Vintage Synth Repair

Hi group.
I'm searching for a pdf file of the service manual for the roland JX-8p.
My JX-8p has the following problem:
- When I select a program which uses a square wave and an envelope with 
fast attack (piano like) some of the notes sound weaker than others, 
but in an alleatory way from patch to patch.
This also happens when notes are played from an external keyboard thru 
MIDI IN.

Any suggestions?

Best regards and merry christmas to you all!
José
Portugal