Marshall MG100 HDFX problem???

[TarheelTechinTraining]

Hey folks I got a MG that I'm "fixing" for a friend. It was blowing the internal fuse and I traced it to the output, TDA7293. Replaced the part , fired it up and we have sound. So now he says it doesn't sound the way it did before the problem. I have no reference if the sound is wrong because it wasn't working when I got it.
Could I have over-heated the IC when soldering it in ? Or could I have gotten a "bad" IC? The sound at the headphones is the same, to me, as the sound at the speakers. Any help would be great!! Thanks,Greg

[Jazz P Bass]

Maybe the bad TDA IC challenged the speaker.
Truth be told the TDA's 'usually' blow rather than sound bad.
Put the rig on the bench, insert a 100mv sine wave at the input, apply an 8 ohm dummy load and scope the output.
If the output looks good on the scope then there are other issues.

[soundguruman]

There is a certain zener diode in the PS that blows
this puts the amp in standby all the time. Or part way in standby.
it may be malfunctioning and not allowing the output to turn ON all the way.
The output module can be purchased already soldered to the board.
Power Module - Marshall, MG100DFX | Antique Electronic Supply LLC
Ya just plug it in, and I would recommend that.
also the heat sink grease must be applied
This soldering may be difficult for an inexperienced person, and you may not have soldered it in, entirely correctly.
(plated through holes) requires some pretty good skills.
also the speaker could have been blown / damaged, or the speaker cable is bad.

You see when a module blows, it's usually because there is something wrong with the speaker
or the speaker cable. I think the minimum load allowed is 8 ohms. A 4 ohm load will toast this amp.
So if you replace one, you have to check the speaker and cable to make sure it won't happen again.

Open and inspect the plugs on the ends of the speaker cables, there can be no loose wires or hardware / terminals
check to see that the tip wire is not touching the sleeve of the plug or touching each other

[TarheelTechinTraining]

Thanks guru. I'm guessing you mean D14, zener diode for the stand-by. I've already installed a new output and those double sided pcbs are a pain but I'm pretty sure its in correctly. I'll check with my friend about his cabinet and replace the diode.Thanks again,Greg

[soundguruman]

Thanks guru. I'm guessing you mean D14, zener diode for the stand-by. I've already installed a new output and those double sided pcbs are a pain but I'm pretty sure its in correctly. I'll check with my friend about his cabinet and replace the diode.Thanks again,Greg

Yes it's D14 don't use a 4 ohm load it will flambe.
use 8 or 16 ohm load.
The speaker cable must be 2 separate wires (like lamp cord), not a shielded cable (not a guitar cable).
Shielded wire for speaker is bad.

[Jazz P Bass]

Before 'replacing' the zener diode check it's function.
Usually they work or they don't.
The zener is a part of the mute circuit.
If you monitor the mute the pin of the TDA7293 you should see a slow ramp up voltage.
Consult the datasheet but I believe 3.5 to 4 volts will turn it on.
This circuit (mute) has no bearing on the ultimate sound of the amp.
Off/ ON. That is what it does.