[volt-nuts] 540b Arrival...........
cfharris at erols.com
Wed Sep 30 13:04:10 EDT 2015
I have several 540B's that have what is probably this same
modification. The circuit board doesn't say Fluke, but it is
very nicely done.
What I find is a 1x1 circuit board that is attached to a standoff
from the front panel. The circuit board has an LM317 adjustable
3 terminal regulator, and uses a 1.3K and 100 ohm resistor divider
to create the 1.35V. It also has a 1uf electrolytic capacitor to
filter the output a bit.
There is a Keystone aluminum double AA battery holder that is
double stick foam taped to the chassis. The holder has a pair of
lithium *1.5V* AA cells in it.
The lithium 1.5V AA cells have ordinary 3V lithium cells in them,
and also have a small switching supply that drops them down to
1.5V. At low/no current, the cells produce more like 1.8V.. which is
handy for this modification... mostly because the LM317 is specified
to need a minimum of 3V input to output for good operation. It is
also specified to need 10ma minimum load, and this circuit is loading
it around 3ma.
And yet, it seems to work as provided. I think it would be better
if 3 AA cells were used instead of the two they applied.
The board is tapped into the circuit by taking the battery output,
and running it to the power switch terminals for the reference
battery... and then taking the switched reference battery output,
and running it to the little circuit board... They just spliced
the wire they removed from the switched reference output terminal
of the switch, to the output of the regulator board.
And, they changed the calibration resistor so that it shows good
at 3V, instead of 1.35V.
David Garrido wrote:
> Hello All,
> My new to me 540b arrived last evening. I have never used one of these so I put
> it on the bench, opened it up and inspected it looking for leaks primarily.
> EVERYTHING is spotless. Fluke factory cal stickers everywhere. AND………….after
> plugging her in over night, all of the batteries take a charge and test in the
> middle of the BATT scale.
> All of the batteries in the 20 cell pack are individually shrink wrapped in light
> blue and the two others on top are AA size lithiums. Two of them side-by-side.
> Was this a factory mod? All the work looks to be factory, but who knows if the PO
> was just REALLY good at clean work.
> Anyhoo………thanks for the bandwidth.
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