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

[Bill Gold]

Roy:

    You are right on the money about the rotary voltage switch.  From what I see these were made with gold plated contacts which should make excellent contact for a very long time, but they simply wear out in time.  The gold plating is worn away.  Mine does the same thing, have poor contact sometimes.  I have put a very light coating of Caig Laboratories DeoxIT and then Caig PreserIT on the switch contacts, and this improves things a LOT.  If I have a problem then exercising the switch a few time usually fixes the problem.

    I just check my 731B and the 10 volt position was doing just fine but the 1 volt was a little flaky.

    Yes, this is why the Fluke 732A has fixed output terminals with no switches in the output path, also the 732B does the same thing.  The worse thing in the 732A is the 20 turn adjustment pots, which really don't cause much of a problem.  In the 732B Fluke didn't use pots, but "digital" rotary switches to set the output voltage.  This does seem better from both the ability to set the output voltage and the stability, but I suspect that age will get to these also.  The old Fluke 332B/D/335A/D voltage standards have exactly the same problems with the output voltage switches, along with the range switch, all of these switches are gold plated.  I have disassembled several of these units, cleaned and then applied the above Caig contact improvers to solve those unit's problems.  The range switch is a real problem because it is enclosed and no way to disassemble it.  This is why Fluke used something else, I am not sure what yet, to setup the feedback string for the 5440/5442 voltage standards.  I don't believe there are any mechanical switches in the feedback string of these units, but I haven't had time to look into this.

    Both the 731B and the 732A use what Fluke refers to as a "Reference Amplifier" to get the stability of the voltage output.  This is a 4 terminal device which consists of a temperature compensated zener with the emitter of a NPN transistor hooked to the cathode of the zener.  If you refer to your 731B schematic, this amp is "U2".  By carefully adjusting the collector current of the transistor you can obtain an almost zero temperature coefficient of the base/emitter/zener voltage over a small temperature range, maybe 20 C to 25 C, I am not sure about this temp range.  My 731B and the one I had at work had a very good factory adjustment of the tempco current.  In the 732A this whole PC Board assembly was enclosed in a very tightly controlled temperature oven running at 45 C.  This vastly improves the changes in room temperature to affect the voltage reference.

    I believe that the 732B started out using the "Reference Amplifier" and then in later units changed to the Linear Technology LTZ1000A references.  Fluke won't show you the schematic of the voltage reference board, it's a "magic black box".  The new Fluke 7000 series Voltage Maintenance System uses the LTZ1000A, probably because they are easier to select and use.  The venerable HP/Agilent 3458A also uses the LTZ1000A as it's reference element.

    As far as I can see from several manufacturers of NiMH cells, it is perfectly ok to keep a 1% to 2% maintenance current flowing just about forever.  Of course don't ever maintain at a rate that gets the cell hot as this will destroy the cell in short order.  This has certainly been my experience for the last 8 to 10 years.

    Sorry for getting too long in this but that is why I am a "Volts-Nut".

Bill