[volt-nuts] Fluke 845AB Null Detector

[Alan Scrimgeour]

I didn't check the detail because the manual warns not to charge for more 
than a week, so I assumed it was just on the high side of a trickle charge 
and so ok for NiMH too.

Alan

----- Original Message ----- 
From: "Brooke Clarke" <brooke at pacific.net>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Friday, March 19, 2010 4:43 PM
Subject: Re: [volt-nuts] Fluke 883AB differential voltmeter


> Hi Alan:
>
> If the factory battery was Ni-Cad it's a mistake to replace it with Ni-MH. 
> During charging they are very different and the Ni-MH might overheat.  Is 
> there a temperature sensor as part of the charging electronics?
>
> Have Fun,
>
> Brooke Clarke
> http://www.PRC68.com
>
>
> Alan Scrimgeour wrote:
>> I also recently restored a Fluke 845AB  Null Detector. I used cheap 
>> Nickel Metal Hydride batteries to replace the originals, but I was 
>> tempted by the improving capacities of Low Self Discharge NiMH batteries:
>> http://en.wikipedia.org/wiki/Low_self-discharge_NiMH_battery
>>
>> The repair seemed to work very well, but a fault quickly developed: when 
>> switched to 'LINE OPR' (mains powered) the meter swings to the extremes 
>> and then settles to zero and won't move again as though it's switched 
>> off. I'm too busy to deal with that right now, but I was wondering, how 
>> much noise should I expect to see in the meter needle with the input 
>> shorted?
>>
>> Alan
>>
>>
>> ----- Original Message ----- From: "Mark Sims" <holrum at hotmail.com>
>> To: <volt-nuts at febo.com>
>> Sent: Friday, March 19, 2010 1:04 AM
>> Subject: [volt-nuts] Fluke 883AB differential voltmeter
>>
>>
>>>
>>> I just finished restoring a Fluke 883AB differential voltmeter (and a 
>>> 845AB null voltmeter).
>>>
>>> The 883 AB needed new battery packs (I used 4 x nine AA cell nicad packs 
>>> and a 4 subC cell pack)...  remarkably cheap from a Ebay vendor.  The 
>>> original 9.6V packs used an unobtainium coin cell and would have cost 
>>> around $200 to replace).  Also the power supply for the AC input 
>>> converter was fried (transistor, zener, cap).
>>>
>>> Final problem was it would not zero properly.  The zero circuit uses two 
>>> diodes to make a +/- 0.6V supply driving the offset pot.  The unit was 
>>> offset so that you could not reach zero and the offset pot had fairly 
>>> little range.  Adjusting the chopper didn't help.  I replaced the diodes 
>>> with three 1.2V reference chips (making a -1.2 to +2.4V supply).  This 
>>> centered the zero adjustment and gave better range.
>>>
>>> Adjusting the KVD is a pain...  you have to unsolder/solder various 
>>> jumpers, etc.  Anyway,  it now seems to be working better than new.
>>>
>>> Now to the Fluke 515A calibrators.  They have two 18 (AAA?) cell packs. 
>>> Replacements can be had for $175...  I think I can do better (or will do 
>>> without).
>>>
>>> I also have a Space Labs digital igniter tester (basically a Kelvin ohm 
>>> meter).  Uses a 6 cell NiMH coin cell pack built into a piece of PVC 
>>> pipe. One place wanted $600 for the replacement!   Since the unit only 
>>> draws 11 ma while you hold the test button,  I replaced it with a 9V 
>>> smoke alarm battery / diode to block charging.  Should last forever (and 
>>> not self discharge in a month)
>>> _________________________________________________________________
>>> Hotmail: Trusted email with powerful SPAM protection.
>>> http://clk.atdmt.com/GBL/go/210850553/direct/01/
>>> _______________________________________________
>>> volt-nuts mailing list -- volt-nuts at febo.com
>>> To unsubscribe, go to 
>>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
>>> and follow the instructions there.
>>
>>
>>
>> _______________________________________________
>> volt-nuts mailing list -- volt-nuts at febo.com
>> To unsubscribe, go to 
>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
>> and follow the instructions there.
>>
>>
>
>
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.