Vintage Synth Repair

Hi, I've just acquired a Roland RS-202. It was described as fully working, but unfortunately it's a little too fully working. I'm not sure if the original owner realised that when you play a note, the bottom F# of the upper half of the split keyboard also gets triggered. It works great in the right key, but not good in others!

Before I open it up, has anyone got any tips? Am I looking for a duff diode, or a simple short? Can diodes fail as a short circuit? I've found a schematic which seems to imply the problem is with a diode or transistor. What's most likely to fail on these old machines?

Thanks for any help
Susie