Unlike a photocopier where absence of a main charging corona voltage gives a no black output,in a laser printer the opposite is the case and an all black sheet is printed. A black sheet with just odd patches of white visible indicates the absence of the developer voltage. For a mainly black sheet with some white and text indicates low main charger voltage. All white can be a lack of the final transfer corona voltage. White sheet with some broken text can be due to low transfer voltage.
Sheet having mixed black and white background with patchy text can be low developer bias voltage.
Text with precise width but random horizontal lines can indicate the start sensor being out of adjustment or noise in this part of the system. Inconsistent distortion of text can indicate inconstant deflection mirror rotation. Single thick black bar on the page can be the OPC and paper being out of sync.