asked Apr 4, 2014 at 9:49pm
Hp HP LaserJet 4

HP Laserjet 4 will not power up

I have an HP Laserjet 4 - one careful owner since new (1992)with light home office use. no serious problems till now.
When I switch on the Ready,Online and Formfeed lights flash repeatedly together with a clunking noise.
The display does not light up.
The rod connecting the on/off switch to the ? at the rear of the printer works ok.

Any ideas whats wrong please
After allowing sufficient warmup time, stick something in the engine test hole
Does it print a lined test page?
by moe on Apr 6, 2014 at 1:41pm Add comment

I have now tried this. Nothing happens
by russ44 on Apr 6, 2014 at 3:04pm Add comment
Wish I could hear the noise. Never seen this happen in all the thousands of this model I've worked on. Power it off and open the back door. Power it back on and stick something in the interlock switch. You can find it by looking on the right side of the door (facing the front). There is a plastic tab sticking up. Move the door in an upward motion and take note of where it goes into this black plastic assy. The plate inside is what triggers the door sensor. If you gently push something in it will think the door is closed. You can then both watch and listen to what is happening. You should see a light emanate from the fuser assy. ends and you should see the rollers turn. Probably the clunking noise is emanating from the fuser. You can also take the fuser out by removing the 2 screws at the bottom and manually rotate the gear on top to see if it moves smoothly. The fuser won't cause the other issue with the panel lights, but it would make the noise. You seem to have more than one problem.
by moe on Apr 6, 2014 at 7:44pm Add comment
Thanks for your help
I have taken the fuser assembly out.
I can move the grey gear on the top LHS and the roller goes round.
I powered up the printer with the fuser out and can now see that the noise I heard is made by some white nylon gears at the Left Hand Side of the fuser slot. They move regularly like a clock ticking slowly. When I turn the power off they make one more tick after the power is off. Strange. The 3 lights flash on and off in time with the gears ticking.
by russ44 on Apr 7, 2014 at 1:22am Add comment
That's something I've never run into and I thought I'd seen everything. That gear assembly is the coupling gear. The top gear on the bracket interfaces with the fuser gear. It wears out, turns yellowish/brown and teeth break off. It would make noise. You can take the toner cartridge out and turn the gear inside the printer with your thumb and watch the gear and also listen. I kind of suspect the lights flashing would be caused by the DC controller. If the test page had printed, I would have blamed the formatter board.
by moe on Apr 7, 2014 at 2:08pm Add comment

I have taken the toner out and turned the gear as you suggest.
The gear seems fine - it turns smoothly with no noise - and will even build up a bit of momentum and will continue to turn a bit after I stop pushing.
I am not technical. My feeling is that the problem is electrical as if what I described in the last post was caused by something starting to charge up and then ? tripping out before it had enough energy to power things up, turn the gears etc and then starting again.
Somewhere I saw something about if the fan does not come on (which it does not) then it could be the DC controller (as you mention) or the DC power supply.

I am really grateful for your continued help
by russ44 on Apr 8, 2014 at 1:19am Add comment