I have found some service manuals that may help with repairing your printers. Hopefully these with be helpful to you.
https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.partsnow.com/docs/service-manuals/hp-laserjet-4000-4050-service-manual.pdf&ved=0ahUKEwiv9YeG0NrTAhWj7IMKHXI_DCcQFggkMAA&usg=AFQjCNHplMSbbd887R7iokWkOx1tDWzz4Q&sig2=nSIzNzjH345qSvcpwxABgg
https://www.google.com/url?sa=t&source=web&rct=j&url=https://diyoffice.files.wordpress.com/2010/12/tuckers-guide-to-40004050-printers1.pdf&ved=0ahUKEwjUj_yg0NrTAhUl6IMKHVxTAhMQFgg8MAM&usg=AFQjCNHA5emp1iHsXkYCSehOTNhdJmEhVw&sig2=GK75JIhTbms4QNEk3PeXHg