Marshall MG100HDFX - TD7293

[WB4SQI]

Thanks Enzo, you're a prince. I wish I had this schematic to begin with, had to troubleshoot using only my ham radio experiences.

If Marshall are using RC timing then that could be altered if needed to fit the chip manufacturers requirements. Still need to read that data sheet.

I'll try your fix on the fan before I go to the trouble of locating another one. I have to wonder though, it seems to move a lot of air when not in the cabinet. I would think if there were an exhaust port somewhere near the transformer end the fan could move much more air. When in the cabinet it creates a positive pressure but I'm sure air flow is fairly restricted.

The complete PC board assy. from Antique Electronic Supply is $17 plus shipping. Seems pretty reasonable under the circumstances. I guess after you rehab enough boards though you get pretty good at it. Thanks again for the schematic. I have my Digikey amp chips now and will rehab my board when I get time to work on it.

[Enzo]

The little board has a couple caps on it. Most of them are between rail and ground. A chip failure doesn't affect those. A couple are from inputs to ground, again no stress. C5 is the only one potentially threatened, and I don't see them fail. SO I reuse the boards.

[WB4SQI]

Thanks Enzo, I've bought one replacement board and will repair the original to keep as a spare.

[guitarfan]

MG100HDFX Schematic

Any idea where I can find a schematic for the Marshall MG100HDFX? The one on the Enzo site is unaccessible.

[WB4SQI]

email me nmar at earthlink dot net

[Enzo]

I just reposted it there. Recently a bunch of files somehow were deleted. Try now.

[gspear]

Looking for the DFX Circuit Board

I've found this thread searching for a source of the small DFX circuit board for my sons MG100HDFX. He too has blown the TD7293 chip. Any source where I can by a new pcb would be appreciated and the part number too. Thanks, Greg

[Enzo]

Any Marshall dealer can order the little board with chip for you. or a parts house like PArts is Parts - www.marshallparts.com

I think the part number for it is PCBCD0053 - about $20 I htink.

But I usually just replace the IC on the little board, the TDA7293V.

Mouser sells tham for $6.48 each. Currently has them in stock.

www.mouser.com

[gspear]

Thanks Enzo, just ordered a few chips.

Greg

[Starman]

TD7293 vs Mankind!

I bought this amp which may have been worked on before. Plugging into may Variac @ 60 Volts ac resulted in over 5 amp draw (no sound). Unplugging the connector to this board w/ the chip removed the short. Using the mentioned manually followed techniques w/ a Solder Sucker resulted in a clean reinstall of the chip. Also replaced the Dead fan (hint!). Hooking up to variac resulted in far less than 1 amp ac draw, no sound. Fuse in both cases did NOT blow!
Also replaced 3 caps on chip board. I thought the replacement process would fix it.

[Enzo]

AH, I thought you were concerned with the low current. If it makes no sound, look on the power supply page of the schematic. Bottom center of the page. There is transistor TR2, which makes the STBY (standby) signal. If that transistor or the cap or zener in parallel is shorted, the amp remains muted. There should be a few volts on the collector of TR2. If it is zero volts, there's the problem.

SOmetimes when a TDA7293 burns up, it damages that little circuit.

[lowell]

hey all, got one of these amps and the fx send is good. the speaker out has tons of hum but still some signal coming out. I figure this TDA7293 must be bad. How do I test it? The supply voltages seem good, they are 42v/-42 but the schem does not say what they should be. The filter caps are 63v so I figure it's right but not sure. There is an odd looking input signal then a clipped signal riding some AC it seems on the outputs. Grounds on the board are good. I'd just throw a new chip in there and see if it's fixed but have to order it. Is there a way to test the chip?

[Prairie Dawg]

Logic in this sense doesn't mean some digital circuit. I think he means the timing between those functions, and that is simple RC timing.

Tight in the middle of the schematic is a 12 wire ribbon to the DFX card, so if you found a small board with multipin tiny ICs, it is the DFX.

Peel the little label off the fan hub to expose the axle. put a drop of WD40 on there or even a drop of Deoxit. See if the fan doesn't perk up and quier down.

I just got a couple of the littel 24v fans from Marshall, for $5 each, they list at $10. The 12v fans I just but at Radio Shack when I need one. I can get them cheaper elsewhere, but then I have shipping and my money tied up in parts stock. They are just CPU fans. SInce there is a 16v cap across the supply, I have to think yours is one of the 12v fans. ANy place that sells CPU fans will have them. If they come with heat sinks, toss the sink.

I don't recall what Marshall wonts for whole power chip boards, I always refurb the old ones.

It is perfectly OK to run these without a load. Just don't run it with improper load.

I thought you had a schematic. It is posted over at ampix - Home in the Enzo gallery.

+1 on the need for good fan health on these things. I had the same issue on one I repaired for a guy and I did the same thing Enzo recommends with peeling off the label and putting in a little lubricant. I took a q-tip, wetted it with LPS3 and dabbed the end of the shaft/bearing. If I owned one of these things I'd probably buy a spare fan.

Shoulda took pix.

[Jack1962]

I've found this thread searching for a source of the small DFX circuit board for my sons MG100HDFX. He too has blown the TD7293 chip. Any source where I can by a new pcb would be appreciated and the part number too. Thanks, Greg

You can buy the entire assembly for the Marshall HDFX at Antique Electronics (Antique Electronic Supply) for $14.95

Rock On

[scooperman]

Just got my brother-in-law's MG100HDFX amp to work on. The story is that it occasionally will cut out, i.e., no sound, then it will come back on and start producing sound again. From your comments above, I think I will start by looking at the mute circuit and also check for fan<--->heat issues. Any other clue is appreciated, thanks. Looked for Enzo's schematic but have not found the right one yet.