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 <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 > > 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. > _______________________________________________ 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.