That darn MG100DFX "output module"!

[waspclothes]

I really like this amp, but the output module in my MG100DFX gave up the ghost. When I tried to remove the TDA7293, a few traces detached from the PCB.

Thankfully, I have a friend willing to make me a new PCB!

Before I have him copy it, is there any additions or modifications that should be considered?

Schematic is here: http://www.amparchives.com/Amp%20Arc...20MG100DFX.pdf
Page 3 - Top-Right is the output module.


Maybe beef up the capacitors on the power rails? Higher heat tolerance at least?

Sort of curious why there are 22pF caps going from the inverting input to signal-ground, but then another going across the inverting input to the non-inverting input.

These things seem to blow a lot, is there any protection that can be added on this board?


TDA7293 datasheet is here: http://www.st.com/content/ccc/resour...CD00001887.pdf

[drewl]

I made up one of those output boards for one that had the whole output module missing.
I used caps with a higher voltage rating.

In the future, to remove a bad chip, clip all the leads, and pull them out from the bent side gently with tweezers or needlenose pliars while heating with soldering iron.

[waspclothes]

I made up one of those output boards for one that had the whole output module missing.
I used caps with a higher voltage rating.

In the future, to remove a bad chip, clip all the leads, and pull them out from the bent side gently with tweezers or needlenose pliars while heating with soldering iron.

Higher voltage rating is a good idea, thanks.

Thanks for the advice on removing the chip too.

[Mick Bailey]

See post #18 and #19 here;

http://music-electronics-forum.com/t41641/

EDIT: I've recently had a couple of much later amps in and the modules are quite different to the original design and far more electrically robust.

[waspclothes]

See post #18 and #19 here;

http://music-electronics-forum.com/t41641/

EDIT: I've recently had a couple of much later amps in and the modules are quite different to the original design and far more electrically robust.

Hey Mick, looks like that larger module has a different connector too.. is that for different amps, or did they totally change the MG100DFX in later years?

[Mick Bailey]

They used the 'old' design for quite a few years. The larger module looks like it's used in the successor to the MG100DFX - I looked on the Marshall Imagebank but there's no schematic for the latest boards and the last drawing revision for your amp shows the old module as being fitted. It would be worthwhile understanding what changes have been made to the latest boards, as the IC is still the same.

From what I can gather there are three versions of boards - the original, a version with separate power connector (usually found on the AVT-100) and the larger one that also has diodes fitted (I see these on the AVT-150). I'd like to know what changes were made for the AVT-100 apart from the separate power connector. It may give a clue to reliability improvements.

In the main I see far fewer failures in amps fitted with a single module than I do where there are multiple modules fitted. The worst being the MF-350.

[Enzo]

What fails seems to be the IC, I have never seen any of the small caps fail on those boards. I doubt voltage is an issue, I always like to use 105 degree caps instead of 85 when I can, because amps run hot. I tend to think a lack of good thermal dissipation is what kills these chips anyway.

I still have a handful of boards with the chip removed. I see two layouts with 15 pin connector, unfortunately the pinout is different. One could easily rewire or reconfigure the cable to accommodate this, but if one assumes the two styles were interchangeable, fuses will be blowing.

Electrically the difference I see in the 15-pin is the one has the "BS Loader" cap up in the corner next to pin 15 of the IC. The other runs pins 6 and 12 of the IC out to pins 7 and 8 of the connector and the same cap is external somewhere on the main board.

[waspclothes]

What fails seems to be the IC, I have never seen any of the small caps fail on those boards. I doubt voltage is an issue, I always like to use 105 degree caps instead of 85 when I can, because amps run hot. I tend to think a lack of good thermal dissipation is what kills these chips anyway.

I still have a handful of boards with the chip removed. I see two layouts with 15 pin connector, unfortunately the pinout is different. One could easily rewire or reconfigure the cable to accommodate this, but if one assumes the two styles were interchangeable, fuses will be blowing.

Electrically the difference I see in the 15-pin is the one has the "BS Loader" cap up in the corner next to pin 15 of the IC. The other runs pins 6 and 12 of the IC out to pins 7 and 8 of the connector and the same cap is external somewhere on the main board.

105 Degree caps makes a lot of sense.

The one I'm replacing has the BS Loader cap onboard in the corner as you described. The one that doesn't, does it still use the same 15-pin connector?

My friend that is making the PCB for me is wondering if he should make a bunch since Marshall told me that the replacement part is discontinued. But it seems like there's a lot of variations, and most of the time the repairs are just done by swapping the IC out right?

[olddawg]

Am I missing something? Is there some reason why you simply can't jumper the bad traces with lead wire? I've had to do it enumerable times where techs and customers have damaged board traces with attempted repairs.... Even cracked boards. It may not be pretty, but in most cases it's reliable.

[drewl]

Right.
A simple job if you take your time and be careful.

[The Dude]

Am I missing something? Is there some reason why you simply can't jumper the bad traces with lead wire? I've had to do it enumerate times where techs and customers have damaged board traces with attempted repairs.... Even cracked boards. It may not be pretty, but in most cases it's reliable.

I was going to chime in with a similar response earlier in the discussion, but it seemed the solution was already in hand. I haven't met one of these I couldn't salvage, at least yet.

[Enzo]

I have repaired many of them as well.

I mentioned in the post you quoted that unfortunately the 15 pin connector pinouts are different. Compare the schematic for say an AVT150 and this one as used in the MF350.

[waspclothes]

Am I missing something? Is there some reason why you simply can't jumper the bad traces with lead wire? I've had to do it enumerable times where techs and customers have damaged board traces with attempted repairs.... Even cracked boards. It may not be pretty, but in most cases it's reliable.

Yeah absolutely this could have been done, but my friend was keen to design a PCB for the experience. I received several of his boards and populated them with all new components from digikey and they turned out great. His boards have plated through-holes so they can stand desoldering easily (unlike the original board) and there's a bit more room to fit the larger capacitors.

I'll post some pictures soon. I know most of these boards can be carefully desoldered, or salvaged with bridges and stuff.. but if anyone needs a replacement board for this amp, I'm sure more can be made easily.