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.
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Marshall Mode 4 (mf-350) IC's blowing up
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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!1937 Gibson L50 "Black Special #4"
1978 Gibson Melody Maker D Reissue
2004 Ibanez SZ720FM
Epi SG '61 with 490R & 498T Pickups
Couple Marshalls, Crate Blue VooDoo
Couple 4x12 cabs
Couple Orange combos
TONS OF FREAKING TEST GEAR- SCOPES, METERS ,ANALYZERS
SIG GENS, ETC, ETC, ETC.
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Originally posted by Jazz P Bass View PostThe 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.
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Update Slave Modules to latest Datasheet - detailed instructions & pics!
Originally posted by davman View PostThanks 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
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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.
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Originally posted by davman View PostQuote 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.
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.soldering stuff that's broken, breaking stuff that works, Yeah!
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