Obviously, TiN has blown up his unit by or during replacement of all carbon resistors:
Doh, I replaced carbon (which were 5-10% high) resistors with modern ones in my 845AB, and now unit is dead
It was working before, just noisy, but now it's just overflow either way and just stays there, on any range.
Battery is ok +9.4V, +15V and -15V are ok too.
My own 845AR sometimes also shows signs of instability.
When I'm in the lowest ranges, i.e. 1 or 3µV, and have a probably noisy input signal, the 845AR just jumps into overload, and does not get back to zero, even if the bridge I'm measuring is probably nearly balanced.
So I have to range to 100µV or higher, and after the 845AR is displaying zero there, carefully go back to full sensitivity, where it really shows a few 100nV only.
I always assumed, that my unit is having problems with oscillations.
The story told by dietert1 now supports my assumption, maybe some carbon resistors also drifted too much.
Up to now, I only replaced several electrolytics and the neons, and didn't want to touch a running device, because I only want to get the alignment of my bridges done, at ~ 200nV
pp noise level, and <<1pA bias, which is fully sufficient for me.
Maybe I will have a closer look now, how the different circuit parts are behaving.
Frank