Marshall MG100HDFX - No sound

[Rotus623]

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??

 

[bob91343]

Measure the power supply output. I would get a diagram first, though.

 

[Rotus623]

Thanks Bob. Should I measure that in volts? And would I do that on the output side of the transformer?

 

[Rotus623]

BTW, if anyone can help at all, here are the schematics if it makes it any easier. : )

http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf

 

[llwhtt]

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

 

[Rotus623]

Well thank you Craig, that sounds like a great idea. How exactly can you tell if an IC has gone bad, or should I just change it out and cross my fingers? I was thinking the DG212CJ was the one I should start with.

 

[llwhtt]

Check for signal at the input of the TDA7293, if the signal is good there replace the TDA7293. You might even check to see if there is DC on the output.

Craig

 

[Rotus623]

I didn't see a 7293 in there, just a bunch of JRC's.

 

[Rotus623]

I just realized, that schematic must be for an older version that the one I have. Hmmm.

 

[Rotus623]

Nevermind, thanks man, I found it. Ill give it a whirl and let you know how it goes. One more thing, when I am checking for signal, I need an analog tester don't I. All I have is a multimeter. Is there a way to check it with a digital multimeter?

 

[Rotus623]

Alright, so I just realized that the TDA7293 is actually in the power module that I changed out entirely. So that has been changed, and still no luck.

 

[llwhtt]

Pull up the data sheet for the TDA7293 and see what the stby voltage is supposed to be. Check all of your voltages on the IC. The 6.8V zener was shorted in the last one I repaired causing the IC to remain in stby mode. Do you have signal up to the TDA7293?

Craig

 

[Rotus623]

Gotcha. Ill let you know when I figure all that out.

 

[Enzo]

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]

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?)

 

[Rotus623]

Enzo, speaker cab works fine, headphone jack works, and spare cord does not restore sound.

 

[Rotus623]

Craig, where is the 6.8v Zener at. I believe that I do have power up to the Input. I think that you are giving me great advice here, the IC seems to be okay (especially since I changed it out) so I think that it is in fact stuck in standby mode.

 

[llwhtt]

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

 

[Rotus623]

So, I checked the input again and it measured at 96.9 volts. The output is reading okay, but I noticed the M104 (labeled C4) is a little burnt. I figured its just a 10,000 capacitor, but why is it labeled 104J63? It is in color, and looks like a polyester cap.

 

[Rotus623]

Zener (D14) checked out good.