Hi Paul, Hi Ivan, Hi 'Paul',
I repeated the calibration process, and found no problem.
I can now close the file. Thank you for your help and compliments.

I should have noticed the negative bias problem much earlier.
The reason why I haven't is because almost every time I changed the measuring
point, I turned off the unit to consult the diagram to confirm the point.
So the faulty capacitor became 'resumed'.

Masaya Suzuki