Marshall MG100HDFX - TD7293

[WB4SQI]

I am in the middle of repairing my MG100HDFX, the TD7293 shorted.

Is this typical? Any way to prevent chip failure?

I usually don't have a problem with soldering, been a ham operator for 37 years but the pc board looks like it has plated through holes, which makes de-soldering a bit more difficult. Any tips to remove the old chip.

Appreciate any feedback (pardon the pun) on the correct way to repair my amp.

Thanks,

Nick

[drewl]

Cut the leads from the old chip, but leave enough to "grab" with needlenose pliars or tweezers and pull out what's left of the legs as you heat them up.
Then clean out the holes with solder wick or a solder sucker and install new device.

[Enzo]

I use a desoldering station myself, but in the absence of one of those, I'd do what drewl said. leave as much leg as possible.

Mouser sells them reasonably. TDA7293V for $6.06 Don't get the HS version, the legs are cut for horizontal moint.

[WB4SQI]

Replacing TD7293

Thanks Drewl. Do the new pins need to be resoldered on the top of the
pc board? Looks like several pins are soldered on top, as well as the bottom.

Enzo, I've got 3 chips coming, TD7293V as the picture looked identical to my
original.

Either of you have any ideas on prolonging the life of the new chip?

On the DIY amp site they never run over 80 watts on their DIY amp.

What is the best cable to run from the amp to speakers? I was thinking maybe
twisted pair of #14 wire?

thanks again for your help.

[drewl]

Solder will flow from the bottom through to the top of the board, just don't glob a ton of it on there as to create a short.


I also have a killer de-soldering station, but I thought I'd explain how to do the repair with layman's tools!

[WB4SQI]

Thanks Drewl. I've got a 38 year old 25 watt Weller iron I used to build my
Heathkits and it's stood the test of time. The tip is a bit broad for these
pins though. I think I have another 25 with a smaller tip if I can find it.
I don't do enough desoldering to warrant a station, just solder wick when
needed.

[Enzo]

Oh I'm with you on the laymans tools.

The original chips are inserted in the holes as far as they go, resting on the kink in the legs. I prefer to mount the new ones a little higher, with the legs just peeking out the other side. This allows a little more room underneat in case I need to touch up the solder. The solder should flow through from the bottom and wet up the leg on top, but if the plate through is damaged or the soldr is not happy flowing, I will touch up the topside solder

[WB4SQI]

This one is mounted down to the leg bends and the board does not appear to have gone through repair before, no signs of resoldering at all, but the original owner (from ebay, where else?) called the amp, "smoke machine". Maybe he
just replaced the whole board/chip assembly?

Any ideas on extending the chip life?

[drewl]

Maybe Enzo knows why they go bad, I'd guess it's being pushed to it's limit and upgrading to a higher output device may extend it's life since it wouldn't have to work as hard.

Just like some amps eat tubes by running them hot, maybe lowering the output of the amp would allow it to last longer, but then it probably wouldn't sound as good.

[WB4SQI]

Enzo? What is your opinion on reliability of Marshall amps, specifically using the TD7293 chip?

Maybe running the amp at lower power levels? Better cooling?

[Enzo]

I wish I knew. I see a lot of amps with this chip come in blown up, but that alone says nothing. There are a bazillion of the things out ther these days. AFter all if 5 of 1,000 fail, that is not as good as 25 of 10,000 failing. Just seing a lot of them doesn't make them unreliable.

Still, I am not sold on them. The 7294 is the same thing at 20 volts lower - no help.

Potential for counterfeit chips? Possibly.

I think they are inadequately cooled, but that is opinion. If the fan chatters, it is not running full speed, and so not cooling. The fan need to be unblocked, as with the vent holes. The chip supposedly has its own internal thermal management, but I don't trust it.

Marshall doesn't sell just the chip, they sell the little board complete. One screw and a connector or two, and your warranty repair is complete. I repair the things because I can stock the IC, instead of the two or three little board formats Marshall uses - not to mention other brands using the chip.

Note the jacks are not labelled the same. It is important to use the proper jack for the speaker conection. Also, connect the speaker before powering up. The amp doesn't mind being run wihtout a load, but seems not to like connecting a load once powered.

[WB4SQI]

I agree on the cooling aspects. The fan on mine sounds like it is winding up to speed when you first turn it on. When it finally gets to full speed it seems to move a fair amount of air. Marshall designed a nice inlet, but where does the air go when the amp is in it's cabinet. I realize the chassis is not air tight but it does present a lot of resistance to air flow. A 2" hole in the end, probably
under the transformer would allow for unimpeded air flow.

Do you have a source for replacement fans?

Do you know Marshall's price on the complete pc board assembly?

I didn't look at the speaker connections when I powered it up as I didn't have a suitable load for the amp. I just turned all the gain controls to zero. When I get the chip installed I will have a suitable load.

Speaking of loads, I have to wonder if many of the failures are caused by unsuitable loads on the amp?

[teemuk]

Some people claim that these chips are delicate to proper sequencing of stand-by and mute functions. It is said that improper sequence might even destroy the chip. I haven't verified this myself. The datasheet does not discuss this but there is a picture related to the topic, which suggests the proper sequence. In many Marshall amplifiers either mute or stand-by has a constant reference to one of the supply rails while the other function is used for start up delay or for protection (such as in MF350). This could be the answer to issues with the chip. Fender amps with the chip seem to follow the configuration suggested by the datasheet, I have an impression they have far less problems than Marshalls. In guitar amps these chips are likely subject to far greater abuse than in any other application so undoubtedly problems become more eminent.

[WB4SQI]

Thanks for the insight teemuk, very interesting. The only onboard logic I could find was on a daughter board located all the way to the left end of the main PC board. I assumed (maybe incorrectly) that this daughter board would handle the effects. It was so far away from the amp PC board I thought Marshall would not run extra long pc traces providing amp control logic over that distance. Do you think they control mute and standby from the daughter board?

Life would be a bit easier if I had a schematic diagram.

I DL'd the data sheet this week, probably should take a read. Thanks again.

[Enzo]

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 www.ampix.org in the Enzo gallery.