[volt-nuts] Battery pack info for Fluke 731B

Roy Phillips phill.r1 at btinternet.com
Mon Aug 22 11:08:42 UTC 2011

I was very interested to read your notes on battery change for the Fluke 
731B.  I have two of these units, and have made identical changes with NiMh 
cells with the same good results.   One of the shortcomings of this unit 
would appear to be its rotary voltage selection switch. which would seem to 
introduce a "variable" resistance to the output ?  Am I right about this, 
and is this why the subsequent models had separate terminals ? The power 
supply section is relatively simple, and in my case it was damaged a little 
by the original  NiCad's, another good reason to get rid of them. I assume 
that the accuracy/quality was due to the high stability resistors and the LM 
type voltage reference.

From: "Bill Gold" <wpgold3637 at att.net>
Sent: Sunday, August 21, 2011 9:50 PM
To: <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Battery pack info for Fluke 731B

> David:
>    Faced with the same problem with the battery pack, I disassembled an 
> old battery pack and found that there were indeed 12 NiCad cells in the 
> pack.  I then reasoned that replacing them with 12 AAA NiCads would 
> probably just have me replacing them a few years down the line.  So I got 
> 12 AAA NiMh cells right off the shelf at the local Home Depot, probably 
> around 1000 mAh, and arranged them in two rows of 6 cells each and stacked 
> the rows so that the second row of cells would fit into the "depression" 
> or space between the cells of the first row, because the cells are round. 
> This reduced the overall thickness of the two rows.  I then wrapped this 
> with Duct tape, a little difficult to hold the shape, but not impossible. 
> Then I soldered wires from the "+" and "-" terminals so that the cells 
> were in series so I got the 14.4 volts necessary (12 X 1.2V). I then took 
> the two original wires and plug from the original battery pack and 
> connected them coming off of the end of the string.  I covered the top and 
> bottom of the pack to insure no shorts occurred.  I made no changes to the 
> charging circuitry and I simply plugged it in and measured the current 
> flow and voltage.  I saw no over current or voltage problems.
>    I then got two longer screws for the "U" bracket that holds the battery 
> pack so that I could clamp the new pack inside the 731B.  This keeps 
> everything just like normal from the outside.  And it fits very nicely 
> inside as there is plenty of room.
>    Be careful when soldering the ends of the AAA batteries as they are not 
> intended to be soldered (if you can buy "solder tabs" then so much the 
> better) and don't get them too hot.  I make sure there is no corrosion on 
> the ends of the batteries by cleaning them up with some fine sandpaper, 
> then tin the ends, wait for the battery to cool down and then solder the 
> wires on quickly.
>    I checked the 10 volt output before doing this against a Fluke 731B 
> from work, left the 731B plugged in while changing the batteries, and then 
> checked the 731B again after all was done and it had a chance to settle 
> down after being opened.  No change I could measure other than a possible 
> room temp change effect.
>    I did this to my personal 731B and then to a 731B at work.  I then sent 
> the 731B at work into the local Fluke Cal lab, when Fluke had a lab around 
> here, and they returned it with no change noted.  The one at work had 
> always been returned with less than a 0.5 ppm change for about 10 years, 
> and never adjusted.
>    I did this about 8 years ago and have had no problems since, including 
> having to change the batteries every few years.  And the batteries last 
> longer when not on line power.
>    It looks to me like you have selected the correct ten turn dial 
> indicator.
> I hopes this helps everyone.
> Bill
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