Laserjet 400 m401n printing all black

Question

We have a lot of these black Laserjet Pro 400 m401n's.  We have a few that have been dying where they only are pingable for a few minutes and then they lose all network connectivity.  So we have been replacing those as they go.    I have a spare unit that came to me because it was printing all black.  Yes all black.  We tried different toner first but it was still all black.  Note, it prints all black from the demo page on the front panel - not even connected to the network, so this is strictly a hardware issue as no drivers are involved. I dissasembled the printer and I swapped the control board on the left hand side with a known working one.  Still prints all black.   I went through the painsteaking process of swapping out the fuser assemebly, as buried as it is in these things (note these must NOT be meant to be taken out!!!!).  Still prints all black.   I swapped the laser scanning unit at the top of the printer thinking maybe its faulty and always on full blast, causing black printouts.  Nope... its still prints all black. I swapped the small connection board on the right side of the printer where the laser assembly connects into (amongsts a few other wires), thinking maybe one (or both) of the transistors on that board could be bad.  Nope... it still prints all black. What else left is there?  The power supply board maybe?  Could that cause it?  Is there another part you would reccomend I look at?  I have this one that prints all black, and I have another "doner" one that just randomly jams all the time (even with new rollers)  So I'd just like to get one good working printer out of these.

Answer 1

> so this is strictly a hardware issue as no drivers are involved. Yup. > Still prints all black.   Ok its not the controller. > Still prints all black.   Ok its not the fuser. >  its still prints all black. OK its not the laser itself > it still prints all black. Not that part either. > What else left is there?  Formatter board hasnt been mentioned.  Something is sending a command to the printer that is telling it to open up the doors and charge the entire page.  Normally the laser is responsible for charging the media.  As the media passes over the toner doors it atracts toner particles.  Then as the paper continues it rolls past the fuser where heat melts the particles into permanent place. > The power supply board maybe?   Perhaps but not likely.  If you have a spare you could try it but power supply problems usually result with the machine unable to boot or print correctly, not printing too much as you have described here. > Is there another part you would reccomend I look at?
You could try the old Halfway test just to verify that the problem is coming from teh laser/toner part of the printer.  http://www.printertechs.com/printer-troubleshooting/halfway-test Your paper may have somehow gotten a staic charge that opposes the charge of the toner, thus auto attracting all particles in range as it passes over the toner doors.  Might want to grab a fresh ream of paper to test and rule somethign crazy like that out. Not very many options left after that.