hkp7asked Apr 11, 2016 at 6:39pm
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.
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.