asked May 4, 2012 at 9:40am
Hp HP LaserJet 4

HP 4+ won't power up

HP 4+ attempts to power up, lights up a couple times a second, but won't turn on.

Beyond unplugging it and trying again, I have no clue what to do next. Any ideas?
That's kind of a strange symptom. Usually they either power up or are dead. I'd suspect the power supply. That's the unit on the bottom right where the AC plug connects. You can flip the side panel down to see it.
by moe on May 4, 2012 at 11:57am Add comment
could be a faulty on/off switch "in" the power supply unit. i believe i've seen one many years ago. ended up bypassing the switch with solder and switch on/off the machine using the power switch at the wall socket. ya i know it's a backyard job, but saved the customer hundreds of dollars back then. moe could possibly supply you a working one for quite cheap these days?
by seriously on May 4, 2012 at 3:34pm Add comment
I always get the strange ones. I think it's my job to put techs to the test. :)

Before I dive in, how do I check if power supply is bad? Is it as simple as ckecking output with voltmeter? What should output be?
by unknown on May 4, 2012 at 3:37pm Add comment
The power supply puts out both DC at the front connectors and AC at the side connector to the fuser assy. It's the DC that powers the display and logic circuits. Sorry, I don't know what the pin outs are supposed to be. I just swap in a replacement when I have problems.
by moe on May 4, 2012 at 4:57pm Add comment
there should be a +5v and a +24v dc outputs from the power supply unit. use the power supply's metal casing as the ground/earth point.
by seriously on May 5, 2012 at 6:44am Add comment
It actually fired up today, but alas it is back to flashing again.I'll check it out.

Do you have any on hand Moe? New -used? Cost?
by unknown on May 7, 2012 at 5:48pm Add comment
strange coincidences happen all the time.....

i just fixed one today with the psu seemingly dead. the customer said before the machine went completely dead it was coming up with an error 52, which is a faulty laser unit.

the psu outputs measured nothing when it's connected, but works properly when taken out of the machine and powered up on it's own.

turns out it was the laser unit that's shorting out the psu. disconnect the laser unit completely and the machine sprung to life!!!

found an old laser unit on my shelf, put it in and the machine now work perfectly :)

try that,ie disconnecting the laser unit, and power it up. good luck......

note: power flashing in general could sometime be caused by worn-out/bulging capacitors in the psu, but i've yet to come across this happened to the lj4 psu. so at this stage it could still be the psu itself, or something else shorting it out.

edit: maybe even try powering it up without the fuser unit and the toner cartridge first as those are the easy ones to do.
by seriously on May 7, 2012 at 10:15pm Add comment
One thing you can try along those lines would be to remove the formatter assy. You won't get any display after you do. Then unplug all the connectors on the DC controller board except the connector going to the motor. Power up the printer and the motor should run for a few seconds. Turn off power. Plug in one connector at a time each time powering the printer on until it fails to spin the motor. That would be the component shorting out the power supply.
by moe on May 7, 2012 at 10:42pm Add comment