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Replacing Zener with vreg?
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ogden:

--- Quote from: GigaJoe on November 09, 2018, 09:13:23 pm ---"bias" - ?   
precision zener , 10bit DAC - ?   for bias ?
I'm skeptical,  I rather say  it a precision meander, generated by switch and then feed ADC to measure

--- End quote ---

You can be believer or not, but this is what it is. Get service guide from HP site, check schematics, page 303. Pay close attention to name of the module. You can check file name of the attachment in the OP as well.

[edit] DC bias specs (two ranges): +/- 40V, 0.1V step, +/- 10V 0.01V. Quite a bits needed for that.
exe:

--- Quote from: GigaJoe on November 09, 2018, 09:13:23 pm ---precision zener , 10bit DAC - ?   for bias ?

--- End quote ---

Well, that's what in the original schematic :). I still feel no shame suggesting lm329 as it is cheap (from TI).

But I agree, it's unlikely DC bias circuitry needs precision.
GigaJoe:
well, ok,  probably a reason of famous HP bulletproof design.
lm329 - still large tempco,  LM369 - also old one , 

how about some modern 10V +  divider ?

ogden:

--- Quote from: GigaJoe on November 09, 2018, 09:41:18 pm ---how about some modern 10V +  divider ?

--- End quote ---

Just noticed R7 (2K) trimmer named "FS adj". Modern 5V with adjustment of particular trimmer could be option as well. If trimpot range does not reach 40V with 5V reference, then R1-8 could be increased.
Andreas:

--- Quote from: TERRA Operative on November 09, 2018, 04:39:52 pm ---I think it shhhoouulldd be relatively simple to drop in a vreg set to 6.3V, but I just want to get someone more knowledgeable to make sure there's not a trick hiding in there.

--- End quote ---

Hello,

before LM399 the 1N829A zeners were state of the art for up to 6.5 digit instruments.
These zeners are selected for T.C. (zero T.C. current) and usually also for noise.
Typical you have around 2-3 uVpp with these so you cannot replace them by a standard
bandgap (like TL431) which has minimum a factor 5 more noise.

I also guess that R1 (resistor array)  is specially paired/selected (with low T.C.)
together with the zener to get accurate output.
So it will be difficult to use a standard voltage divider (with 50-100ppm/K T.C.)
without degrading the cirquit.

1N829A are still available (e.g. at Farnell) even if they have now much higher price
as they used to be at the time when they were state of the art.
Eventually you will have to adapt the 1K resistor to get optimum
zero T.C. current for a different device.

So before replacing the temperature compensated zener with
a different device I would carefully check if I can maintain the specs
of the instrument with a different (low noise, low drift) voltage regulator.

With best regards

Andreas

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