Thanks all for the suggestions and input.
* The reference is warm to the touch, so the heater is active
* I have checked both front and rear input terminals and the behavior is the same
* I have replaced the electrolytic capacitors and input switch and still see the same behavior
I've run some other comparison tests between my two 34401A's, and below is what I've done so far. All measurements were done after turning on the meters for a few minutes (it takes me a while to get Benchvue loaded and setup before capture). All readings were normalized against the average reading for the hour to make it easier to compare between the two meters. The suspect meter is in blue and my reference, presumably good meter, is in orange.
In summary, the issue seems to be present on DC Voltage in toe 10 V and 100 V range, and to a lesser extent in the 1 V range. It doesn't seem to be an issue in the AC Volt .1 V, 1V, and 10V range. It also doesn't seem to show up in the 2W resistance mode testing a 1K resistor.
DC Voltage
.1 VDC Input / .1V Range: Meters agree almost exactly
1 VDC Input / 1 V Range: suspect meter's readings change more over the hour, but overall is a very similar shape to to the other meter
5 VDC Input / 10 V Range: suspect meter's change significantly over the hour, while other meter is nearly flat
10 VDC Input / 10 V Range: suspect meter's change significantly over the hour, while other meter changes very little
30 VDC Input / 100 V Range: suspect meter's change significantly over the hour, while other meter changes very little
AC Voltage
.1 VAC Input / .1 VAC Range: Meters agree very closely
3.2 VAC Input / 10 VAC Range: Meters agree very closely, but there is some odd differences in the first 5 or so minutes. (periodic dips in the readings are likely due to my cruddy signal generator)
DC Resistance
2W / 1 KOhm Resistor: Suspect meter takes about 20 minutes to settle, then it gets to within the range of the other meter
4W Short: Suspect meter is pretty flat, but my other meter takes about 40 minutes to settle down and shows periodic oscillations in readings (but that odd behavior will be for another thread...)
Sorry, that comparison stuff is much too complicated and (for me) not understandable.
The error shows up in the 10V DCV mode /range already. That's the most simple one, and therefore you should only track the error in that mode. That's much more systematic, then.
Supplementary to the reference voltages, now check the input path, also.
For that, please put the DUT 34401A in DCV, 10V range, 6 digit SLOW mode, .
Apply
10V externally, from your 731B. 5V externally, from your DMMchecker.
Check briefly the stability and value of the
731B DMMchecker by the other, known good 34401A.
Then check if the input amplifier / multiplexer is ok, you should see exactly these
10V 5V also on pin 4 of U101.
It may be, that this buffered output is also switched on /off, due to AZ measurement, then put your other 34401A in fast DCV mode, so you will see 5.00V briefly flashing, maybe.
You may also measure the 5.000V input voltage before the buffer (maybe relays K103 is defect), that is marked Buffer input, pin 5 of U101.
For that measurement, you have to put your known good 34401A in high Z mode again.
Frank
PS: The last step, if that up to now would all be ok, would be checking the MC output of the multiplexer, and the DCV amplifier.