[volt-nuts] Datron 1281 Repair Help

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I am working on a Datron 1281 meter.
DC voltage calibration at 10v functions ok, but the Internal source 
calibration failed.
This Meter has the following errors after Internal Source Calibration.
Error codes:
2202 1KV    Range zero magnitude test   (P028).
2192 100V  Range zero magnitude test   (P021).
2162 10V    Range zero magnitude test   (P006).

What would cause these errors?