HP LaserJet 4 Service Manual - Page 242

Printer Message Troubleshooting Procedures When the following procedures require removing and replacing an assembly or PCA, refer to the applicable section in Chapter 6 for instructions. Blank Display Table 7-3 Blank Display Checks Is the power on? Is the fan working? Action 1. Check the ON/OFF switch and verify the power is plugged in. 2. Press a Control Panel key to verify problem still exists. 1. If the fan is working, check the cabling to the Control Panel. If the cabling is good, replace the Formatter first, then the Control Panel. 2. If the fan is NOT working, measure the DC voltage levels (refer to Table 7-39). If the voltage levels are good from the power supply to the DC Controller and the fan is not working, replace the DC Controller. If the voltage levels are not present at TB1, replace the DC Power Supply. 7-12 Troubleshooting