asked Jan 2, 2001 at 10:21pm
Hp HP LaserJet 4P

HP 4P Laser Problem

I just purchased a used HP 4P Laser Printer from a friend. The problem is
simple, yet difficult to diagnose. I am not getting any errors, the menus
work. I get a ready light. Here is the problem, the printer will print only
a black page(demo) or several black pages(Cont. self test). The fuser seems
to be fine and I've installed a new cartridge. But the problem persists.
I've stripped the entire unit down to check for bad connections to the toner
cartridge and everywhere else, burnt traces and components to no
avail.Everything appears to look new.
Can you help us?
Thank You,
Peter Johnson
Laser printers operate with high voltage. There are 3 things that get charged in the printer. The photconductive drum, the toner and the paper. The laser erases the charge off the drum and wherever the charge is gone the like charged toner sticks to, it's repelled from the like charged areas of the drum. The paper receives an opposite charge and the toner is pulled down on to it, then it continues to fusing. If you're getting a black page it would indicate one of 2 things; a laser firing all the time or a lack of high voltage to the photoconductive drum. Either of these conditions would cause the toner to stick to all the drum surface, resulting in a black page. HV would be my first choice. Check the spring contacts on the right side of the printer after you pull the cartridge.
by moe on Jan 3, 2001 at 6:13pm Add comment
Hi Moe,
Thank you for your help. I have a much better understanding of what should be going on inside of this printer now. Do you sell any of the assemblies(High Voltage and/or Formater?)This has become my new project. I've started testing the diodes on the HV board. But I realize that troubleshooting to individual component level is usually not done. Although it would tickle me to death to find a bad diode or capacitor and fix it. I just may not have enough time to do it. I've spent 30$ on the printer and another 50$ on a new toner cart.When do I reach the point of diminishing returns and just buy a new laser printer. I could dump another 150$ into it max.It did print a partial light page about 50 black pages(sometimes it will print a blank white page) ago and I could read that the page count was 2,450. Which makes me want to fix it. What do you think?

Thanks Again!


by Peter on Jan 3, 2001 at 8:09pm Add comment