Renovation of the historic Rochar A.1335 digital voltmeter

https://radiokomponent.com.ua/en/product/stabilitron-d818d-1/

Parameters here. I once managed to buy one of these at a bazaar, took 10 pieces, made a standard on it to mark 3.5 digit voltmeters.

One might be tempted to insert at least a REF01 or LM399 into the V560. I think that would be more than enough.

koko0 wrote:

I might be tempted to insert at least an REF01 or LM399 in the V560.

I think that would be more than enough
In my opinion, there is exactly no point in putting a super stable voltage source into the V560. The temperature drift of the amplifiers, the comparator in the A/D converter and the drift of the resistors in the divider is rather greater than such a source. A temperature-compensated Zener diode will suffice here.
The manufacturer even recommended in the manual that the instrument should be allowed to pre-heat for 30 minutes before measurement. This already proves that the temperature stability of the whole thing is not so good.