Voltage Indications: The Green Leds - Agilent Technologies 8719ET Service Manual

Theory of Operation
Power Supply Theory

Voltage Indications: the Green LEDs

The nine 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. For troubleshooting information, refer to
Troubleshooting."
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.
Variable Fan Circuit and Air Flow Detector
The fan power is derived directly from the +18 V and −18 V supplies from the A15
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. (Full
speed is normal at initial power on.)
Display Power
The A8 assembly supplies +5VCPU to the A22 GSP interface board. The +5VCPU is routed
to the A19 GSP where it is regulated to +3.3 V and sent to the display. The A19 GSP also
controls and supplies power to the A20 backlight inverter. The voltages generated by the
inverter are then routed to the display. Display power is not connected to the protective
shutdown circuitry so that the A18 display assemblies can operate during troubleshooting
when other supplies do not work.
If blanking pulses from the A19 GSP are not present, then +3.3 V will
NOTE
not be sent to the display.
12-8
RF Network Analyzers
Chapter 5 , "Power Supply
8719ET/20ET/22ET
8719ES/20ES/22ES
Service Guide