[volt-nuts] Fluke 735C - Follow Up

[J. L. Trantham]

Thanks to everyone for their thoughts about the 735C 10.00000 VDC standard.

 

I cleaned up the 'corrosion', rebuilt the NiCd battery packs, and plugged it
in.  It measured about 10.00010 VDC after being on for 24 hours or so.

 

I reviewed the manual for the 732A and found quite a similarity, including
the resistor 'decade' of 2K, 1K, 500, 250, and 125 ohm resistors to select
from to achieve calibration.  On the 732A, these are brought out to a
separate (sort of external) PCB to make selection easier.  However, on the
735C, all are mounted on the 'oven unit PCB' and I had to open the oven
several times in order to come up with the correct combination of resistors
in parallel to achieve calibration.

 

I used my recently Agilent Cal'd 3458A and was able to adjust the 'CAL' pot
on the front panel (R20 per the 732A schematic) to bring it 'on scale'.
Looking at the output, it seems to be +/- 2 uV when watching it over about a
week.  Amazingly stable, compared to my 731B's.  I wound up using the 125,
250, and 1K ohm resistors in parallel to get the 'CAL' pot to bring on scale
as measured by the 3458A.  The 'TH' resistance is about 3.3307 K ohms and
very stable.

 

The 732A has a front panel LED that illuminates when the unit is 'IN CAL'
while the 735C has an LED that illuminates when there is 'NO CAL'.

 

To get the 'NO CAL' LED to go out, I just used a copper 14 ga wire as the
'twiddler' and when I touched the internal 'Oven' case (likely shorting it
to the front panel case) the LED went out (after everything had warmed up
for a day and adjusted to 10.0000000 VDC).  The 'NO CAL' LED has been out
now for about a week and looking at the 3458A it is about +/- 2 uV from
10.0000000 VDC.

 

I have some pictures of the 'innards' of the oven if anyone is interested.

 

The NiCd's (2800 mAH compared to 2500 mAH original batteries) seem to power
the unit for only about 3 hours.

 

Joe