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

[Dave M]

> 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