Power Supply/RF Board
Principles of Operation
Force FX-8C Service Manual 4-17
When the ALARM signal is high, the U1C output transistor is turned on
while the output of U1D floats. When the U1C output transistor is on, R10
is pulled to ground and creates a fixed voltage divider with R32 to
produce the alarm volume level at the input to audio amplifier U7.
Meanwhile, the output of U1D is allowed to float, thus removing the
variable resistor divider from the circuit. In this case, the volume control
potentiometer becomes a small resistance in series with the high
impedance input from the audio amplifier, negating the effect of the
volume setting.
Audio amplifier U7 and speaker SP1 comprise the final stage in the audio
system. The audio signal is AC coupled to the amplifier by C25 to
eliminate the need for well-controlled input biasing. The voltage gain of
U7 is set to about 20 by floating its gain select pins. Because the U1 output
signal is internally biased to Vcc/2, it is necessary to AC couple the
speaker through C2 to prevent the amplifier from DC biasing the speaker.
Power Supply/RF Board
Refer to Section 9 for components and the Schematics Supplement for
Board Drawing and Schematics.
The Power Supply/RF board is the main board of the generator. It
contains the high voltage power supply and the RF output stage. Circuitry
on this board performs several other functions:
• Output voltage monitoring
• Output current monitoring
• Leakage current sensing (RF leakage sensing and reduction circuits)
• Spark control circuit
• REM impedance monitoring (REM circuit)
• Handswitch closure detection (IsoBloc circuit)
• RS-232 connector
• Expansion connector
• EKG contact closure connector
• Output high voltage relays
• Temperature monitoring and fan control.
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