Vintage Synth Repair

Hello all

I've got a problem with Roland D50. When turned on, I'm faced 
with a frozen display - no button presses change anything, no 
sound is heard by playing the keys. The only thing that changes 
anything on the display is by pressing any of the transpose, 
chase or portamento keys on the left. The joystick can then be 
used to change these values.

I've checked PSU and battery voltages, cables and connectors, 
and taken the battery out and replaced, with no luck. Does 
anyone have any  information on the D50 - any factory resets, 
ideas on faulty ICs, etc?

Any help greatly appreciated.

Andy Thompson