[volt-nuts] Newer Fluke 845ab manual with chopper mod?

Dallas Smith dosmith at outlook.com
Tue May 6 13:15:15 EDT 2014

Did Fluke ever make the 845ab Null Meter with the optically-isolated 
Bilateral Analog FET chopper? Is so where's the schematic? The 335D is 
not optically isolated. Will not having this isolation effect the 
leakage? What is the part number of the optical-isolated Bi-FET?

My Fluke null meter new neon's are now starting to flicker with unstable 
zero again.
I modified the meter to use curve tracer selected INS-1 Russian neon 
dots. Selected for equal break down voltage and brightness. Worked well 
for about two years. The NE2? with the isotope in it is impossible to find.

Has anyone used some type of circuit modification for this meter?


On 5/3/2014 12:18 PM, Dallas Smith wrote:
> Does anyone have the schematic for the Fluke 845 chopper modification 
> as stated in front of manual to be released approximately Dec 1993?
> Manual statement:
> 'The new circuit now employs optically-isolated Bilateral Analog FETs, 
> and is fabricated on an additional circuit board.'
> Frank, from Volt-Nuts 21 Mar 2010, suggested that: The 335A_D manual 
> contains the coil chopper for the calibrator circuitry, (page 119/120) 
> and the opto chopper version for the included Null volt meter, same 
> circuitry as of the 845A/B. Compare this with the 335_D manual,(page 
> 169) where you can find the MOS FET version for the calibrator amplifier.
> Has anyone tried this circuit for the Fluke 845ab?
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