3-74 Phaser 7300 Color Printer Service ManualThe +5 VDC LoopRefer to the wiring diagram, "The +5 Volt Loop" on page 10-258 for all procedureslisted under this topic.Upon power-up the low-voltage power supply (LVPS) generates a +5 volt signalwhich is routed out of the LVPS through the engine control board to the toner sensorboard and then back out the toner sensor board to the low-voltage power supply. Oncethe +5 volt signal is returned to the LVPS, the LVPS will then turn on all other supplyvoltages.If the low-voltage power supply does not detect the return of +5 volts, the LVPS willimmediately shut-down. You may see the Engine Controller Board LED flashimmediately after power-on.To verify the +5 volt loop is complete:1. Test fuse F6 on the Engine Controller Board for continuity. If the fuse is blown(open circuit), check the circuit for a short to ground.Note: Disconnect the POWER harness from the EngineController Board prior to checking for a short.a. If a short to ground is found, systematically disconnect wiringharnesses on the Engine Controller Board to isolate the short. Replacethe defective wiring harness or component and the Engine ControllerBoard.2. Verify all wiring harnesses are connected, seated correctly and not damaged.Replace any damaged wiring harnesses.3. Check for continuity between the Engine Controller Board OPTN and theinterconnect wiring harness between the toner sensor board and LVPS. Ifcontinuity is not found, the problem is the OPTN wiring, the toner sensor board orthe interconnect wiring harness.4. Verify proper operation of the LVPS by bypassing the Engine Controller Boardand the toner sensor board.a. Disconnect the power connector from the Engine Controller Boardand disconnect the interconnect between the LVPS and the tonersensor board.b. Run a jumper wire between pin 5 on the POWER harness and pin 14on the wiring harness interconnect, as shown on the +5 Volt Loopwiring diagram.c. Turn the printer on. If the jumper connection is correct, thelow-voltage power supply will start and the power supply fans willrun. This indicates the problem lies with the Engine Controller Boardor the toner sensor board or their wiring. If the low-voltage powersupply does not turn on, it is probably bad.5. Replace the Low Voltage Power Supply if all other components are operatingcorrectly.
