As I recently acquired a Fluke 5450A, I'd like to revive this old thread, adding some interesting information.
My instrument came from the German Air Force, last calibrated in 2003 (coincidentally by Air Force cal lab, where myself worked 35 years ago).
It was obviously stored in a mining tunnel depot since then, and now sold as surplus.
The manufacturing date is about 1987/88, 2 years older than Daves instrument.
When I checked the resistors by a 34465A and a 3458A, I found the same drifts, as Dave did, mostly all lower resistance values drifted upwards, 1k and 1k9 more than the others, and the high ohmic ranges, 10M, 19M and 100M read extremely low, between -1000 and -5000 ppm.
So I quickly found the fault by 'nosing': The transport box and the instrument itself smelled like an old, musty cellar.
It was obviously stored under high humidity conditions for an extended period of time, which might well cause leakage paths.
When I opened the instrument, see picture 1, it contained moulded ELECTROL reed relays for these high ranges, instead of the COTO types, like in Daves instrument.
Moulded components can very well suck water vapour inside, which would explain, where the leakage is located.
So I heated the relays indirectly by an incandescent lamp to about 55°C, for about 5h.
These gross errors now have vanished, still reading a bit low, to the same level as shown in Daves video.
I will come back to that point later.
During the heating period, I monitored the 10k value, which was extremely stable to < 2ppm, meanwhile the temperature of these resistors also had increased by about 15°C.
That means, they really have a T.C. of around 0.15ppm/K.
Great performance, but how is this accomplished?
I now had a closer look & calculation on the T.C. matching, which is really very tricky.
You can see in the pictures, that the individual T.C.s are written on the resistors, with sign and in multiples of 0.25ppm/K.
So, the two 450k resistors are marked P0.75 and N0.50, which denotes +0.75ppm/K, and -0.50ppm/K.
For the calculation of the combined T.C. of the resulting 900k resistor, the difference of their T.C.s has to be averaged also, so the resulting T.C. is 0.125ppm/K only, opposed to the expected 0.25ppm/K.
For the lowest resistor arrays of this chain, 10 Ohm, 9 Ohm, 81 Ohm, one can easily see the individual T.C. markings.
The paralleling of four resistors gives 4 fold improvement of the T.C., and also mitigates self heating effects when higher measurement currents are applied.
These big vias are used as the resistor reference points with Kelvin connection on the PCB bottom side.
This pairwise matching / averaging 'trick' can be found in all Fluke instruments, like the 332B/D, 752A, and the 742A, which explains their extremely low T.C.s, even after all these years.
I took the readings of all resistors from 10 Ohm to 9 MOhm and calculated the overall T.C.s.
So the 5450A resistors have T.C.s of -0.54ppm/K worst case, and 0.1ppm/K for the 10 kOhm resistor, which is in full agreement with my rough measurement.