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Marshall Mode 4


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4/24/2005 8:40 AM
Steve Dallman
Marshall Mode 4
I just got my second one in for service. The first one just had a loose fan (which buzzed).  
 
This doesn't really qualify as a "tube amp" but I found the failure of the second one of interest.  
 
Why a guitar player would ever need a 350 watt amp is beyond me.  
 
The power amp in the Mode 4 consists of four TDA7293 power amp chips. Each is capable of producing 100watts RMS so the 4 in parallel yields the power Marshall claims.  
 
Like the pretty common LM3886, these chips fail easily if overheated.  
 
In the amp on the bench,there are two power supplies...the high power for the power amp chips, and the lower power for the preamp op amps.  
 
A shorted component in one of the preamps took out the -15v regulator and the pair of 2.2ohm resistors just before the rectifier of the low power supply.  
 
Unfortunately, Marshall put the cooling fans for the power amp chips on the +15v supply, so when the 2.2 ohm resistors burned up the fans stopped.  
 
Without the fans, the power amp modules all fried.  
 
This design insures catastrophic failure caused by what should have been a minor problem.  
 
Once back up, I am adding a dedicated power supply for the fans...something Marshall should have done.  
 
The schematics are dated early 2002, so evidently Marshall worked on these things for a while before releasing them.  
 
I will say, for a SS amp, the build quality and ease of servicablity is above average.  
 
The two 12AX7's are run at 160V, but as Marshall has been doing for many years, the actual distortion is mostly from clipping diodes following the tube's.  
 
I usually hate working on SS power amps, but the use of monolithic power amp chips makes troubleshooting much easier. It's a shame that these chips, which are supposed to have internal protection against overheating, fail so easily when overheated.  
 
I had been looking for a simple, small 100 watt SS power amp that could be dropped into old SS guitar amps using obsolete components to at least get them going with a minimum of cost and effort. The TDA7293 looked promising, but I'm not sure now.
 
4/24/2005 9:58 PM
rooster
Look at the repair records on any of the new Marshall G-series(I think it is) crap, and maybe the new valvestates as well, but I think the VS's are built quite a bit better. The good news about these amps is that the guy from System Of A Down uses the Mode 4, so there's a chance that if I ever am unfortunate enough to hear them live, his amp will melt down and cut the show short.  
 
rooster.
 
4/25/2005 10:20 PM
Enzo

I have one in here that flamed all four. Glancing at the schemo, there are separate circuits for 8 and 16 ohm it looked like. Not sure what they were doing, on the other hand I didn't look very close.
 
4/25/2005 11:37 PM
Dr. Photon

I don't have the schematic on hand, but why would they be paralleling the four chips (using some .1 ohm load-sharing resistors?). Are they using two chips in parallel per side on a bridged arangement? personally, I wish more equpment had thermal sensors on the main heatsink to cut out the amp when it overheats, or just a huge passive heatsink...  
 
I am currently working on (in addition to a backlog of tube amps for my friends and my personal transformer split class AB2 monster!) a solid state power amp that uses 4 of my free sample LM3886 chips in parallel-bridged primarily into an 8 ohm speaker. I intend to be able to reuse the power trannies, rectos, and filter caps for a true discrete design once I get around to getting all the complementary pairs I need
 
4/27/2005 3:54 AM
Enzo

Never paid attention before. The heat sinks are hot - -45v on them. There is no insulator. I'll be darned.
 
4/28/2005 9:25 PM
Steve Dallman

The one I'm working on has plenty of silicone grease and a good heat sink as well as a fan on each pair that is supposed to be on all the time. The problem was that a short in a preamp killed the low voltage power supply, which stopped the fans and fried the power amp chips.  
 
I am adding a separate supply for the fans.  
 
When did these come out? The schematic are dated 2002. It seems they were working out bugs for a while before they released this monster.  
 
A 350 watt guitar amp makes as much sense as putting a spoiler on the rear end of a front wheel drive car.
 
4/29/2005 1:40 AM
Enzo

I doubt they were "working bugs out" for three years. I think it was a design that had no market at the time. PLus it takes time to get a design into hardware. it ain't draw it up today and build it tomorrow.  
 
Those chips SHOULD shut themselves down for thermals. DOn't know why they didn't.  
 
My repair runs OK, but now I had to order 8A slow 20mm fuses. I only had fasts in that range.
 

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