Marshall Modefour MF350

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Carlosraj

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Joined 2010
Hi


I am working on these amp which had its fuse keep blowing. I noted that one of the 4 power ICs has cracked open. I have checked all the resistors, elect caps, diodes and transistors on the main board and they are OK. I noticed some bad joints, so I resoldered the whole board. Now I have put in 4 new power IC modules, and replaced the two 47uf caps on each power board. I have not powered the amp yet. I plan to use a variac, without plugging in any signal and any load, I intend to connect a DMM at the speaker output and bring up the variac slowly while monitoring for any DC offset. If no DC offset, then turn off the amp, plug in a signal and speaker and then turn the amp back on while bringging up the variac slowly. What I want to know is, Is it safe to turn ON the amp without plugging in any load. Will it damage the power ICs. Please advice.


Thanks
 

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Carlosraj

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Joined 2010
Enzo said:
Actually, as you bring up the variac, monitor MAINS current draw. If it starts to ramp up, back off.
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I put back all the caps onto the main board and reconnected the board, then plugged in all the 4 connectors for the power module. Connected the speaker and thru the variac powered the amp. I was monitoring the DC offset at the speakers. I brought up the variac slowly, checking the tempature of the 4 ICs with my fingers and everything seem normal. I went up 3/4 way on the variac at 180V AC. The 4 ICs were just feeling warm. I plugged in a CD player and there was perfectly normal sound. I then decided to turn OFF and ON back the amp. After turning off the amp, I almost instantly turned the amp back ON again. The very moment I turned the amp ON, all 4 of the ICs broke into pieces at the same time. I really dont understand why this happened as I did not touch any of the connectors or components. I am very sure that there was no residual charge on the caps as I had just installed them. What probably could be the cause.


Thanks
 
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Enzo

Member
Joined 2004
First, you really should test an amp with NO LOAD first. ICs getting warm? Not a good sign.

I hate this amp model, and in fact I refused to take them in at my repair shop. I could fix one, but could not tell anyone the amp will be reliable. IF I can't do a good job and put my name on it, I won't do the repair.

Working from memory, the four modules are working in bridge. The two sides of the bridge are pairs of TDA7293. One acts as an amp and the other a follower in each pair. So DO NOT ground either side of the speaker or load. DO NOT connect your scope probe ground to either side of the speaker/load.

If one module burns up, I always replace all four, even if the others seem OK.

Never connect the modules when there is any residual power supply voltage present.

Do not connect or disconnect a speaker/load while the amp is powered.

ALWAYS match the load impedance to the output jack.


The reason I suggested watching the mains current while variac-ing, is because you can see excess current flowing - the very excess that was heating your ICs I would wager. If you start up and see any more than the basic cap charging current draw, STOP and turn it off. No reason to "see how high I could go".
 
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Carlosraj

Member
Joined 2010
I am trying again to fix this amp. I got 4 new TDA7293 modules. I realise that these modules are screwed directly on to the heatsink. There is no mica insulating piece. The metal tab of the IC is in direct contact with the heatsink. Is this OK. Please advice.


Thanks
 
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