Vintage Synth Repair

Hi, group.

I was fortunate enough to acquire a Roland Jupiter 8 w/Encore MIDI
recently.  I've determined that it is the 12 bit version with variable
split point.  (serial 120978)  Do the first two digits of the serial
number refer to month (1 = January) and year (2= 1982)?  Assuming the
numbering started at 0100 for the remaining four digits, would it be
reasonable to assume mine is the 878th unit?

Also, what is the procedure for updating the D/A converter to 14 bit?
 Is it as simple as replacing the 12 bit DAC or will it require more
extensive modification?

Last, it is in need of calibration as there are some slight tuning and
level variations among the voices.  Does the JP-8 have a 'voice
defeat' feature so I can effectively mute the bad voices until I can
calibrate it?  I saw no mention of it in the manual.

Any other information on this particular version or any tips and
tricks would be appreciated!

Thanks,
Stephen