I've got a working HP 8485A power sensor and another one which I suspect is dead, but am curious as to the failure mode. Most of the dead one I have seen won't zero. This one zeros fine and can be calibrated but the reading is unstable. It's easiest to see on an analogue meter like the 435 the needle wobbles significantly (rendering it unusable of course).
Is this a known failure mode? I'm aware they are basically unrepairable!
Alan