Korg M1 - Ringing, distortion and reverb on all programs

[SargFowler]

I've got hold of a Korg M1 that will play OK, but all sounds are ringing, distorted and have reverb. Also very loud though the headphones.
I've changed the battery, reset and reprogrammed successfully via SysEx Librarian, however the sounds are the same.
If I go into the effects for each one and turn down the parameters, the sound is OK, but still distorted and eventually the distortion comes back.

Does anybody have any idea which board/fault could be causing this?
Someone suggested I reflash the EPROM chips with the latest software.

 

[SargFowler]

Bit of an update.
I've got some new EPROM chips and programmed them with V1.29 (M1EX firmware).
Replaced the chips on the board with sockets and installed the new firmware EPROM chips.
Did a reset, then SysEx Librarian download over MIDI cable.
The sound is the same, but when I turn off Reverb settings, the cacophony of sound is gone.
So whilst not fixed, it's certainly better. Anyone else experienced this?

 

[SargFowler]

Another update.
I've read a lot online about this fault and messaged a few repair shops.
They reckon the RAM chips tied to the 87405 chip responsible for effects could be faulty (common fault).
I've ordered some from China for $5 and will try these when they arrive.