It's an old thread I know but this seems to be the most useful source of info on fixing these Yamaha units. Having not worked in electonics for a LONG time I'm basing my efforts on what's out there on t'interweb.

Seems likely that my unit has the ST P14NF12FP's are blown based on the symptoms and common faults reported so I'm going to try to replace those. Seem easy enough to get hold of the ST P14NF12FP's relatively cheaply from Honkers if you are prepared to wait. I've ordered four of them and will solder them in somehow. Might re-site them on the heatsink though to avoid having to strip a load of components off the PCB mostly buried in mounds of rock hard white suff, presumably epoxy.