577 AC collector supply not reading zero on display

DW
 

With the collector supply set to AC, the collector voltage set to 0 volts, and particularly with a lower volts / division selection (1V, 0.5V) I notice a horizontal line about 1 division wide appearing across the the display as if the collector supply isn't quite going to zero. Is there an adjustment to compensate for this? The line becomes a dot in either + or - collector polarity. The line becomes longer in length when selecting higher collector voltages in AC.

Thanks

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