Resurrecting this now rather old thread.
Given that i now have a calibrated (
ish) Keithley 2001 I thought I would look at adjusting the 522, especially as it seems to have drifted somewhat higher.
The first step in the adjustment procedure is to use R181 to get the reference zener to the voltage on the tag - in my case 6.1606V with a forward current of 4.6mA
Checking the reference voltage it was high - BUT I can't get the zener voltage down as low as 6.1606 - it's stuck at 6.1609V - measuring across the 1K "trim" resistor R1A gives a forward current of 5.114mA which is higher than the current on the tag as well.
The reference circuit is the same as the one previously posted for the 521, but the 522 manual is clearer.
The voltage across D101 is 6.17 so that's OK, the voltage across the combined "R1" is 6.18V (the forward drop on D101 more-or-less countering Q102's V
BE )
However, given that a 1N825 has a zener voltage of 5.9-6.5V I can't see how the reference circuit could
ever have provided 4.6mA of forward current for D103 - the minimum is approx 5.9/1200 = 4.9mA
I'm guessing my reference 1N829 had a very low voltage/current for it's temperature coefficient "sweet spot".
What would others do - I can replace R1A with a 1.2k resistor, which should be enough to get 4.6mA or do I ignore the fact that the reference isn't quite running at it's optimum and adjust the output to be in spec with the trims on the voltage divider chain?