[volt-nuts] HP-419 and Fluke 845 Modifications

[Randy Evans]

For those interested, I have largely finished the design of the HP-419A
modifications to replace the current active circuits with a more modern
chopper amp; e.g., LTC2054.  The LTSpice analysis shows the circuit draws
0.3 mA at full scale meter readings using a single 6V battery.  I think I
will use a 6V 225mAH NiCad rechargeable battery (because I have some) that
should last many many hours on one charge.  I am still working out the
board layout and unit mods but the circuit is extremely simple and will use
all the existing range and mode switching circuits.  The circuit should
also work for the Fluke 845 also.  Keep tuned.

Randy


On Mon, Aug 4, 2014 at 10:05 AM, Dave M <dgminala at mediacombb.net> wrote:

> From: Randy Evans <randyevans2688 at gmail.com>
>>
>>
>> I am looking at modifying my (defective) HP419A and Fluke 845A Null
>> meters
>> using the LTC2054 zero drift op amp.  I am planning on using a single
>> 4.2 V
>> Lithium Ion battery to power them (no AC input required and may not
>> include, TBD).  The LTC2054 has a very low bias current of typically
>> 1pA at
>> room temp and the bias plots vs. temp  show no appreciable increase
>> until
>> about 50C.  Conceptually, it looks pretty straight forward and, based
>> on my
>> Fluke 887A modifications, likely will be very stable.
>>
>> Since this is a very astute group, does anyone have any comments on
>> the
>> feasibility of this modification before I get too involved?
>>
>> Regards,
>>
>> Randy Evans
>>
>
>
> Hi Randy.  Good to hear from you again.
> I have modified two 883A models and one 887A model DMMs using your
> details, with great results.  I have a model 845A null meter that is
> working fine right now, but who knows how long that will last.  I would
> surely welcome some research on modifying it with a zero-drift opamp.  One
> concern that I would have with that mod would be that some mechanism would
> need to be added to retain the isolation of the recorder output and guard
> integrity that's afforded by the photo-modulator/demodulator circuit.
> Perhaps some kind of optically-isolated interface might be in order
>
> Some Spice runs on the original circuit would be interesting to see.
>
> Dave M
>
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