Sure thing, the service note will come in handy: http://technopolitan.se/manuals/Boss...e%20Manual.pdf
I can't write much right now about the issue but iy seems to me that you should check the diode D5. I wonder if the memory (IC1, TC5501P-1) has not been affected by the reversed voltage(i.e partially erased). Just a quick theory... don't take my idea for cash.
Find the RAM datasheet here: http://www.datasheetarchive.com/pdf/3913144.pdf
Hope that helps