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 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
> 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 wrote:
> >
> > > From: Randy Evans
> > >>
> > >>
> > >> 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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