I'm starting a topic which might be interesting for volt nuts.
I'm going to share some thoughts on null voltmeters as I own three units.
For me there are a beautiful example how the engineers in the 60's managed to overcome the lack of high precision components.
Repairs:
In meters that use neon bulbs most common failure are the neon bulbs
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They get metalized inside and their flash threshold voltage gets much higher. They start to flicker. The flickering can be seen off course however You should remember about the 'dark effect' in neon bulbs (dim the lights in the room while observing).
Off course electrolytic capacitors and leaking batteries are a common issue.
Fluke 845AB- difficult access to the components because of it's mechanical construction . Batteries inside can cause problems due to leaking electrolyte. Goes down to 1uV full scale.
Fluke 845AR- easier to repair, no batteries inside. Goes down to 1uV full scale.
For both Flukes, notice two 47nF capacitors between LO- Guard and between Guard- mains earth. If You connect directly a output of an OPAMP to LO terminal it is highly probable that it will start to oscillate at high frequency. I warned You!
HP419A- very good design. Goes down to 3uV full scale. Seems to have lower noise than Fluke models. Watch out when disassembling and assembling because the screws have different lengths (about 3mm and 5mm ) and if You put the longer one in place where it shouldn't be you will have a short between LO terminal and mains earth (it will disturb floating measurements). The meter utilizes a Mercury battery as a floating voltage source, but You can use the meter without this feature as well.
If the meter is operated without batteries, two Zener diodes should be soldered in place of the batteries to create a low impedance ground path.
Keithley 155- I heard that it is a very good design. If there is any owner, please write an opinion.
Let's also consider using modern good digital multimeters with the autozero function at 100mV range. What are the pros and cons?