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

[Todd Micallef]

Randy,

I sent you a copy of the file to your email address.

Todd

On Wed, Nov 26, 2014 at 12:18 PM, Randy Evans <randyevans2688 at gmail.com>
wrote:

> 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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