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

Dallas Smith dosmith at outlook.com
Tue May 6 14:29:04 EDT 2014

Thank you Charles,

The Dec thread looks interesting, maybe a good solution. But was not 
looking forward in replacing the amp circuit. I think I'll try a optical 
isolated version of 335D circuit. This MOSFet is easy to apply to the 
null meter circuit without a major overhaul. If successful I'll share my 
simple mod. If the performance fails, I'll try the Dec 2013 method.


On 5/6/2014 2:06 PM, Charles Steinmetz wrote:
> Dallas wrote:
>> Did Fluke ever make the 845ab Null Meter with the optically-isolated 
>> Bilateral Analog FET chopper?
> I have never seen an 845 with FET chopper, despite the engineering 
> change specification most 845 owners have seen.  I've heard of one or 
> two 845s that may have been repaired (by Fluke) with a FET chopper 
> assembly (perhaps using 335D parts ??), but I'm not sure any 845s left 
> the factory that way.
> These days, there are integrated chopper-stabilized ("zero offset") op 
> amps that can do better than chopper amplifiers (e.g., LTC1050, 
> LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239).  Randy 
> Evans started two threads here back in December 2013, one on modifying 
> the Fluke 887 and one on modifying the 845.  He made available his 
> notes, which were quite thorough. Check the archives starting on Dec. 
> 22, 2013.
> Best regards,
> Charles
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