All done and working. Actually it was working before, but seeing as I had to open the Matrix 6R to replace the old battery that had died, I figured it was worth replacing the PSU caps and voltage regulators as well.

Readings from the PSU board 15 pin output connector J03 are as follows.

Pin No.    Schematic (Volts)    Before Replacement (Volts)    After Replacement (Volts)   
1 & 2           -5                               -5.05                                     -5.04
3 & 4          +5 B                             5.11                                      5.10
5 & 6          +5 A                             5.06                                      5.09
7 & 8          +12                             12.02                                    12.31
9 & 10         -12                            -12.30                                   -12.14
11 to 15       GROUND

So only minor differences in these readings, perhaps with the exception of the +12 Volt pins 7 & 8, but I guess this may be due to fluctuations in the Mains supply and/or tolerances in the voltage regulators and capacitors, as specified in their datasheets. I would be interested to hear from others with more experience in electronics, particularly if anything here looks out of spec. But so long as the Matrix keeps working for many years to come, then this was worth doing, I hope. :)

Next job is to replace the tact switches for the front panel buttons, as several of those are playing up.

Cheers.