Marshall MG100HDFX - No sound

Rotus623

Member
2010-09-20 4:34 am
I bought this half stack used for an insanely cheap price (Marshall MG100HDFX) I know, bad reviews, but I couldn't refuse. $125.00 used. It worked great for a few months, and then the head just stopped producing any sound. I am not sure whether this amp is considered a chip amp or solid state, although it has a ton of JRC 4558DD 2004C chips in it (5 to be exact). It has one JRC 5532D 210BB, and one DG212BDJ. It also has an effects board that is surface mounted on a separate PCB. None of the electrolytic caps are leaking, and the ceramic discs appear to be fine. The input jack is clean and all solder spots look great.
The amp cuts on, and the cooling fan immediately runs. I replaced the fuse (just for kicks), and the power module (located on the heat sink of the fan) which are the most common problems. If I run my headphones in the headphone jack, I get a tiny bit of sound, but all other output jacks do absolutely nothing. I am lost at this point. Should I start at the transformer, capacitors, IC's??
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
99% of the time the only thing wrong with these amps is the output IC. The remaining 1% was the last one I repaired the output IC fried like no other I'd seen it also wiped out part of the stby circuit. Check the 6.8V zener, D14, and the other parts around it. The output IC maybe stuck in stby.

Craig
 
Don't overlook the basics. Make sure the speaker and speaker wiring is OK.

The headphones have their own driver, so if they are not working, that would mean there is not much signal even at the master volume point.


Also, plug a spare cord from FX send to FX return. If that restores sound, the return jack needs service.
 

Rotus623

Member
2010-09-20 4:34 am
Sooooo. I tested the uninverted input jack in front of the TDA IC in the power module. It read 97.0 when I had the multimeter set at 200V. Does this mean I am in fact getting all of the power in front of the IC? If so I am going to order the IC and change it out. (Is it very possible that the chip I just switched it out with was a dud?)
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
IIRC if you are in front of the amp looking down at the PCB it is on the right side close to the front. Lower right corner if you look at another way. When find it just measure across it for a shorted reading, or if it doesn't read short turn the power on and measure for a DC voltage.

Craig