Thanks, Trag, that'll be my next step!

So far:

Opened up the PSU and only discovered the lingering scent of magic smoke. :-/

Replaced the LC475's PRAM Bat with the new one from the IIsi and it works. The 475 is again happy with me, driving the APD beautifully at 16bit GS, without complaint.

Tested the Full Height IBM Q700 drive (that I'd been using in the IIsi) in the LC 475 and it's fine.

Found my extra PAS 16 inside the LC475, so that made me happy!

So good! )