Hey guys! Dreaded Marshall M350 (mode four head) related question

[sunny84]

Hey folks, great forum (I kinda ended up here by chance)!

I have a Marshall Mode Four head thats been sitting in the garage for a couple of years now - the output TDA7293 modules went up in smoke (at a bloody gig just before we were due to go on).

After a load of digging around on the internet, I've learned that this is apparently a common problem with the Mode Four, and that the IC's may go pop anytime even after they have been replaced.  I've disconnected the power stage, and the preamp section and all the functionality works as it should (via the tuner out or FX send).

I've been looking at ways to improve the reliablity of the amp so that I can gig with it in confidence that it won't produce some onstage theatrics of its own!  Is this something worth doing or should I simply replace the 4 IC's and hope it holds out?

I've attached copies of the power amp stage and power modules schematics, thanks in advance!

[phatt]

Note the warning on the schematic ,, these are *Bridged* outputs and both speaker wires are *Active*
You may have inadvertently grounded a speaker connector plug back to Amp chassis and hence smoked the chips.
Phil.

[g1]

  Not sure what you could do to increase the reliability, sorry.
But I will mention that before you connect the new chips, the power supplies in the amp must be fully discharged.
  Otherwise you can blow the brand new chips before you ever turn the amp on.

[J M Fahey]

Problem is, the basic design idea is stupid and flawed.
The idea behind chipamps is to *simplify* design and production, so for amps up tp, say, 80W RMS which is the practical limit today, fine.
Now, to make a 320W RMS amp combining 4 (four) chipamps, with 19 connections each for a staggering 76 wires, it's CRAZY.

If you add that the configuration chosen is inherently unstable , the words "stupid crazy" come to mind, wonder why.

I've never had one of them in my bench, but if I did, I'd try to see whether I could junk that crap power amp, and fit a normal, reliable, discrete one in the same space.

[sunny84]

Cheers guys, there's a ton of threads over at music electronics forum on them and how "suspect" the actual design is.  Tbh, the amp was purchased in my youth (depressing when I think of what I could've got for the same money, but hey ho, you live you learn); it's not a bad sounding amp, I figured I try and get it back to working condition. I've been using an old H&H poweramp with a POD XT Live for ages now.

"J M" as you say, the design seem crackers - add to that the floating ground and somethings bound to go belly up on the road.  This had been my thoughts, to shoe horn a fully discrete circuit in there (or even a kit type if that'd be applicable).  If anyone has any suggestions on where I can go with this please feel free to chime on in!

Once again, cheers!

Sunny

** Just a quick one, could I slot something like this off the shelf kit (http://www.velleman.eu/products/view/?country=es&lang=en&id=360242) in without too much fuss?

[Roly]

I didn't respond sooner because I was kinda hoping that one of the guys would have some bright ideas about this.  I haven't had the pleasure of working on one of these before either, but my general thinking was similar to JM's - chip amps are fine as far as they go these days, but 350 watts is a big ask, and I'm not surprised that these have reliability problems.

While Velleman generally have a good rep the heatsink shown is very "Hi-Fi duty" to my eyes.



For guitar work I'd be fitting something quite a bit bigger if possible (or maybe the amp already has a suitably brutal heatsink).  The old rule that "you can never have enough heatsink" applies double to guitar amps (hot venues, stage lights, driven witless for hours at a time).

Note the spec;

200W music power @ 4 ohm load
100Wrms power @ 4 ohm load
70Wrms power @ 8 ohm load

Even down rated to a 100W_RMS amp it's still going to be plenty loud, and the reliability would have to be improved.

When a design develops a rep for being a bit crook then giving it a transplant becomes a practical idea to recover the situation.  It's sad to see a great company like Marshall producing more than the occasional poorly deigned boxes these days, but it's head in the sand to ignore an obvious trend.

Also, just be careful of your supply voltage.  The Velleman needs 2x 30VAC and I would treat that as a hard upper limit.

[Enzo]

100 watts is way more than enough for a serious musician.


If you like the preamp, you certainly could cobble some power amp into it.   Or just take a line out from the FX send or other preamp output and use an external power amp of your choice.  Leave the stupid modules out of it.  It won;t care.

[sunny84]

Thanks guys.  Having mulled this over (again) and was thinking, would it be more feasible to use simply 2 of the existing Marshall boards in a bridged design, lowering the output - my thinking is that the heatsink and fan asssemlby would be sufficient for cooling a single IC. 

In terms of sounds, I haven't used the amp in donkey's years, I figured I'd get it working and then look to mod the preamp stage (if I can work out something more akin to a hot rodded JCM 800 I'd be happy - though channel 2 does an almost credible job).

[Roly]

If cooling is the design problem.  It may be, but it could also be related to all those connectors, as JM hinted.

Also, what's the load/speaker arrangement?

{I should confess to a bit of residual bias here because I had some really painful experience$ of power amp module reliability back in the early days which has left me with a somewhat jaundiced view.  They seem to be a lot better (and cheaper) these days, but I'm still not thrilled with having to replace the entire output stage when ones built out of discreet components are quite repairable.  But that's just me.  :duh }

[Enzo]

Yes, of course you could rewire the whole thing for two modules or even just one.  But it won;t be trivial.   This is not like a 100 watt tube amp.  You can;t just take out two modules.   As it sits, the thing is bridged, two on a side, but in those two, one is wired as a follower to the other.    You'd basically be taking the existing power supplies and making up a new power amp.