[volt-nuts] HP 3420B calibration assitance

[Stan Katz]

Hi Dick,

If you haven't posted to Yahoo Groups hp_agilent_equipment because you're
exercising proper internet ettiquette, iwaiting for a first reply here,
don't wait too long. The agilent group is terrific.

I have no experience with the HP 3420B, although I have a much older 740B
diff. meter. Fortunately, all its internal alignment could be accomplished
(excluding ref. voltage) with onboard resistor networks.

Good Luck,
Stan

On Thu, Mar 18, 2010 at 12:24 AM, Dick Moore <richiem at hughes.net> wrote:

> Hi, volt-nuts -- I picked up an HP 3420B diff. dc meter for *real cheap*
> and it works pretty well. I've replaced the batteries with AA NiMH units in
> 4 each 10-batt plastic holders (there was just enough room to shoehorn them
> in -- the A version is line-power only, the B version is line or batt), and
> have done a little clean-up and some repairs, replaced some knobs, etc.
>
> This unit has cal pots to adjust the total resistance of each of the
> resistor sections for the first two decades of the six decade divider
> (modified Kelvin-Varley constant-R) string. HP supplied a special test board
> for this purpose that plugs into a socket on the divider assembly, but the
> test board is long gone from this unit. The manual copy I have shows a
> *very* convoluted process for adjusting these pots in the first two decades
> and this depends on having the test board plus some precision resistors and
> some connectors.
>
> Is there a more direct way to adjust these pots using a precision variable
> reference voltage (from the Fluke 5440B that I own) as input? Any help
> appreciated. This is a very nice meter with an excellent ratio function. Be
> nice to have it working at or near spec. I do own an HP 3458A to use to look
> at internal voltages and resistances too. Because all of the string
> resistors in a decade are always all in series, I can't wrap my head around
> a procedure to adjust the pots, since they interact....
>
> The general form of the first two decade dividers is as follows: the first
> decade has five resistors which are switched in and out in combos to make up
> Ra and Rb, and to maintain a constant 11k, using 5 resistor sections, each
> with a tweak. The second decade uses 4 resistors, each with a tweak, to
> maintain 10k total. The dividers use a 4k, two 2k and one or two 1k
> resistors for the Ra and Rb, each with a tweak, so there are five pots for
> each resistor of the first decade and four pots for each resistor of the
> second decade. The remaining four decades are fixed with each having a 10k
> total R. I formatted the following diagram with a constant width font, so I
> hope it displays OK... worth a try....
>
> I---------|
>          |
>          Ra
>          |
> Ref input |
>          |
>          |--------O
>          |
>          |
>          Rb       Out to next decade
>          |
>          |
> I---------|--------O
>
> As Ra goes up 1k ohm, Rb goes down 1k ohm, & vv
>
> Dick Moore
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