Years ago I bought a battered DX7 IIFD on the cheap. Bizarrely, externally it looked OK'ish but internally it was a mess.

Anyway, one problem I ran into was with the micro-switches for the buttons. Many of them didn't work well. I don't know whether I was particularly unlucky or whether Yamaha used switches that were prone to failing on this model but either way, I ended up desoldering and replacing all of them. That took a while.

Works great now.

When buying a series II you may want to test each button to be sure it works well and you don't have to push a hole in the synth to get it to register.