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5.6 Input/Output Filter, 800–193AD
All input/output circuits connected to the accessory connector as well as the AC line input have radio–
frequency filters. The filter circuitry is located on I/O Filter Schematic 800–193AD.
5.7 Front Panel Circuit Board Assembly, 953–2130–001/–002
The front panel circuit board contains three switching power supply circuits, decode circuitry for the SR–40A
frequency programming switches, the metering circuitry, and a digital–to–analog converter (DAC) circuit.
+12V dc from the switching power supply is applied to the front panel circuit board. U13, U14, and U24 use
this voltage to create the following dc voltages: 1) +5V, 2) +14V, and 3) –12V. The +14V supply is routed to
the IF detector, audio, and decode circuit boards. On SR–40A units, the frequency programming information
is converted to serial data and routed to the microprocessor by a decode circuit consisting of U1, U2, U3, U4,
and U20. A DAC consisting of U16, U17, U18, U19, U22, and U23 is used to control the program audio
output level. The DAC is controlled by the microprocessor.
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