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. -Chuck Harris 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. CRAZY!!!!! 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. Cheers, David _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
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