[Test-Equipment] Simpson 270 VOM

[k4pf at juno.com]

Hi, Bill

I have a Simpson 270 as well.  
I suppose you have a different series than I do (Series 4, 
about 1975 vintage) since the pot numbers you referenced are different than my schematic.

To calibrate it, you need a 50uA dc current source fed to the 50uA
jack on the front panel, with negative to the common jack.
I would set the range scale on the meter to 500VDC for max isolation
from the remaining terminals during this procedure.
Adjust the pot across the basic meter movement so it reads 
full scale.

Then adjust the pot in series with the basic meter movement 
(using a high impedance voltmeter), so that there is 250mV 
between the 250mV pin and the common pin when the 50uA 
is applied to the pin.

That completes the calibration for DC,
since all your resistors checked OK.

For future reference, if you need to touch up AC calibration,
the pot that connects to the junction of the two diodes
in the AC rectifier is used to adjust the 2.5VAC scale.
The pot connected at the junction between one of the rectifier
diodes and a resistor is then used to trim one of the higher voltage 
AC scales for full-scale calibration with that voltage applied.

I remember calling Simpson around 1999, when I needed 
to recalibrate my 270, and realized the info wasn't in the manual.
The woman on the phone said "Our calibration procedures
are for our technicians."  I eventually found the info
I needed on line, in a military tech manual.

73,
Ed Knobloch

> Bill Abate wrote:
Need help on the correct procedure for calibrating my 270.
It started with the ohms scale zero adjustment 
needing to be set at minimum resistance with the probes 
shorted to zero the meter. Batteries are fine.  
The service info I have indicates there is no adjustment 
for resistance scales.  Traced out the circuit and all resistors 
are at the specified values.  Did a calibration on the 1 ma scale as the R27 pot is in series with the basic movement. 
That did not help.  The only thing 
left to do is adjust the R32 pot that is across the basic movement.  I can't find any info on how to adjust that pot. My best guess 
is to use it to calibrate the 50 micro amp scale but I'd like confirmation before trying that.  
When I checked the 50 micro amp scale the specified 
current only showed 44 on the meter..