Agilent Technologies HP 8719D Service Manual page 333

The A8 post regulator filters and regulates the DC voltages received from the
voltage supplies. It distributes regulated constant voltages to individual
assemblies throughout the instrument. It includes the overtemperature
shutdown circuit, the variable fan speed circuit, and the air flow detector. Nine
green LEDs provide status indications for the individual voltage supplies.
Refer to the Power Supply Block Diagram located at the end of Chapter 5,
"Power Supply Troubleshooting", to see the voltages provided by the A8 post
regulator.
Voltage Indications: the Green LJ3Ds
The eight green LEDs along the top edge of the A8 assembly are on in normal
operation, to indicate the correct voltage is present in each supply. If they are
off or flashing, a problem is indicated. The troubleshooting procedures later in
this chapter detail the steps to trace the cause of the problem.
Shutdown Circuit
The shutdown circuit is triggered by overcurrent, overvoltage, undervoltage, or
overtemperature. It protects the instrument by causing the regulated voltage
supplies to be shut down. It also sends status messages to the A7 CPU to trigger
warning messages on the analyzer display. The voltages that are not shut
down are the + 5VD and + 5VCPU digital supplies from the preregulator, the
fan supplies, and the display supplies. The shutdown circuit can be disabled
momentarily for troubleshooting purposes by using a jumper to connect the SDIS
line (A8TP4) to ground.
The fan power is derived directly from the + 18 V and -18 V supplies from
the Al5 preregulator. The fan is not fused, so that it will continue to provide
airflow and cooling when the instrument is otherwise disabled. If overheating
occurs, the main instrument supplies are shut down and the fan runs at full
speed. An overtemperature status message is sent to the A7 CPU to initiate a
warning message on the analyzer display. The fan also runs at full speed if the
air flow detector senses a low output of air from the fan. (Pull speed is normal
at initial power on.)