[Test-Equipment] Simpson 270 VOM

[k4pf at juno.com]

Hi, Bill

To follow up our discussion on the Simpson 270 calibration -

I looked at the military manual you referenced, TB 9-6625-2352-35.
At the bottom of page 13, in the procedure for the Simpson
model 260 series 3, they explain how the 50uA jack 
is also the 250mV jack in footnote 2.

One error in the chart is that they lead off with '250'
at the top of their voltage chart.  It should be labeled 250mV.

The adjustment for 250mV is also relied on for the current metering
function. The current shunts have 250mV voltage drop 
across them for full scale reading.  Unless that series pot 
is first adjusted for 250mV with 50uA applied, all the current
readings will be off. 

My method, using a 5V supply with a 95K series resistor allows 
both the 50uA and 250mV adjustments to be done simply,
using one set-up.  Of course, you could use a different supply
voltage, requiring a different resistor than 95K,
but I assume everyone has a regulated 5V supply handy.
Just don't forget to account for the 5K resistance of the basic
movement and its adjustment pots.

You never mentioned what series 270 you have, so I couldn't
reference the pots by number for your unit.  

If you download a 270 series 4 manual, the 50uA current adjust pot
across the meter movement is R1.  The pot in series with the meter
movement to adjust the 250mV setting is R2.

73,
Ed

> Bill Abate wrote:
Thanks for the info.  I hope the recal helps the ohms zero problem.
I found a military manual, TB 9-6625-2352-35, for the Simpson meter 
service, but I did not find the info that you provided.  One thing is 
confusing me, I can't follow what you are calling the 250MV pin.  Where is that?  The calibration that I have for the meter series pot is for the 1 ma scale.