Another thought:

I believe the service manual for the Matrix 6 has some errors with regards
to the display.
If I recall correctly the service manual specifies 2x9VAC as coming from
the transformer (at least on the block diagram) but in reality it's 2x42VAC.

What I also found a problem occasionally is the multi pin connectors
between the boards. Contact problems with those can result in strange
things including garbled characters.

Of course "Reset and initialize" should probably be performed first. At
least in one case this was already enough to bring a Matrix 6 back to life
that otherwise behaved very erratically.