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HARRIS
888-9058-001
WARNING: Disconnect and lockout AC primary power prior to servicing
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9.2.2 Six Phase SCR Rectification
The T1 WYE secondary voltages are applied to the six SCR full wave bridge mounted
on a fan cooled heatsink, the SCRs come in pairs called power blocks. Wires from the
SCRs are connected to a PC board containing a RC snubber network. The snubber
protects the SCR from noise and line transients. The RC time constant also controls the
SCR commutation time. CR1, mounted on the front of the heat sink furnishes a path for
current flow when the SCRs are not conducting. A thermal switch on the heat sink
closes when the heat sink temperature exceeds 100 degrees centigrade. T2, a smaller
transformer, has two functions. Six small rectifiers generate the unregulated V+ for the
power supply and the three phase line voltage sample for the SCR gate control circuit.
9.2.3 DC Supply Filtering
The rectified output is filtered by a choke input filter. The L1 inductor is follow by an
external capacitor network consisting of six 240,000 mF. capacitors, 1.4Farad total. The
large value of capacitance is needed for video bypassing in visual service. This value is
retained for digital service because of control logic timing. The return current passes
through a metering shunt allowing the controller board to provide a current metering
sample to the control and metering system.
9.2.4 Control Board
Figure 9-1 50 Volt Supply Control PC Board
The control board has several basic sections, control 24 volt supply, control logic and 50
volt DC voltage regulation.
