Printer blues

I am having trouble with a HP IIIsi. I am getting about 2/3 of a page with nothing but black lines, and an error code 51. I have tried replacing the DC controller, DC power supply, laser scanner and the fiber optic cable. The proper voltages are being dropped at the scanner, and I have tried a known good toner cartridge, all to no avail. I have done an engine test, and it does the same thing. I did a half page test, and I noticed a lot of toner where the printing stopped. Could a high voltage power supply fix this, or could the DC controller be defective, or what could it be? I have been fighting this for 3 weeks, and am contemplating using it for a boat anchor, although it’s not my decision. Thanks