Hi, thank you very much for the follow up! I've tried the steps above and unfortunately there seems to be no change in the behavior. I've removed the grounding strip for this minimal "bench test" (so as to rule out corrosion around/on it), but I have tested the PB fully assembled as well. Wiggling boards, etc, checking continuity, and reassembling the whole "apparatus" has no effect either.
The power supply (EU model) is not refurbished but works; I have measured the voltage and it also works perfectly with a PB150 for hours on end. If I might add, the display contrast and brightness sliders are fully functional. Other things I've done include removing and replaced the floppy drive w/ its flex cable, and installed/uninstalled a RAM expansion card.