I was tending to think of the 1281 as a bench instrument which wouldn't be measuring rapidly changing voltages on the DC ranges, but of course the 1271 is designed for systems use where this is very likely to be the case.

I don't suppose anyone has any good references as to how the autocal works on the 1281 as there doesn't seem to be anything in the service manual? Is that big transformer for AC calibration only?

I suppose, that 1271 and 1281 are practically identical, apart from the double references.

There's a complete service manual available for the 1271, and the complete schematics also.

I did not find the description of this selfcal /autocal circuit, but if I remember correctly, T701 is used for precision DC ratios of 9.93: 7.23 : 1.83 : 1.00 : 0.19.

This is accomplished by a precision synchronous rectification scheme, delivering these precise DC ratios as listed.

transformer ratios can be made extremely precise and stable, so this will be the key transfer method inside the 1271/1281, but I can't estimate the precision of this transfer method.

Anyhow, this selfcal circuit is omitted in the FLUKE 8508A, so it was not that convincing, obviously.

Frank

PS: Just discovered the full service manual, on ko4bb.com, including schematics for the 1281.. this ratio selfcal module is similar/identical, but also not described in detail.

There's a brief hint, that this selfcal module is capable of resetting the calibration to the 90day uncertainty.

That's the 'extended specification', having half the 1 year uncertainty.