DEKABOX DC-40 Zero Capacitance.

[rastro]

I have a DEKABOX DC-40 decade capacitor.  Problems with zero and lowest setting.

My "zero capacitance" setting consistently reads ~30.6 pF which is twice as high as the label "15uuf" (15pF). The unit seems very repeatable after initial cleaning.

I've cleaned the contacts on all rotary contact switches using 98% IPA and contact cleaner.  Accounting for the nominal 30pf offset, all selections come in under 1% error - with the exception of the lowest value (100pf) which is 1.8% high.

I measured input of both sets of rotary contacts separately (disconnected) and they each read close to 15pf when set to zero capacitance.  Since they are connected in parallel inside the unit this would come out to the 30pf (makes sense).  It seems odd they both read the close to the same 15pf. It leads me to believe both have a similar problem.

Measured with DE-5000 (setting: (Cs at 1 KHz) using TL-21 (alligator clips).  Shows 0.3pf with no connection - so I'm confident it's not an offset issue with the test setup. 

Does anyone have a DC-40 they can read the capacitance when all is set to zero?

[ebclr]

Did you try connecting guard to gnd ?

[SeanB]

Try to zero it again with the leads as shown when measuring. Also, connect the ground lead, it will make a difference.

Most of the error is just a connection issue, as the rest of the selections do read correctly with a fixed offset.

[rastro]

You're both right - it was because the guard not connected.

After I plugged in the guard the reading went to 10.8pf from 30.6pf.  Thanks for pointing me in the right direction.

Still the lowest setting 100pf is over 1% - it reads ~101.5pf after subtracting the offset (So close).  I took the cover off and read the 100pf capacitor directly (with the guard) and it reads 100.2pf - so it's well within it 1% marking (probably in the noise range of the meter also).  I doesn't look like the capacitor is faulty.  I've gone through and cleaned and pushed on the contacts.  I don't want to chance damaging the contacts.

Overall I really can't complain since everything else is well within specifications and each switch setting gives very repeatable readings.

-rastro