If the output voltage is correct at the 100 V setting but inaccurate at the other settings, the main cam switch can be the culprit. The bottom of the precision voltage divider is not directly grounded but runs through the switch to allow for the current loop position. These contacts need to be clean to provide a low resistance ground path and accurate output voltages.
Also note that the output voltages measured with a high impedance voltmeter will be off! Don't forget to put a 1 MΩ load resistor in parallel when using a high impedance voltmeter.