On Sat, 5 Nov 2016, mike wrote:

I have an HP Laserjet 4MP.

I have no idea. I got my HP 4P about 2004, $15 at a Rotary Club
garage sale, and it's worked fine ever since. It even had a low
page count, I have no idea why it got donated in the first place.

But I know back when I got it, I was able to download a service manual for
it, so at least do a search now. Maybe that gives hints.

Michael