[volt-nuts] Fluke 731B parts xref

WB6BNQ wb6bnq at cox.net
Thu Nov 22 06:43:16 UTC 2012

Hi Chris,

If you study the schematic you will see that S3, the front panel lock-open-momentary switch, is
in series between the standard cell (J10) and the null meter (J8).  So what is happening is the
voltage from the 750A goes to one side of the null meter and the reference (Standard Cell) goes
to the other side of the null meter.

Remove the wire from S1201 going to J7 and cover it with heat shrink.  Then connect a wire
between J7 and the 10 volt TAP (top of R12).  From this point on, the "Standard Cell" banana
jacks is where you connect the 731B 10 volt source, the null meter goes to its jacks and the S3
switch is still normal.  The kelvin decades for the 1.xx voltage are no longer in the circuit and
have no effect.

Should you ever want to sell the 750A (you shouldn't) it is easy to reverse back to the factory
arrangement.  If you don't have a manual for the 750A, let me know and I will forward it to you.


Christopher Brown wrote:

> On 11/21/12 6:44 PM, WB6BNQ wrote:
> > Hi Chris,
> >
> > Do you mean the Fluke 750A ?  It has connections for standard cells.  The 752 is a ratio
> > device designed to be used with a 10 volt reference standard.  No need for the 1.xx outputs
> > from the 731B or the use of standard cells..
> Your are correct, not the 752 (would like a 752 and a 720) but do have 2
> 750A.
> >
> > The 750A standard cell arrangement can be bypassed and set up for use with just a 10 volt
> > reference using the original banana jacks for the reference and null meter as normal.  All
> > that is needed is to (non destructively) remove one connection and add one wire.
> >
> > Bill....WB6BNQ
> That sounds like a very worthwhile mod.  How is it done and how does it
> change operation?
> As far as the 731B(s), I have 2.
> One is in good working order and overall great shape.  Came out of a
> standards lab in the czech republic that recently switched to a 732A.
> Have a replacement NiCd pack currently cycling.  Other than cleaning,
> and replacing the dead pack the only changes planned are replacing the
> IEC socket with a small filtered socket (have a box of them) and
> replacing R30 (if needed) to adjust the float current for the new pack
> (old is 450mah, new is 2100mah.
> For the second (received this morning), intend to do the same, and
> repair.  I may just hard-wire around the failed switch, but beyond that,
> not fixing it would just bug me.
> The ref board is in good shape and does not appear to have been messed
> with.  The main issues being the pot and the P/S section.
> Near as I can tell the Fairchild 1N5248B is a good match for CR1, and
> for 2 cents each I can just grab a batch and select.
> I have a bunch of 4148 and 1n914 fairchild parts around here somewhere,
> curious as to suggestion of 4448 over 4148?  Looks like 2pf v.s. 4pf and
> the 4148 may start conducting a few hundred mv earlier from the datasheet.
> Thanks
> Chris - WL7CLA
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