Available schematics for the Marshall MG50CFX amp?

[plugbark]

Hi!

I'm looking for the schematic of the Marshall MG50CFX amplifier in order to fix my amplifier. I already have done some researches on Google and this forum but it seems that the MG CFX Series amplifiers do not have available schematics on the Web (as far as I know!)

Thank you in advance!

[Jazz P Bass]

Seeing that the MG50 CFX is the latest & greatest current model, I would not expect that a schematic would be available on line.

Authorized service stations will have access to it though.

Speaking of: isn't the amp under warranty?

[plugbark]

Hi, Jazz P Bass.

No, the amplifier is no longer under warranty :-(

By the way, the problem is that the amplifier's output signal ("Loudspeaker" output) saturates (clipping), not only on the Clean channel but also on the Crunch and Overdrive channels. However, that's not the case with the "Send" output (I recorded the signal on my audio interface and it is quite good).

I also tried to connect the Loudspeaker output to my Fender Super Champ speaker but the problem remains the same (although less present than on the Marshall speaker).
It seems that it's mostly low frequencies that generate distortions of the output signal, but it's maybe because the last strings (4th, 5th and 6th strings) generate a signal whose amplitude is higher than the first strings?

I get the same result when I play a music from my audio interface's output, so my electric guitar is not the problem.

What do you think about that?

[Jazz P Bass]

Try to locate the output IC.

It will be bolted to the heatsink.

Try to identify the part #.

[plugbark]

It's the TDA7293 (STMicroelectronics): "120-volt, 100-watt, DMOS audio amplifier with mute and standby". I think that's the IC you're talking about.

[oc disorder]

Night Shift here (well its morning here) I think JP Bass has deduced your pre-amp is ok " the "Send" output" being ok is the clue.

Depending on your skills you could check the voltages around the TDA7293 but seeing you tried to connect an additional speaker,
from where I'm looking (maybe we are looking???) that o/p Ic is probably half fried! For the cost .. probably worth changing.

Unfortunately to preserve the printed circuit board its best to clip out ... cut out the old IC leaving enough to grab hold of and desolder the pins one by one.

If it is on a separate board if you heat it up and throw it on the ground the solder and pin will fall out (best one at a time).
Other than that sometimes when on a separate board with a multi pin plug the whole assembly is available and just plugs in.

Like this

Marshall Mode 4 Output Module TDA7293, Pro Audio Service Ltd

I haven't seen your model so I don't know if the above is compatible. However the chip is readily available from electronic stores like Mouser etc.

If I misjudged your train of thought JP Bass lemme know

[plugbark]

OK, I'll try to check the voltage around the IC with the associated datasheet. Also, how can you be sure that the problem comes from the TDA7293 IC?

[Enzo]

We can't be sure, but those power amp ICs are pretty much the entire power amp.

Do you have or have access to an oscilloscope? or alternatively a signal tracer?

Even without a schematic, we can go into the amp and see if the power supplies are both present and clean. Since the preamp works, we can assume the +15 and -15 are OK, but whatever the power amp IC uses, one side might be missing, for example. Or have very high ripple.

[plugbark]

Yes, I have a DSO. I'll check the IC after my work day, and post the results here (if I succeed!) :-)

[Enzo]

OK, what I was going to suggest was look at the amp output waveform., then look at the waveform at the input pin of the power amp IC. If clean comes in and distorted comes out, that is reasonable to assume the IC is bad.

If you have also a signal generator, try a sine wave into the FX return. That bypasses the preamp and should be fairly pure into the power amp.

[J M Fahey]

Does it distort at *all* levels or , say, it sounds clean at bedroom levels (say volume set at 1 or 2 ) but distorts at rehearsal levels, even on the clean channel, if you play alongside a drummer?

If you can feed it a continuous signal, say a 400Hz to 1 KHz sinewave tone, set volume to 0 and rise it slowly.

Do you get a distorted signal at all levels? (send us an image) or you can get, say, 5 or 6 V RMS clean and it istorts at , say, 15 or 20V RMS?

As of schematic, get *any* MG50 version; I guess they are all the same as far as premp and power amp, difference comes in the effects secton: no reverb / mechanical spring reverb / digitalreverb / digital effects and even presence or absence of "CD input" but your problem apparently resides on the power amp, so any will help.

In fact, even MG100 uses a single TDA729x ... which is the cause of much trouble, that chipamp is on the edge of what can be done without exploding ... no matter what the datasheet claims.
Let alone crazy MF350 ,which uses 4 of them in unstable parallel bridge mode.

Now up to 70W RMS it can be trusted.

EDIT: here´s MG100 DFX which should be quite close http://www.amparchives.com/Amp%20Arc...20MG100DFX.pdf and MG50DFX is available on Elektrotanya.
Download it to your own computer and upload a copy here.

[Jazz P Bass]

Uhmm, that schematic is for the MG100FX.

[J M Fahey]

Uhmm, that schematic is for the MG100FX.

Yes ....so? ....
We are worried by a TD729x as used by Marshall; effects and even preamp are minor matter in this case since no problem was stated in that area.

Every designer has his own pet theories and way to do things, I can see an unsigned schematic and know whose "hand" designed it, so seeing a similar product by the same hand is often very close.

As an example , long ago I was very surprised when I saw Acoustic use split rail supplies, while for ages they were the kings of capacitor output, single supply amps.
Also used no emitter ballast resistors to equalize current in multi transistor outputs ... obviously becuse they could source tightly matched or selected transistors straight from Factory ... and so on.
Same with many other manufacturers, once you see a few you can predict what an unknown one is using ... and not miss by much.

[plugbark]

Hi everyone!

All the levels are affected by the distortion.

The funny thing is that I could not perform tests on the output op-amp because it blew up when I turned on the amplifier! Since I do not think I made any electrical mistakes (i.e. short-circuit), I assume the chip was dying the whole time.

I tried to order samples from STMicroelectronics but my order was rejected (which was predictable). I'll keep you in touch once I'll receive the new chips from Farnell.

PS: It seems that the integrated circuit in my guitar amp is actually the TDA7295, not the TDA7293.

[Jazz P Bass]

The TDA7295 is an 80Volt part.

The TDA7294 is a 100Volt part.

The pinouts are the same.

The 7294 can be used wherever the 7295 is (not vise versa)



The TDA7293 is a 120Volt part with added pin functions.

https://www.findchips.com/search/tda7294

https://www.findchips.com/search/tda7295

If you are not sure which IC is in the amp (scorched lettering) look to see if pins 5,11 & 12 are connected to anything.
If so, that is the 7293.



If the pads go nowhere, then the 7294/ 95 is called for.
The power supply voltages will dictate which IC to use, although, as noted, the 7294 can safely be used where a 7295 was.