Service Guide 12-7
8719ET/20ET/22ET Theory of Operation
8719ES/20ES/22ES Power Supply Theory
RF Network Analyzers
Line Power Module
The line power module includes the line power switch, voltage selector switch, and main
fuse. The line power switch is activated from the front panel. The voltage selector switch,
accessible at the rear panel, adapts the analyzer to local line voltages of approximately
115 V or 230 V (with 350 VA maximum). The main fuse, which prevents the input side of
the preregulator from drawing too much line current, is also accessible at the rear panel.
Refer to the analyzer installation and quick start guide for line voltage tolerances and
other power considerations.
Preregulated Voltages
The switching preregulator converts the line voltage to several dc voltages. The regulated
+5 V digital supply goes directly to the motherboard. The following partially regulated
voltages are routed through A15W1 to the A8 post regulator for final regulation:
+25 V +18 V −18 V +8 V −8 V
Regulated +5 V Digital Supply
The +5 VD supply is regulated by the control circuitry in the A15 preregulator. It goes
directly to the motherboard, and from there to all assemblies requiring a low noise digital
supply. A +5 V sense line returns from the motherboard to the A15 preregulator. The +5 V
CPU is derived from the +5 VD in the A8 post regulator and goes directly to the A19
graphics system processor.
In order for the preregulator to function, the +5 V digital supply must be loaded by one or
more assemblies, and the +5 V sense line must be working. If not, the other preregulated
voltages will not be correct.
Shutdown Indications: the Green LED and Red LED
The green LED is on during normal operation. The green LED is off when line power is not
connected, not switched on, or set too low, or if the line fuse has blown.
The red LED, which is off during normal operation, lights to indicate a fault in the +5 V
supply. This may be an over/under line voltage, over line current, or overtemperature
condition. For troubleshooting information, refer to Chapter 5 , “Power Supply
Troubleshooting.”
A8 Post Regulator
The A8 post regulator filters and regulates the dc voltages received from the A15
preregulator. It provides fusing and shutdown circuitry for individual 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.
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