[volt-nuts] Fluke 731B battery & charge circuit

Christopher Brown cbrown at woods.net
Mon Jun 17 03:29:54 EDT 2013

They are not selected, think of that whole assembly as the "pre supply",
the close regulation and the standard are later.

I suspect noise would be had, but exact voltages come later.

I ordered 100 Fairchild 1N5248B (Mouser 512-1N5248B) for $3.70

Not exactly critical, but this is a fun hobby, so I setup a quicky test
setup, ran em all, picked the five closest to 18V, then breadboarded em
with 24v through dropping resistor and let em run for a week (while I
was doing other work on the 731s).

Didn't see any change, picked 2 and used em.  Rest are in the parts bin.

That jfet and the shielded transformer are hard to get.  Everything else
on the power supply board is nothing special.  All of the specially
selected stuff is on the daughter board.

My take on things...

The highest float charging battery voltage should be ~ .8v lower than
the zener.

The "settled" voltage of the battery (where the pack will sit after
12hrs of no charge and where it will sit until the final discharge
slope) should be high enough that there is no change in the output as
read by a 3456A in 6 digit.

The point here is to have a short term stable noise free power source
going to the actual ref circuit.  Longer term, you just need to keep the
changes small enough to minimize output changes.

Based on what I saw with both of mine (came without batteries), that ~
20v actually included about 3V AC, and the meter was calibrated without
the pack. (Take a reading of pre-supply output with the battery
disconnected with and without filter DC and then ACV on the 3456A)

  Both of mine were last calibrated _before_ their batteries were pulled
and were low when running off of the battery charger.  I did not find
the problems with the zener, jfet and current limit resisters until I
built replacement packs and was trying to verify circuit functions.

On 6/16/13 10:48 PM, Orin Eman wrote:
> On Sun, Jun 16, 2013 at 11:20 PM, Christopher Brown <cbrown at woods.net>wrote:
>> You might want to check/replace the current limiting resistor in the
>> batt charge circuit, and the zener.
> I'll check the current limiting resistor for sure.  It's a carbon comp, so
> it could well have drifted.
>> Both of my 731Bs had low zener output.
>> Had to replace the zeners in both and that 5ma jfet regulator in one.
> The E505 has already been replaced with I presume a J505 as it's in a TO92
> case.  Do you remember what your good zener voltage was?  I know they are
> spec'd at +/- 5% and the J505 is +/- 20% (don't know about the E505, I
> can't find a datasheet) so there could be quite a range.  There is no
> indication on the part list that they were selected.  I have some zeners on
> order and I'm going to see what I get with 1mA through them.
> Thanks for the info.
> Orin.
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