Thinking about selling my damaged MT-32 for pieces...

[Cloudschatze]

Hmm...
Those new LCD ribbon connectors ought to fit like gloves onto the CM-32L/64 display headers too...

[Maxime]

Wow, great !

You can't imagine how happy I am to see that you managed to get rid of nearly all the problems I had with this unit!

Well, now you have distortion, but I hope that you'll find out the problem as quickly as you did for the noise! And I think that the distortion becomes less present if you lower the volume of the unit (which wasn't the case with the permanent noise)?

And your LCD ribbon cables look really rock-solid now. Congratulations!

Somewhat ironically, the voltage regulator is the one component Maxime would have been able to replace himself as it isn't soldered. It also means that if he had gotten the extra MT-32 main board from Cloudschatze (which I haven't looked at yet) to work, it wouldn't even have solved the problem (or not all of it) as the voltage supply was bad.

Damn, you're right!

Cloudshatze's mobo, even in excellent condition, would not have been of any help for me if the problem was located elsewhere...

I would be even more happy if you manage to get his mobo working as well. Meaning that you could have a newer mobo with a phone jack, and maybe it would not produce any audio distortion, who knows?

In any case, congratulations for your nice and clean work, patience and thanks once again for helping me to have a fully working MT-32, yours.

I'll continue to watch this topic for news. It became a real pleasure for me since you told me that you could give a second life to my damaged unit :lol:

EDIT: I have a very dumb question: I hear the same distortion level on both of your recordings! Are you sure that it comes from the unit?

[Laust]

You can't imagine how happy I am to see that you managed to get rid of nearly all the problems I had with this unit!

Well, I had a feeling it could be repaired, and am having fun doing so

Well, now you have distortion, but I hope that you'll find out the problem as quickly as you did for the noise! And I think that the distortion becomes less present if you lower the volume of the unit (which wasn't the case with the permanent noise)?

Correct. The whine was there whether something played or not. This problem is relative to the volume level.

EDIT: I have a very dumb question: I hear the same distortion level on both of your recordings! Are you sure that it comes from the unit?

Yes, because the sound level isn't even close to the maximum input level on my sound card. Here are two pics from a "sound card" oscilloscope program, showing a 1KHz sine wave from the test mode:

Bad MT-32


Good MT-32


The clipping is quite obvious, but it's not due to my sound card or it would have clipped on the good MT-32 as well.

I'm guessing there's something wrong with the amplifier circuit or it may even be the DAC itself.. I need a real oscilloscope to check this, so it won't be this minute.

Those new LCD ribbon connectors ought to fit like gloves onto the CM-32L/64 display headers too...

Have you actually looked at the headers in the CM-32L/64? Anyway, they swapped the pins around so it's not as simple connecting what I've got, but yeah, it's easier to make an adapter now

[Cloudschatze]

Have you actually looked at the headers in the CM-32L/64? Anyway, they swapped the pins around so it's not as simple connecting what I've got, but yeah, it's easier to make an adapter now

Yes, I know about the swapped pins. I had hoped that you might have wired up an adapter while you were working on the MT-32. :smt110

[Laust]

Two transistors later and the MT-32 is finally repaired. I can neither hear nor measure any difference compared to a healthy unit:

http://gallium.prg.dtu.dk/~lbn/mt-32_repaired.flac

The MT-32 has a small circuit, the purpose of which it is to mute the output while the machine is being turned on or off. This is to prevent a nasty click in the output as the opamps adjust themselves right after you power on/off the device (for the first second or so). You can actually still hear the click on a normal MT-32, and it is especially noticable when you turn a unit off, but it's nothing compared to how bad it could have been, apparently

Anyway, there is one such transistor for each channel, and one of them (or both) had gone bad. I replaced both and now everything is fine.

What caused it we will never know. Best guess is that someone attempted to put an input on the MT-32's outputs, and quite possibly something stronger than a normal signal output (like an amplified speaker signal) and the transistors took the fall.

[Maxime]

Well, Laust, once again, I can only congratulate you for your work. You gave a second life to my noisy unit, and now, we both have working units, thanks to you!