Followup to the Agilent E4418B saga:
Apart from the half-baked soldering on the sensor connector I reported earlier it seems to have more issues which I am addressing one by one:
I had a broken 74HC14 hex inverter in there which is part of the power on/off circuit. Essentially it had turned into a non-inverting inverter.
Never had that before. The power switch circuitry IMHO is far more complicated than it needs to be but ok.. Apart from the fubared inverter chip I had to modify the circuit a bit following the approach I found here on an EEVblog posting.
Result sofar: it now reliably powers on, passes all self tests, detects a sensor.
But it sofar refuses to Zero a known-good sensor.
I need to re-enter hardware rev and option info too.
More debugging to do..