I had to do something similar to fix the audio on a Sega System 24 board:
Broken audio on System 24
This also has a YM2151 but it's paired with a YM3012 (similar to the System 16b you mentioned). I used an audio probe on various parts of the circuit and I found I was getting something recognisable as the music on the SO pin (21) of the YM2151, which is likely wired to the SD pin (4) of the YM3014. Even though it's serial data, you could still hear the music with the probe (compared to say, what MAME sounds like) so I knew the YM2151 was good.
Turned out it was the YM3012 that had gone bad, but at the time I ordered both a new one of those and a new TL084 Op-Amp which was the next chip along in the circuit.