot' attached shows each fan section marked 'T' (top) which helps with easier re-assembly after fan is removed before vacuuming the h'sink.

Good luck!



Modifying the Slave PCB modules.doc

[Jazz P Bass]

Very well documented.
Thanks!

[davman]

Quote Originally Posted by Mars Amp Repair page1 #13
OHHH NO!
You cannot run this amp unless all 4 of the ic boards are connected! One IC bad or disconnected will throw off the balance & immediately blow the others! etc...............................



This is not quite right!
This amp has 2 'master' chips & 2 'slave' chips (TDA 7293).
You can run the amp without the slave chips & it will work fine, but at half power.
If all is well,then you can add the slave chips one at a time if you want.
To identify the slave chips, see my contribution 9/16/2014 & subsequent.
davman.

[nosaj]

Quote Originally Posted by Mars Amp Repair page1 #13
OHHH NO!
You cannot run this amp unless all 4 of the ic boards are connected! One IC bad or disconnected will throw off the balance & immediately blow the others! etc...............................



This is not quite right!
This amp has 2 'master' chips & 2 'slave' chips (TDA 7293).
You can run the amp without the slave chips & it will work fine, but at half power.
If all is well,then you can add the slave chips one at a time if you want.
To identify the slave chips, see my contribution 9/16/2014 & subsequent.
davman.

I just completed this repair on a MF350. And the above is true. I performed the mod to the slave chips though I'm unsure what it does.
Hardest part of this repair was clipping the old IC's out and removing the pins.

Though the number one thing from this thread is the draining of filter caps before connecting the connectors. That right there saved a lot.

So 4 IC's and some time and it breathes fire again. Somehow I ended up with 16 ic's of which I used 4 and only ordered 8.

nosaj

I would recommend this thread a sticky for this particular amp.