I have a HP LaserJet II & a III. My II had the same symptoms you
describe and the problem turned out to be a small electrolytic
capacitor mounted on the print density card. This is the small card
above the toner cartridge with the slide switch.
I know this sounds strange but some of the interlock circuits feed
through this card.
In my case the capacitor was open and would not give any kick to my
meter.

Van Gardner