The power supply noise is caused by the mains voltage transformer, it's a known problem.
How to fix it is well documented on another forum, basically replacement with a more modern switch-mode power supply.
On my M1000, I never noticed the problems as I got mine with a European power cord & I was using it for years with no safety earth. It was racked with other gear which did have earth leads. I never noticed the hum, but used to wonder why the front panel often seemed to vibrate. Seems its the transformer cores vibrating as they are not secured with enough varnish.
Does not seem to cause audio problems.
Regarding the MIDI chip modifications, without a picture of your board it's hard to tell, but there were many official factory modifications
carried out (called ECO's I believe) to correct hardware problems and deficiencies.
One such mod was made to pin 24 of the ACIA chip (MIDI chip) which tied it to the output of another chip so that the pin 24
state could be read by the firmware during calibration routines. Early boards did not have this, so maybe the mod was done by someone
who wanted to used newer firmware. I believe that version 1.11 & newer won't work unless this is done.
Gerry
---In oberheim@yahoogroups.com, <jan@...> wrote :
This guy in Sweden
used to be a great source.
(Don't let it irritate you that he sells the chips as Cheetah MS6 replacements, the Cheetah was more or less a Matrix-1000 ripoff, and the chips are the same.
Hint: before ordering, rotate the chips to find out whether it's really the CEM3396 that's faulty.
Activate Voice Display Mode
(Ext. Func. 7, Number 1) to find out which one is the culprit.