Keysight E3631A, Service Manual

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4 Theory of Operation
98 Keysight E3631A Service Guide
AC Input and Bias Supplies
The ac mains are connected by a fused power module. This module incorporates
the functions of mains connection, fusing, and line voltage selection (100/115/
230 Vac). The line voltage selection function of the module selects which primary
winding of power transformer is energized. The transformer secondary windings
are connected to the main pc board through connectors.
The bias supplies are consists of four sections; ±15 Vdc for the +6V power circuits;
±15 Vdc, +5 Vdc and -5.1 Vdc for the ±25V power circuits and floating logic; ±16.4
Vdc for the display; and +5 Vdc for the earth referenced logic. Power-on reset
signals are provided by the +5 Vdc supply of the floating logic.
The ±15 Vdc for the +6V power circuits are produced by rectifiers CR15 and CR16,
filter capacitors C43 and C44, and voltage regulators U24 and U25 on the bottom
board. These supplies provide bias voltages for the +6V power circuits.
The ±16.4 Vdc for the display circuits are produced by rectifier CR20, filter
capacitors C44 and C45, and voltage regulators U24 and U25 on the top board. A
separate winding of power transformer provides a center tapped 6 Vrms filament
supply for the display.
The ±15 Vdc for the ±25V power circuits are derived from ±16.4 Vdc by diodes
CR15 through CR18 on the top board. These supplies provide bias voltages for the
±25V power circuits.
The fl5 Vdc and -5.1 Vdc supplies are produced from ±16.4 Vdc supplies.
VR2 on the top board forms a clamp circuit to provide over voltage protection in
the event of a regulator failure. The ADC REF signal is derived from +15 Vdc
supply and the TURN_ON RESET signal is derived from +5 Vdc supply. The FAN
FAIL signal is asserted when the fan current through R73 on the top board is not
detected. The TURN_ON RESET signal holds the main controller and other logic in
a reset state until after the +5 Vdc logic power is fully operational. This signal is
generally active only following application of line power to the instrument.
The +5V dc earth referenced supply is produced by rectifier CR3, filter capacitor
C15, and regulator U11 on the bottom board. The GPIB (IEEE-488) and RS-232
computer interfaces are powered from this supply.