Cropico ESC1 Electronic Standard Cell

A few days ago I managed to buy one of these for £5.50 on Ebay, it was was one the first zener diode replacements for the Weston cell,  but when I powered it up from my bench power supply it was dead!   so,  I cracked it open and after a while checking through the components I discovered one of the tantalum caps had shorted,  so I replaced that with an electrolytic one of the same value and when I powered-it-up,  it was working once again!.

The bad cap which was replaced!














The unit powers-up with 27 volts (or 18 x 1.5 volt C cells! ) and consumes ~16mA

From some measurements I took with my 7150 it seems that my 7150 has drifted quite a bit from the cal I had done a few years back;   the good thing is I can reasonable confirm this drift because the 1.0000v reference from my Time 2003S reads exactly the same on the 7150 as the Standard Cell.

The 7150 readings were..

Standard Cell  @ 1 volt
0.999815 volts

Time 2003S @ 1 volt
0.999815 volts

IDM305 DMM 1
0.9989 volts

IDM305 DMM2
0.9986 volts

Ambient was approx ~19.5 degrees C.



Inside the cover of the unit was a historic record of the measured Standard Cell voltages.

An identical unit was taken apart over on one of the popular electronics forums eevblog, the linked page also contains some useful PDFs.

http://www.eevblog.com/forum/metrology/cropico-esc1-electronic-standard-cell-a-look-inside/


This is another useful link to a site called "Johns Virtual Museum of Interesting Gadgets" where gives a short bit of information about the unit.

https://sites.google.com/site/johnhurll/home/cropico/ecs1-electronic-standard-cell