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Trying to salvage a MU90R (help welcome)

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First post, by BeastOfSoda

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Howdy,

As title says, I am trying to rescue a MU90R unit that was about to be thrown away. I would love some help in figuring out why it's acting up; I did try posting in the Yamaha forum, but the site isn't working that well, and I thought I'd try here as a Hail Mary 😁

The issue is that the drums channel is heavily distorted, to the point of being unplayable. Here's what I tried:

- Factory reset;
- Checked battery (OK);
- Ran the tests (nothing weird comes up, but the 1KHz tone generation does not sound clean: admittedly, I do not know what it's supposed to sound like...)
- Done a test by hooking up the A/D input to the PC: audio sounds fine there, so that seems to rule out issues with the preamp and passthrough circuitry...

Given the above, I am thinking there might be an issue with the actual tone generator side of the unit.

Is it possible that ROM data may simply get corrupted on its own over time? Or could this be due to hokey electronics? (I did check the capacitors, and nothing looks out of the ordinary to me...)

Thanks, hope someone can help!

Reply 1 of 4, by nopik

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did all of the drum sets distorted the same way?
is distortion similar after GM, GS and XG init?

Reply 2 of 4, by nopik

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did you clean the pcb with alcohol or aceton?
did all other instruments banks sound clean?
did you selected drums on any channel, or just 10?
did you try other instruments banks on channels 10,16?

Reply 3 of 4, by BeastOfSoda

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I can't decide if I fixed it. I gave it a gentle scrub with acetone, as you suggested, and it *seems* to be working okay (thanks for that); now, however, I'm noticing more weirdness.

The screen started flickering, and as it does I hear a soft thumping coming through the headphones: this is pointing towards a power delivery issue. Admittedly, I do not have the original brick, but the distortion issue occurred with its own adapter as well; seems like I need to check how this poor thing is being fed, as it's not happy right now.

Reply 4 of 4, by BeastOfSoda

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And... nope. I put it back together, thinking that I was having a grounding issue: the screen flickering did go away, but the distortion came back. I'm thinking faulty caps or VRMs, and I am this close to shotgunning them, but that does not seem like an intelligent diagnosis.

Also, upon further listening, the distortion is not limited to the drum bank, but it happens on the whole track. It's rather subtle, and more audible around bass frequencies.

Finally, as I was typing this the flickering came back, but when it does the audio clears right up 😒

Anything else before I desolder components willy-nilly? 😁

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