3. Theory of Operation
Functional Block Descriptions
146
A1 Motherboard Circuit Description
A1 routes signals and power within the instrument via interconnect lines and
buses. A1 has seven slots for plug-in boards (each plug-in board has two
connections to A1), and a power supply section. A1 also has jacks for
power/control/monitor functions for all the modules in the instrument. Refer to
Figures 3-3 through 3-6 for A1 schematic, interconnect, block-diagram, and
component-location information.
The power supply section consists of the ac line voltage input, overvoltage
protection, and external dc input circuits as shown in Figure 3-3 and 3-5, A1
Power Supply Schematic and Block Diagrams.
AC-Line Voltage Input Circuit
A1J5 receives the output of the toroidal power transformer (T1) and delivers it
to full wave bridge rectifier A1CR3 for rectification. The rectified voltage is
then filtered by filter capacitor A1C4 and routed to A11 Power Steering Logic
Assembly via connector A1J4.
Overvoltage Protection Circuit
The Overvoltage Protection circuit across the ac input lines is a crowbar
circuit comprised of silicon controlled rectifier (SCR) A1CR1, zener diode
A1VR1, resistors A1R1 through A1R3, A1R5, A1R6, and capacitor A1C1.
This circuit not only protects the components within the Overvoltage
Protection circuit but also the dc-dc converter integrated circuits (ICs), A12U1
through A12U3, on A12 Dc-Dc Power Converter Module rated at a maximum
input voltage of 56 Vdc, (72 Vdc for Opt. 048).
When the input ac-line selector switch is set to the 120 Vac position, A1CR1
is active if the input voltage is roughly 165 Vac. At this level, the secondary
side of the circuit (across A1C4 or the output of the full wave bridge rectifier
A1CR3) exceeds 50 Vdc, (65 Vdc for Opt. 048).
This causes zener diode A1VR1 to breakdown and supply current to the gate
of A1CR1. A1CR1 conducts and temporarily shorts the ac voltage to ground
blowing the main fuse inside LF1 line filter/cable assembly. When A1 CR1 is
conducting, the current in the secondary circuit flows through A1R1, R2, R3
and A1CR1 to PS_COMMON
(non-chassis ground return).
NOTE
Figure 3-4 uses a dual-row 1-48 or 1-96 pin format to show A1
slot signal-pin designations. The actual A1 connectors are three-
row DIN connectors (A, B, C, 1-16 pins). Pin A-1 on any DIN slot
connector corresponds to pin 1 of the slot connectors shown in
Figure 3-4.
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