Electronics > Metrology

General Radio 1403A 1000pf capacitor adjustment

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charliedelta:
I have a General Radio 1403A 1000pf capacitor. Its supposed to be a 1000pf capacitor with 0.1 % tollerance. When i measure this capacitor with  several accurate LCR meters I  am getting values readings of around 750pf.

I cant believe that such a well constructed capacitor can be so badly off. There appears to be no easy way of adjusting this capacitor.

I seem to think I am  misunderstanding how this capacitor was calibrated or measured. Can anyone shed some light onto why this capacitor is so badly off?

There is no signs of physical damage to the plates.

Thanks

antintedo:
What is the DC resistance between terminals? There might be whiskers shorting some of the plates.

Stray Electron:
  I might be wrong, but it looks like there is white corrosion on the plates and around the top where it joins the cylindrical shell. And it looks like there are dents and dings in the plates!!

Stray Electron:
  The connectors and the plates are simply stacked on metal rods and are held in place with nuts. Have you checked to see if all of the nuts are still tight?  You probably need to check the torque on them with a torque wrench instead of just tightening them with a pair of pliers.  But I have no idea what the torque requirement is. But you could probably find a recommended torque value for that thread size, pitch and material in Machinery's Handbook or other reference.

  To me, it looks like there are clear signs that the inside of that unit has been exposed to moisture. If so I suspect that there's some dissimilar metal corrosion between the aluminium plates and the stainless steel shafts and nuts and I expect that that's what is altering your readings.

Stray Electron:
   Did you already take one of the plate supporting rods out of the capacitor assembly?   it looks like there's one missing.  There is a empty hole shown near the top OF THE PHOTO in the bottom photo.

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