Marshall MG100HDFX - TD7293

[Enzo]

Look for the simple stuff first. Cutout contacts on headdphones jacks, contacts on speaker jacks, solder on speaker jacks, dirty FX loop jacks.

[scooperman]

Good tips, thank you. I will probably get started on it this weekend, will post if I find anything definite.

[Jazz P Bass]

Marshall MG100HDFX Schematic

Here is the schematic.

[Ronsonic]

I've had two of these hit my bench over the years with component leads that were cut too short to make it all the way through the circuit board so the wave solder missed them. Both caused an intermittent signal situation as you describe. Use your finger or stick to push on things with a safe light and monitoring signal.

Two secrets to keeping the 7293 alive - always have the amp off when connecting and disconnecting speakers. One major manufacturer found that in their self contained gear the 7293 lived long and well but died horribly in stuff with speaker jacks. The other thing is that crappy little fan. If it makes any untoward noise or functions in any way less than perfect, replace it.

[haunt26]

Hey guys, I know this thread is old but I was hoping I could get some info from you guys. This is a long post but it's an interesting one.

So my Marshall MG100HDFX head has a blown TDA7293 chip and thanks to the info I found on this forum I was able to find and order a replacement TDA7293 PCB assembly from Antique Electronic Supply. So I installed the replacement in my amp (used thermal compound, replaced fuse, cabled everything correctly to my MG speaker cab etc.) and it blew again, but this time a small capacitor on the PCB board exploded. I watched it happen because I had the amp assembly outside of the head when I powered it on.

This is what it looked like after I took it out of my amp:



I then realized that the wiring on the replacement they sent me was backwards and thought maybe this is what caused it to fail. So I did some research on why capacitors explode and found out that its due to backwards wiring because some capacitors are "polarized" and can only be used one way, so when they're wired backwards they can fail and explode.

This is a picture of the stock TDA7293 PCB assembly that came with my amp with the correct wiring of a yellow wire on the far left and a black wire on the far right(I already removed the TDA7293 chip on it BTW):



As you can see in the picture, the wire on the far left is yellow and the wire on the far right is black. This is the correct wiring because my amp worked fine until it blew due to a friend incorrectly cabling it when recording. If you go to the Antique Electronic Supply website, you can see in the picture they have that the PCB assembly is wired like this as well. All of the pictures I have found online of the TDA7293 PCB assembly have this same wiring pattern so I'm pretty confident that this is the correct wiring pattern.

So I contacted Antique Electronic Supply and told them that the part failed because it was wired backwards so they sent me another replacement and...........lo and behold this one was ALSO wired backwards.

Here is a picture of the second replacement part they sent me, pay attention to the wiring and compare to the one above:



As you can see, the wiring is backwards. So my question to you guys is am I right about the wiring? Does it matter if the wiring is backwards? I'm no expert, but I'm pretty sure it does matter because the colors of the wires represent different things like positive and negative charges so when it's wired incorrectly and connected to the mainboard of my amp it will blow, right? Hopefully some of you with more knowledge would be able to give me more info.

I'm not going to install this second replacement they sent me because I'm pretty sure it will blow again. Let me know what you guys think!

[Marshmathers]

Hi Everyone,

I figured I would bump this old thread as I am having the exact same problems with my marshall mg100hdfx amp burning up the chip on the small daughterboard pcb. Aka the audio amp chip.

I too bought the amp used and the prior owner tried to resolder the new chip himself. When that didn't work he bought a brand new replacemenr pcb with new chip and harness. That burned up on him as well, and it's how I ended up with the amp.

Well I sent out the daughtrboard pcd for repair and to get a new chip soldered in. It came back to me repaired and looked great. I remounted the chip and pcb and thought it was odd that there was no spaced between the heatsink and the new chip itself. I applied the thermal paste to the chip and the heatsink where I could see traces of it before.

I made sure all cords were properly inserted and nothing was powered on untill then.

As soon as I flipped the power switch, pop sound and then magic smoke....UGH!!!!!!!!


So my question is, can someone please open their amp and take a pic of how the chip is mounted to the heatsink?

I read in another forum that there is supposed to be a small micron plastic spacer separating the chip from the heatsink, and that you are supposed to have a silicon based lubricant for the screw, before screwing the chip into the micron washer, and then to the heatsink.


Also if someone knows where to get this micron spacer, and what type of lubricant for the screw that would also be helpful.

Please post pictures, so we can prevent this from happening to future owners of the MG100HDFX.


Thank you in advance!

[MarkusBass]

It's not correct what they told you in the other forum. The heatsink is isolated from the amp's ground and does not need any plastic spacer. It is true that there is negative power supply connected to the tab of the chip but since the heatsink is isolated, it doesn't matter. Look at the photo to see that the heatsink is isolated. Maybe in your case the amp was "modified" and this caused the smoke.

Mark

[g1]

First off, there are a batch of replacement pcb's currently out there that have the wiring harness backwards. Make sure the yellow wire is on the left when looking at it from this angle (viewing front of chip):

So you will have to replace the chip again, and if the harness is backwards, rewire it correctly.
As far as I know, this model amp does not require the mica spacer or insulating washer. There is no lubricant for the screw, just the thermal paste (or heatsink compound) between the chip and the heatsink.
It is hard to say whether anything else in the amp will have been damaged if it had the backwards harness installed.

edit: I believe this is also the type of chip that must have it's power supply discharged before the harness is connected, otherwise the chip can be damaged.

[haunt26]

I also have a blown TDA7293 in my MG100HDFX and there WASN'T a mica spacer (the plastic insulator that goes between the chip and the heat sink) installed in mine and everything in my amp is stock. If your amp is stock as well, take a look at the back of your amp where the fan and heat sink assembly are screwed together to the amp chassis. You'll notice that those screws go through little plastic spacers that keep the screws from making contact with the chassis of the amp keeping the heat sink isolated from the ground of the amp's chassis. So no, you don't need one.

You might want to take a look at my thread regarding the backwards wiring on these parts:

http://music-electronics-forum.com/t32253/

[Marshmathers]

First off, there are a batch of replacement pcb's currently out there that have the wiring harness backwards. Make sure the yellow wire is on the left when looking at it from this angle (viewing front of chip): [ATTACH=CONFIG]22645[/ATTACH]

So you will have to replace the chip again, and if the harness is backwards, rewire it correctly.
As far as I know, this model amp does not require the mica spacer or insulating washer. There is no lubricant for the screw, just the thermal paste (or heatsink compound) between the chip and the heatsink.
It is hard to say whether anything else in the amp will have been damaged if it had the backwards harness installed.

edit: I believe this is also the type of chip that must have it's power supply discharged before the harness is connected, otherwise the chip can be damaged.

Thank you for the info! How do you discharge the power supply? The amp had been completely powered off without the cord even plugged in yet when I hooked it up and it blew the chip.

Also I noticed that all the connectors in pics for the daughterboard pcb are multicolored. Mine is just Grey for every wire in the harness except for the black one. Does anyone else have this harness? I have no yellow wire?

I also have a blown TDA7293 in my MG100HDFX and there WASN'T a mica spacer (the plastic insulator that goes between the chip and the heat sink) installed in mine and everything in my amp is stock. If your amp is stock as well, take a look at the back of your amp where the fan and heat sink assembly are screwed together to the amp chassis. You'll notice that those screws go through little plastic spacers that keep the screws from making contact with the chassis of the amp keeping the heat sink isolated from the ground of the amp's chassis. So no, you don't need one.

You might want to take a look at my thread regarding the backwards wiring on these parts:

http://music-electronics-forum.com/t32253/

Hi, actually your thread is what led to me joining the forum. It was very informative and helped me diagnose what was going on with the amp in the first place.

Does your screw for the tda chip have a small metal washer around it? It's very very small...i'm wondering if this washer is what led to the meltdown...

[ATTACH=CONFIG]22644[/ATTACH]
It's not correct what they told you in the other forum. The heatsink is isolated from the amp's ground and does not need any plastic spacer. It is true that there is negative power supply connected to the tab of the chip but since the heatsink is isolated, it doesn't matter. Look at the photo to see that the heatsink is isolated. Maybe in your case the amp was "modified" and this caused the smoke.

Mark

I don't think the amp is modded at all. Looks to be all original with the exception of an aftermarket wiring harness? I will post a pic of it tonight. It's all grey with one black wire on the correct side.

[Enzo]

The wire colors do not matter, what matters is the wires go one by one straight across to the connector. 1 to 1, 2 to 2, etc. His defective ones were cross wired so the connector was backwards. It is easier to tell with the colors, but as long as your connector is wired like his correct example, you are OK there.

[g1]

If you do not have that color harness, then it is unlikely you have the type that is wired backwards. But like Enzo said, make sure the harness does not cross over when installed.
Make sure the heatsink mounting screws (which also mount the fan) are insulated from the chassis with plastic washers (see post #39).
To discharge the power supply before you connect the harness, you can short out R16 & R17. Use a piece of wire or something similar, connect across R16, then R17.

[Jazz P Bass]

[QUOTE=Marshmathers;297523]
Does your screw for the tda chip have a small metal washer around it? It's very very small...i'm wondering if this washer is what led to the meltdown... QUOTE]
The washer has nothing to do with it.
The metal back plane of the TDA IC is at B- voltage potential.
Therefore the heatsink itself will also be at that voltage.
The fan has isolating plastic bushings that keeps the heatsink away from ground.

[haunt26]

Hi, actually your thread is what led to me joining the forum. It was very informative and helped me diagnose what was going on with the amp in the first place.

Does your screw for the tda chip have a small metal washer around it? It's very very small...i'm wondering if this washer is what led to the meltdown.

Yes, the little screw in my amp also has that tiny little washer, but like Jazz P Bass said, it doesn't matter since the heatsink is isolated so you're okay.


Also, just in case you need to buy a replacement TDA7293 PCB assembly, you can buy them from Antique Electronic Supply, they're the cheapest at $40 and they received all new correctly wired replacements from Marshall (that's what they told me) so that's your best bet. Amp Repair Parts (Part# MG100-PM) also has it but for $53, pretty pricey. Every other place I've found online is out of stock.

[g1]

Just to be clear about how the heatsink is insulated from the chassis: where the heatsink mounting screws pass through the rear panel (chassis) of the amp, there are 4 plastic insulating washer/bushings. These make sure that the screws do not come into contact with the chassis. Then the screws pass through the fan and into the heatsink. The tab of the power IC, heatsink, and screws are all live. The plastic bushings in the rear panel prevent the live screws from contacting chassis ground.