[volt-nuts] Fluke 887A Modifications

Randy Evans randallgrayevans at yahoo.com
Thu Dec 26 11:54:24 EST 2013

I have received a lot of interest in the modification to the Fluke 887A that I posted earlier.  I have sent out packages with tthe following information to help people understand the modification.  

- schematic
markup of the changes 
- a hand-drawn schematic of the new circuit
- LTSpice analysis of the new circuit
- picture of the installed maodification.
The mod on 887A is
really easy and it only took me about 2 hours to make the circuit and install
and test it.  Fortunately, I had all the parts on hand (but it requires
very few parts).  The only time consuming part was removing all the
existing components but even that was only about an hour.  
I used the LTC1050
precision op amp but it required +/- 5V so I had to add voltage regulator chips
to drop the -18 and +8 V lines down to  +/- 5V.  The +8V line is listed as
+6 on the schematic  but I measured +8 on my unit.  If it’s really +6
on your unit then you would need an LDO regulator chip for the +5V line. 
 If you use the LTC2057 precision op amp (recommended) instead of the
LTC1050, then you wouldn't need any regulator chips at all, making the circuit
even simpler.  The parts list would become the LTC2057, two 1uF
bypass caps, a 1 K resistor, and a circuit board.  Note the 1 Kohm resistor is needed since the -18
V power supply oscillated when it was directly connected to the 1 uF input
bypass cap on the 79L05 regulator chip.  The power supply doesn’t like a capacitive load.  The unit also oscillated
until I removed C219 for the output filter but the LTC1050 doesn’t need any additional
filtering.  Obviously, Q206 and C217 need to be removed as
After I finished calibration
of the unit it worked fine.  The zero
adjustment does not ever seem to need adjustment after the initial setting, not
surprising since the LTC1050 has a voltage offset stability of .05 uV per
Degree C.  So far I am very happy with
the modification and it couldn’t be any simpler.
Randy Evans

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