Xerox 7750DXF - Phaser Color Laser Printer Service Manual page 148

The +5 VDC Interlock Circuit
The 5 VDC interlock circuit runs from the 5 VDC LVPS to the L/H door switch, then
to the engine control interface board, out to the R/H door switch, back to the engine
control interface board, out to the front door, through the coil of the LD power relay
and back to the 5 VDC LVPS. This circuit helps identify which door has interrupted
the 24 VDC circuit.
If the circuit is complete, LD power relay activates, and 5 VDC is sent to the waste
cartridge installed interlock switch and through all four imaging unit connectors and
to the engine control interface board. This signal merely confirms that all imaging
units are installed. Individual missing components are identified at power-up or when
a door is closed (completing the 24 VDC circuit) and the engine control board
interrogates the imaging unit NVRAM. If the imaging units are present, then the
waste cartridge is missing by default.
1. With all doors closed and printer power on, check for +5 VDC between P536-4
of the engine control interface board and frame ground. If not present, check the
LD relay and the LH cover interlock switch.
2. Check for +5 VDC between P631-3 of the engine control interface board and
frame ground. If not present, check the front cover interlock switch.
3.
Check for +5 VDC between J568-5 (top pin) of the LD power relay and frame
ground. If not present, check the RH cover interlock switch.
4.
Check for +5 VDC between J400-6 of the engine control interface board and
frame ground. If not present, check the LD power relay, and the circuit through
the waste cartridge sensor and the four imaging unit connectors.
5.
If +5 VDC is present, then replace the engine control board.
4-20 Phaser 7750 Printer Service Manual