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
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