Hi Randy, I have an HP-419A (ser# 948A-06384, seems to be one of the latest revisions ~198x) I refurbished myself including battery pack (NMiH) and it works not bad. Just noise in the highest sensivity range isn't sufficiant.
I have a spare HP-419A (ser# 543-00149, one of the olderst revisions ~196x)with defekt NEON choppers. That I resevered for your announced modifications. I also have an acceptable equipped lab (including EAGLE layout sw V4 [proffessional edition] & V6 [limited edition]. So if you need some support to reduce your work load, just give me a note. I am happy to help. BR PeLuLe Gesendet: Donnerstag, 11. Dezember 2014 um 02:50 Uhr Von: "Randy Evans" <randyevans2...@gmail.com> An: "Discussion of precise voltage measurement" <volt-nuts@febo.com> Betreff: Re: [volt-nuts] HP-419 and Fluke 845 Modifications 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...@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 <randyevans2...@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.len...@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" <randyevans2...@gmail.com> > > > *An:* "Discussion of precise voltage measurement" <volt-nuts@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 <dgmin...@mediacombb.net> > wrote: > > > > > > > From: Randy Evans <randyevans2...@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 > > > > > > > > _______________________________________________ > > > > volt-nuts mailing list -- volt-nuts@febo.com > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > > > > mailman/listinfo/volt-nuts > > > > and follow the instructions there. > > > > > > > _______________________________________________ > > > volt-nuts mailing list -- volt-nuts@febo.com > > > To unsubscribe, go to > > > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts[https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts] > > > and follow the instructions there. > > > > > _______________________________________________ > > volt-nuts mailing list -- volt-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts[https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts] > > and follow the instructions there. > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts[https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts] > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts[https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts] and follow the instructions there. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.