Dr. Frank
40K resistor from dead A1 is Vishay 301031 hermetical
40K on my 3458A's A1 is Vishay VHP101 (photos posted above). I think same is in my second unit, as it's 5 years younger.
Hi TiN,
so that's the original resistor (i.e. technology from 1988) which you tested.
It has 1.4ppm/K only, as specified for the 3458A.
Again, I assume C- or K- foil technology inside, by selection fulfilling the tighter spec.
The VHP101 on the other hand, is different, a series combination of a C- and a K-type resistor element, giving a 5 time better T.C., maybe.
I write "maybe", as this is not reflected in the spec.(which never improved over the version from 1989), and because it's not clear, whether the ~2005 Vishay specification of the VHP101 reflects the same components, as of from 1995 onwards, as used in the 3458As from then on.
I know, that it's a big wish, but maybe it's possible for you, also to measure the T.C. of the 40k, VHP101 resistors, on one of your working A1 boards.
This would give a better baseline about the actual specification also.
Frank