New (old) HP 3478A with a very floaty volts range

[alexwhittemore]

I've been thinking about doing something similar (making a DIY 3478A "calibrator"), and possibly even basing it off of the Gellar Labs SVR or the voltagestandard.com units, perhaps using an LTC1043 to divide or multiply the reference reasonably precisely.  If you start working on this, be sure to start a thread, I'd love to help out!  I just took the first Contextual Electronics course, so I now know how to make a basic PCB design

Cool! I didn't know about the Gellar standard, but yeah, if I get around to it I'll definitely make a thread! I was thinking about designing it with optional most significant digits too, so decades plus a switch for 2/3/4/5/6, but I'm guessing if I follow through on that idea the design will balloon out of practicality pretty quickly.

[Neganur]

And yeah if you abort at 300 V due to lack of a suitable source, it'll fail the entire cal attempt.

Actually, I verified earlier this isn't quite true.
...

yeah, but don't abort the cal

[bingo600]

Maybe I just need to make friends with someone at a cal lab.

I can recommend that 

Got my 3478A,3457A and 34401A cal'ed for 142 us$ , last Sep.

/Bingo

[alexwhittemore]

I can recommend that 

Got my 3478A,3457A and 34401A cal'ed for 142 us$ , last Sep.

/Bingo

Damnnnnnnn. Do want

[mmagin]

Me too.  It's harmless but I've always been curious about this.  I have a perfectly functioning 3478A, and I've verified that it is a very real voltage with my antique Keithley 616 connected to it -- the reading matches.

What's funny is that it creeps up to a little over 3 volts, then it sees that it's overrange and it starts toggling between ranges -- and the 616 observes it going between 3 and 0 volts repeatedly.  It doesn't need the longer period of time to recharge to 3 volts as when you'd shorted the input.  The whole thing is insensitive to the autozero setting too.

If I'm reading the schematic correctly, it looks like there's a 470 pF capacitor in the input circuit which is only switched in for the lower ranges.  Where exactly the charge is actually coming from, I don't yet know.

(Edit: and putting the 616 in the 10^-11 amps range, I get about oh, +2 pA out of the 3478A with auto-zero off.  With Auto-zero on, it's more like +15 pA)