Todd, My 845 has the optical chopper circuit. It quasi-works but the most sensitive ranges are very noisy so I will replace the amplifier circuits with a new chopper stabilized circuit after I finish the 419 modifications.
Randy On Sat, Aug 9, 2014 at 7:48 AM, Todd Micallef <tmical...@gmail.com> wrote: > Randy, > > I am looking forward to your test results. Does your 845 have the separate > board for the chopper amp as mentioned in the change/errata sheet dated > 7/93? I have not seen any further documents from online sources that > references this change. I have two AR models and it appears support for it > was dropped long before 1993. It would be interesting to see how many of > the original parts were removed and how this new board was mounted inside > the AB chassis. > > > Todd > > > On Fri, Aug 8, 2014 at 10:23 PM, Randy Evans <randyevans2...@gmail.com> > wrote: > > > 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.