@ICBM or other people MG50FX

Danny1969

 @ICBM or other knowledgeable folk 

Anyone know the difference between the MG50FX and the MG50DFX ? reason I'm asking is I can only see a schematic for the DFX and want to know how similar they are. 

I've got an MG50FX here which was written off by another repairer ... had the usual broken input jack plus a bad pot plus the IC and bridge rectifier were fried. Amp is up and running now but has an odd fault .... there's 36V DC between guitar ground and real ground and I can't find where it's coming from  .... . any ideas ?

ICBM

There should be a network of two diodes, a cap and a resistor between the audio ground and the chassis - if there's 36V there, at least one of the diodes and the resistor must be blown. If that's the case it might just be floating up to one side of the power rails via the other diode, if I remember right they should be about +/- that.

ecc83

ICBM said:

There should be a network of two diodes, a cap and a resistor between the audio ground and the chassis - if there's 36V there, at least one of the diodes and the resistor must be blown. If that's the case it might just be floating up to one side of the power rails via the other diode, if I remember right they should be about +/- that.

There might also be two diodes on the first IC input as input protection. B's used zeners in some chassis.

Dave.

Danny1969

Thanks for the input guys

@ICBM there is 2 diodes and a resistor but they are all good ..... it's just a bit of a head scratcher because It's hard to see how the audio ground can be 36V up on chassis ground without affecting anything else .... amp is quiet noise wise, no crossover distortion that I can hear, all effects fine. It's bizarre !

I can only find a schemetic for the MG50DFX which is quite different

jpfamps

Have you measured the resistance from signal ground to chassis ground?

Quite a few amps ground lift the pre-amp by inserting a resistance between the signal ground and chassis ground (eg some Ashdown amps). 

I've seen shorts increase this resistance and hence float the pre-amp up from ground.

darthed1981

The DFX models were the model before the MG FX I believe.  I used ot have an MG250DFX, one of my biggest ever wastes of money.

Anyone got any affection for Marshall MG?  I just stuck an old G30R CD in a skip last weekend.

ICBM

Danny1969 said:

@ICBM there is 2 diodes and a resistor but they are all good ..... it's just a bit of a head scratcher because It's hard to see how the audio ground can be 36V up on chassis ground without affecting anything else .... amp is quiet noise wise, no crossover distortion that I can hear, all effects fine. It's bizarre !

One of the diodes and the resistor must be dead if there’s 36V across that network. Either that or there’s a break in the traces leading to it.

darthed1981 said:

Anyone got any affection for Marshall MG?  I just stuck an old G30R CD in a skip last weekend.

That’s a shame... they’re actually OK. Only the DFX series are really crap - because they use the fan-cooled power ICs that are a failure in waiting. Marshall went back to discrete transistors for the bigger models in the new FX series.