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

[Randy Evans]

Stan,

I working for a daughterboard design.  I will be using parts of the
existing circuitry and I am not worried about maintaining "purity".  I just
want a functioning nullmeter.

Randy

On Tue, Dec 9, 2014 at 9:25 PM, Stan Katz <stan.katz.hk at gmail.com> wrote:

> Hi Randy,
>
> I have a very pedestrian and naive question regarding the fab of the
> chopper eliminator board for the HP419A. Is it your intention to replace
> the A4 board with a board containing your new design? If so, you have to
> keep in mind the A4 board is a whopping .173" thick.  Expresspcb seems to
> only have one thickness in their fab of .062". I'm sure that wont mate well
> with the existing connector. There are other fab houses that can provide
> boards as thick as .125" stateside (sunstone.com), or pcbcart.com China
> (.126"). Even using these other fabs leaves almost 50 mil short of the
> original A4 thickness. Maybe the connector can make up the 50 mil
> shortfall? If this is just a 2-sided board, you could split the layers onto
> two circuit boards with thicknesses that come close to the original A4
> board. These boards would have to be epoxied together by the end user.
> In any event, you may not get by using Expresspcb since two of their boards
> combined still leaves that 50 mil shortfall.
>
> All of the above is easily dismissed if you intend to mount a daughterboard
> onto the A4, cut the traces on the A4 down near the connector, and then
> wiring the daughter board onto the trace remnants still running to the
> connector. The only downside would be no resale to a purist who wants to
> run with the David Wise HP H11 linear optocoupler/photochopper replacement
> (see HP yahoo group for HP740b ).
>
> Just curious,
> Stan
>
> 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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