Search for an authorized Yamaha Repair Tech. I recently took my Yamaha V50 (mfgd in the same era) in for service on the back-up battery and audio out (headphones) and it's in tip-top shape.
I also have an RX-120 and am trying to recreate it as a software instrument for use in Garageband. I've sampled the audio of the percussion samples, but capturing the actual patterns in a synchronized manner with my Mac is proving a headache. I have a few utilities and, well, it's just a heck of a lotta work.