asked Apr 11, 2016 at 6:39pm
Hp HP LaserJet 4

Alphanumeric display not functioning.

Maybe someone can help me with this. After not using my Laserjet 4 for some time, I went to turn it on, and the LEDs at the buttons all light up for maybe 30 seconds, but the alphanumeric display at the control panel doesn't indicate anything. I hear the motor cycle the way it always does on power up. I'm figuring that since the motor is running, and I get the 5V, and 27V from the power supply, that the P.S. is not the problem. I bought a used (supposedly good) formatter board, and get the same results. Tried the print engine test, and it did nothing. Any suggestions on what to check next?

If it's not doing the engine test, then the formatter board is out of the picture since that is the only thing not checked by the engine test. The display is powered by the DC controller. That would be the first thing to try.

by moe on Apr 12, 2016 at 12:50am Add comment
Thanks Moe, I'll give that a go. Do you know what the specific pinout voltages should be? I can't find that information anywhere, nor can I find schematics for any of the boards.
by hkp7 on Apr 12, 2016 at 1:29pm Add comment

7-68 on the schematic of the manual. It shows what connector goes to the laser diode, but doesn't give the voltages. I would guess the red and black wires? Voltage would only be present during the print cycle.,%204,%204M,%204M,%205,%205M,%205N%20Service%20Manual.pdf

by moe on Apr 12, 2016 at 5:23pm Add comment
I just found a quick reference service guide from that shows some of the voltages to the various boards/sensors. Some models have more information about them than others. Still no circuit board schematics though. Looks like the only thing(s) on the circuit boards themselves that can be changed out on a component level with 100% certainty are capacitors.
by hkp7 on Apr 12, 2016 at 6:53pm Add comment
Moe, the DC Controller got the print engine running. I now see the proper LEDs lit at start up, but the VFD is still unlit. I saw signs of capacitor leakage under the 100 MFD capacitor, and switched it out. That didn't fix the problem. I'm getting -32V at the cathode terminals of the VFD. I'm assuming a 4 Plus panel won't work to replace the panel on a 4, is that assumption correct? Thanks
by hkp7 on Apr 16, 2016 at 6:48pm Add comment
Just an update. Printer is now operating 100% correctly. The problem with the display was a combination of the leaking capacitor (radial through pin 100 MFD, 10V), and a bad solder joint on one of the VFD pins. In case anyone reading this doesn't know how to spot a leaking capacitor, look for an oily appearing film underneath. That's the electrolyte from the capacitor. If it's left there, it will eat up the traces on the board. I cleaned it a few times using 92% isopropyl alcohol. As Moe pointed out, the DC controller board needed replacing before anything else. It turns out that the original formatter board was still good. Thank you very much Moe for your assistance! :)
by hkp7 on Apr 20, 2016 at 5:40pm Add comment