Korg M1 distorted audio/digital noise

[Bassslave]

Hi,

I'm hoping somebody can give me some support or advice with an M1 I just purchased...

Bought as working, good condition from Ebay. Seller advised the issue is a simple one~: back up battery has failed resulting in a reset of sounds and a broken key. On inspection The audio seemed to work, but sound distorted, you couldn't tell what sound should be playing and was very loud. Thought nothing of it, possibly the sellers headphones caused this or could be normal for a reset keyboard. Once I got it home for testing I found:
Very loud, overdriven audio/noise -Sounds quite digital.
You could not tell what the sound should be due to the digital noise.
Once battery replaced and Sysex file reloaded, the sound names were present but the distorted audio was the same.
The digital distorted noise is not affected by which key is pressed or the mod wheel/pitch. its always the same.
The distorted audio noise will continue despite the patch, eg the piano sound sounds nothing like it should and will continue on once a note is pressed.
changing patches SOMETIMES resets the audio until a key is pressed, but not on every patch, only certain ones (the same ones each time).
Audio is the same for all output stages, headphones, outputs).
Midi in/out is working -will work OK as a controller.
Functions/buttons respond as I believe they should.
My suspicions is the main board is stuffed, hence the digital noise which does not follow the expect sound profile of the patch (the noise continuing consistently suggests the sound formation isn't being done ahead of sending to the VDF/DAC etc). Anyone with any further advice or hints?
I'm really gutted since I've wanted an M1 since I was about 11 (some years ago now!).

Many Thanks,

Matt Bassslave

PS -sorry if this is in the wrong forum index, I could see one spefic to the M1!