[Test-Equipment] Selective Voltmeter W&G SPM-15

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Hello all,

 I have a Selective Voltmeter made by Wandel & Goltermann, model SPM-15
It works well, but from time to time during the autocalibration process 
(either
started manually or automatically), it gives the error code I-202  or  
I-201

I don't have the service manual, but I have been told that, logically 
enough,
those codes indicate the impossibility to reach an accurate calibration 
state.

It looks like the error is more frequent if I leave the instrument 
unused for a few
weeks. Can anybody shed a bit of light on the possible cause of the 
error, and,
even better, suggest a way to fix it ?   Many thanks in advance

Alberto,  I2PHD