Fix
I detected a huge misalignment in the AC 750V range – you could even detect it when measuring around 100V AC and switching between the 100V and 750V range. The difference was about 4-5V AC.
According to the schematics, and some measurements, only R110 or U105 could be the affected parts:
It turned out that the R110 was not okay any more – the 1k part was out of alignment. Getting a new one is not that easy. The right series from Vishay is the MTMP Family, The right value would be MPMT90011001AT1 giving you 1k at one end and 9k at the other resulting in a 1:10 division. Unfortunately this one seems to be not that common and I had to use a MPMT10019001AT1, which has the 1k and 9k the other way around.
Mounting the resistor the other way around is not optimal, but works and the 750V AC range was okay again.
Upgrade
My VFD display was quite dim and reading it from the side was not possible any more. I found a custom LED one (which I my view looks much more original than the OLED ones) from HubEquip in the US.
The LED display replaces only the VFD and not the full display PCB. Mechanically you only have to remove some paint from the display window (I used the recommended nail remover and plastic polish).
Unsoldering the VFD is even with professional gear not that easy as you can’t desolder between the VDF and the PCB on the too side. I had to repair some traces.
Is it worth it?
Just take a look at the article image – this is the new display in action. It really looks great and makes the good old K2000 much more useful!