3458a A9 board details

[mimmus78]

Did anyone noticed that by pin 1 of J400 you can change temperature set point of the board?

Wondering if they use it for accelerated ageing by changing or cycling LTZ1000 temperature.

Also the heater driving pin at base of the transistor is rooted from J401-1 to P401-1 on the A1, than it passes via a relay somewhere to be read. The schematic is to much complicate for me to figure out what happens but it seems this is used somewhere.

I cannot hear any relay click during normal operation. Wondering it it's used and when, maybe during acal to get an idea where the heater is for sanity check?

[Kleinstein]

There seems to be no connection to J400-Pin1. So likely something for the test jig only. Besides changing the temperature, this could also act to monitor for drift of the two critical resistors that set the temperature.

J401 Pint 1 seems to have a path to the ADC somehow - though a little odd. I don'T see relays involved - only CMOS switches. HP uses a little odd symbol that looks a little like a relay. Since the signal would go to the normal ADC, I somewhat doubt they would use it in normal operation. I would guess more like during self test and maybe for a sanity check during ACAL.

[mimmus78]

Maybe they wanted to do some digital temperature compensation in the same way of the "400k*" resistor, but in this case wasn't better to measure voltage drop of the heater like in the 400K resistor?

[mimmus78]

Maybe next 3458a that TiN will repair he can stick a probe on this switch to check when it's used.

[Kleinstein]

It does not make a big difference whether the heater resistor or the emitter resistance is measured. The BE drop depends on a closely related temperature and both voltages go down when the temperature goes up.