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

[Randy Evans]

I have finished the design and analysis (LTSpice) and I am preparing the
BOM and doing a PCB layout now (ExpressPCB).  I am hoping to finish the
work over the Thanksgiving holidays but I wanted to see Dallas's 845A
modifications before ordering parts and pulling the plug on PCB fab first
but it hasn't shown up on K04BB's site yet (still in the upload section).

Randy

On Wed, Nov 26, 2014 at 4:06 AM, <Ludger.Lenzen at gmx.de> wrote:

> Hi Randy,
>
> did you finish the HP-419A modification with a modern chopper amp?
>
> I have to refurbish my HP-419A so it would make sense to replace the
> optical chopper. Any help or made expericance would perfect.
>
> BR
> Ludger / PeLuLe
>
> *Gesendet:* Samstag, 09. August 2014 um 03:23 Uhr
> *Von:* "Randy Evans" <randyevans2688 at gmail.com>
> *An:* "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> *Betreff:* Re: [volt-nuts] HP-419 and Fluke 845 Modifications
> 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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