Do a print calibration in Tools Mode. If this not working you will need a tech to perform a Max Setup in Diagnostic Mode (Procon On/Procon Off, Toner Adjustment ...etc) and to diagnose the printer.
I have a problem with a WC 7830 low density BW and color printing.
I have changed Transfer belt, Drum R1, Second bias,
What i should do next,maybe on Diag mode something?