From
https://xdevs.com/doc/HP_Agilent_Keysight/HP%20478A-H75%20Operating%20&%20Service.pdf I get there's a way to check if 478A is really in good order.
If I'm getting it right, one should make a circuit so:
a) pins 2, 4 and 6 of thermistor mount get connected to ground
b) a power supply giving 29 DC volts is put in series with a 2200R resistor
c) after the 2200 ohm resistor, a battery is placed against the voltage coming from the power supply, I guess it's to lower the 29V so the suggested DVM (HP3440) is able to get in a range where it can resolve 1
mW 1 mV
d) ground of that DMM is connected to power supply ground, like pins 2,4 and 6 of thermistor mount are, completing the circuit.
e) probing for milivolts against pins 1 and 3 of thermistor mount in some kind of voltage divider, connected between resistor and bucking battery, should then show less than 30 mV difference. Otherwise thermistor mount isn't good. If that difference gets up to 150 mV, thermistor mount can't even be repaired.
Now my questions:
1) Is it worth the hassle if thermistor mount is able to be coarse/fine zeroed? I guess probably not.
2) I guess that 2200 resistor is there to limit current/voltage. So having a good modern power supply with CV/CC, one wouldn't need that resistor, provided the power supply is set to the correct V and I output --- or is it there to make sure there aren't any peaks able to damage thermistors?
3) Having not a 3440, but a 3456 6-1/2 digit DVM, surely that bucking battery wouldn't be really needed anymore?