Nice Video, and interesting to see, that in 1964, silicon transistors were very common already.
In the end, the topology of the circuitry is very similar to what Fluke in 1965 (!) designed in the early 332A calibrator, i.e. an ovenized zener, a stable, inverting OpAmp, a trimmable range resistor, and 4 1/2 decades of wire wound resistors for the sample string. (The 1/2 digit consists of a 10k ww resistor, R45).
The Fluke 332 / 335 instruments are also designed "the other way round", if you study the "Theory of Operation" in later manuals.
Therefore - no need to turn the schematic of the 2005 PSU upside down, not even in Australia
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
...
And that transistor pair Q5 is NOT a current mirror, in this sense, it's simply the front end, i.e. the differential input stage of the OpAmp.
Instead of using chopper stabilization, this differential pair is also temperature stabilized to achieve a certain degree of stability. Clever idea, .. I think this was later realized in an integrated OpAmp from National, or Fairchild, maybe µA 714, or earlier.
Certainly, this Zener diode CR13 has drifted over these 50 years, i.e. its voltage has increased. Maybe also the ww range resistor R11 = 5200 Ohm has decreased..
But there's absolutely no need to crack the oven open for calibration.
R80 and R83 are intended to do the coarse calibration of the range.
Simply measure the Zener diode voltage, and chose these resistors, that the value in kOhm of the complete range string R80 + (R11 || R83) + 1/2 * (R12 || R79) (roughly 5.3kOhm) equals the voltage value (roughly 5.3V).
Then, the linearity of the sample string (10k + 10 x 1k / 100 / 10 / 1 + vernier 1 Ohm) should be checked, i.e. whether it still achieves 0.1% linearity..
Maybe, that the gain (beta) of several of the transistors (outside the oven) also decreased greatly, as this is a typical effect of such old components.
This, and also dried out capacitors, may lead to an unstable output.
But anyhow, I also would like to see a characterization of this historic (?)
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)
instrument, after this easy calibration!
Frank