[Test-Equipment] Simpson 270 VOM

[k4pf at juno.com]

Hi, Bill

On my 270 series 4, the 50uA jack is also labeled as a 250mV pin.
It's the equivalent, for a 20K Ohms/Volt movement.

If you divide 250mV by 50uA, you get 5K Ohms.
So a quarter volt full scale would see a 5K resistance load.
Therefore, a one volt scale would be 20K Ohms total resistance.

I see for the 270 original model, and for the 270 series 2 and 3,
 they didn't label that jack both 50uA as well as 250mV,
they only labeled it 50uA.  But it serves the same function.

We can create a 50uA current source using a 5V power supply
in series with 95K resistor.  That will be the correct
current assuming a 5K load by the meter.  There may be a slight
back-and-forth adjustment as you trim the series pot to get 0.25VDC
between the 50uA jack and the common jack, and may need to touch
up the pot across the meter movement to maintain full scale.

Something like a 10Meg Ohm input DMM would be ideal 
to measure the 250mV drop, without disturbing the Simpson
calibration. 10Meg in parallel with 5K is a negligible error.

73,
Ed Knobloch  K4PF


> 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.