Agilent Technologies 8753ES Option 011 Service Manual page 361

Theory of Operation
Power Supply Theory
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:
−18 V −8 V
+70 V (not used) +25 V +18 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 in normal operation. It 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 in 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.
Refer to the troubleshooting chapters for more information.
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.
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. The troubleshooting procedures later in this chapter detail the steps to trace
the cause of the problem.
Chapter 12 12-7