This info came from basher [builder]. It relates to modifying the 845 with LEDs in place of the neons and is something that I will have a look at.
"I got my idea for my 895A (similar chopper circuit to your 845A) from the following,
[volt-nuts] Finally got around to modifying my Fluke 845ab with LED's
Dallas Smith Tue, 09 Sep 2014 08:57:07 -0700
Finally got around to modify my Fluke 845ab with LED 's for the chopper circuit. Used the 17 volt windings for LED's (Mouser 941-C513AMSNCW0Y0511 Warm White Round LED) instead of the 130 volt, move red wire on transformer pin 9 to pin 7.This winding is 180 degrees out of phase, so I reversed the steering diodes (CR106 & CR107) I left in to help make sure the phase was correct for the LED's when connecting. Change R154 to 6K to set the brightness, selected for good operation of the zero control. Then install jumper to replace C119. Also changed the filter integration response caps C111 to .022uF and C116 to 47uF, this stabilized the jitter to a manageable mode of operation. Meter now works as well or better when the original neon's worked.
As the meter originally had this problem, why is the offset reading different when polarity is reversed at the meter input? About 10uV's.
Lamp Blocks.
However, I used H11F3M IRED/FET optocouplers and increased R154 as I now had IREDs not neons. My TVM now has less than 1/2 micro volt jitter and drift, of the TVM, is much less the a microvolt in 20 to 30 minutes.
I can't recall more as I'm away at present.
Hope this helps, Builder"