Thanks for the continuing help!
Have you check all power supply voltages?
No, I should probably do that next. I checked the internal reference voltages at TP501, TP502, and TP503 and they all looked OK.
Was this an external 10V reference? (Not the 10V reference from the meter itself.)
It was using an external Agilent bench power supply. It may not be perfectly calibrated, but its self-measurement and my handheld meter both agree, while the 3456A disagrees by a decent margin, so it loses the vote. ;-)
Is it constantly 20% low? On any voltage and polarity?
I didn't try reversing the polarity, but I tried a range of positive voltages from 100mV to 25V, across all the ranges of the meter, and they all appears to be
approximately 15-20% low. Unfortunately, I didn't record the readings, so I can't check that they were all exactly the same amount off without redoing the measurement.
When you disconnect the cable from A20J19 the voltage is right?
btw: You should get about 0.1V less than the input voltage with your handheld meter. Assuming it has 10Mohm input impedance. There are the four 27kohm resistors in series.
Nevertheless, the voltage should not change when you disconnect A20J19. So the input amplifier has problems.
Yes, when I disconnect A20J19, the voltage at A20R103 jumps from +9.6V to +10V. I haven't observed a 0.1V delta compared to my handheld meter, but the readout on the power supply is a close match to what the handheld meter shows.
Is the reading unstable (noisy)?
Test the supply voltages on the input amp first.
+30V (TP302)
+18V (TP301)
-18V (U302 pin 16)
I don't know how stable I should expect it to be given that it probably hasn't been calibrated in forever, that I have the case open, it hasn't warmed up, etc, etc. But I was observing approximately 4 stable digits with noise in the last 2.
I'll plan to check those supply voltages this evening. Do you know if the service manual has any comprehensive list of test points for those kinds of things, or will I need to read through the schematics to find them?