Just repaired a number of 3456a's at work, (yes a few still being used!). Although most have been replaced with the 3458.
Any way I came across some HP service notes tucked away in one of the old handbooks, so I have attached them here for the reference of owners of these Voltmeters.
They may be of some use, or you may have them already!. There are probably more but I cannot find them on line, (found reference to number 16 in a bench brief, but not the actual note
Also I took a few pictures and attach those as well.
Quick note to those owners that might have "orange sleeved' jacketed electrolytic cap's in the power supply PCB, even if your meter passes all self tests, I would recommend you replace them asap. While checking the meters at work I had one unit pass its tests but inspection of the PS PCB showed corrosion at the lead of one of the orange electro's.