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Marshall MG100DFX repair advice please


SH73
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I recently bought a second hand Marshall MG100DFX. The fan was a bit too loud so I cleaned it out. The fan and the cooling unit were dusty. For a good measure I sprayed a bit of switch cleaner in the fan and all the potentiometers. I plugged the amp in to test if the fan would be quite. I forgot to plug in the speaker. As I switched it on , there was a piping sound and a small flash next to the cooling unit. The amp doesn't work. The T1.6AL fuse on the pcb is blown. I read that replacing the fuse and trying again may cause further damage , so a bit reluctant to replace the fuse. Anyone can suggest what the issue may be or is it money down the drain? 

Edited by SH73
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  • SH73 changed the title to Marshall MG100DFX repair advice please
2 hours ago, SH73 said:

Thinking whether I caused a short circuit with a switch cleaner and will be ok once dried out evaporated?...

 

That board is the power amp; the active part that churns out the 100w is that multi-legged black thing at the bottom, under the heat sink. If the fuse has blown, with a spark or flash, it's very likely shot. It will do no further harm to replace the fuse with one of exactly the same rating; it will in all likelihood blow too. In that case, the amp is dead, but it may be possible to obtain a new power module, which is easy to replace (Plug'n'Play...). The power IC itself (the black thing...) costs about a fiver (TDA7293V...), but may blow again if there's other faults on that board. If you can't do this yourself, take it to a Good, Reliable Repair Man and get an estimate. It's not a hard fix for someone competent.
Hope this helps. B|

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1 hour ago, Dad3353 said:

 

That board is the power amp; the active part that churns out the 100w is that multi-legged black thing at the bottom, under the heat sink. If the fuse has blown, with a spark or flash, it's very likely shot. It will do no further harm to replace the fuse with one of exactly the same rating; it will in all likelihood blow too. In that case, the amp is dead, but it may be possible to obtain a new power module, which is easy to replace (Plug'n'Play...). The power IC itself (the black thing...) costs about a fiver (TDA7293V...), but may blow again if there's other faults on that board. If you can't do this yourself, take it to a Good, Reliable Repair Man and get an estimate. It's not a hard fix for someone competent.
Hope this helps. B|

Thanks, I've been pretty much watching You Tube videos this afternoon on replacing TDA93V or the whole power module. They're suggesting the same as you. The only thing I can't see is any discoloration of the soldering joint suggesting that the IC has had it. I try the fuse first, then the whole power module. 

The only confusion is that the spark was from underneath the yellow capacitor 100 nj63. I ordered 20 fuses so I have a "play" tomorrow. I had the amp less than a week 😁

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Two faults I may or likely have done are.

1. After cleaning the fan , I didn't assemble the heatsink and fan using the screws that keep them isolated from the chassis, hence the spark...

2. The speaker wasn't connect to amp, it should have had a load no lower than 8 ohms.

 

Still waiting for fuses to re test, now everything connected as it should so fingers cross.

 

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1 hour ago, SH73 said:

The speaker wasn't connect to amp, it should have had a load no lower than 8 ohms.

Not connecting the speaker shouldn't have caused any problems as it's a solid state amp and they are usually quite happy without a load.

If it was a valve amp, that's a different story.

The "no lower than 8 Ohms" is the lowest impedance of a speaker when you do connect it.

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So... The fuse arrived, installed the new fuse recording the board with phone so I can trace the potential spark ...Well. The amp works and is loud as flip. The fan is quiet now after I cleaned it. Thanks all who advised. @Dad3353. lesson learnt.." don't meddle with things you don't understand" ..

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23 hours ago, SH73 said:

If anyone reads this thread, where are the places to get spare electronic parts, capacitors, PCB etc. E bay takes a long time...thanks

 

CPC (part of Farnell but prices often cheaper with free delivery over £17 I think atm).

Ebay sellers can be quick if UK stock.

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