Footswitch Board
4-18 Force FX Electrosurgical Generator C Service Manual
Audio Circuit
The audio system consists of an audio oscillator, tone control signals, a volume control
potentiometer, an audio amplifier, and a speaker. A reference voltage (V
ref
) is used
throughout the audio circuit and is generated by dividing the +12 V power supply down
to about 2 V by R9 and R8.
The audio circuit annunciates the presence of RF output and provides an auditory
indication of alarm conditions. A potentiometer adjusts the volume of RF output
activation tones. The speaker volume cannot be turned off entirely. The volume of the
tone issued during alarm conditions is not adjustable.
The audio oscillator is enabled when UP_TONE\ or RF_TONE\ is pulled low. Diodes D1 and
D2 provide a wired OR function for the two signals. Because UP_TONE\ and RF_TONE\
are +5 V (logic level) signals, resistors R4 and R6 divide the +12 V audio power supply
down to about 4.85 V to prevent D1 and D2 from sourcing current into the output pin of
U3 on the Control board.
When either UP_TONE\ or RF_TONE\ is enabled low, the voltage at the noninverting input
of U1B is pulled below the V
ref
threshold present at U1B inverting input, the open
collector output of U1B is turned on, grounding R31 and allowing U6A to oscillate.
U6A is a relaxation oscillator whose frequencies are determined by the RC time constants
of R30, C35, and C18. This design allows the oscillator to produce two distinct
frequencies that can be selected by the state of the LO_TONE signal.
•When LO_TONE is not asserted, R30 and C35 determine the frequency of operation
of the oscillator (approximately 900 Hz).
•When LO_TONE is asserted (+5V), the voltage at the noninverting input of U1A
exceeds the 2 V V
ref
signal at the inverting input, turning on its output transistor. This
effectively connects C18 in parallel with C35 to produce a higher RC time constant
for the oscillator, which results in a lower audio frequency of approximately 700 Hz.
The ALARM signal selects the user-controlled audio volume or the fixed alarm level
volume. U1C and U1D are configured in an exclusive OR arrangement in which the state
of the output transistors of U1C or U1D is complementary. In other words, the output
transistor of one of these two devices is always on, but both cannot be on
simultaneously.
Under normal operating conditions, the ALARM signal is low, allowing the U1C output to
float while the U1D output transistor is turned on. The output of U1D creates a voltage
divider through R11, R12 (the volume control potentiometer), and R32 to attenuate the
audio signal to levels acceptable for input to the audio amplifier.
R32 determines the maximum audio volume and R11 determines the minimum audio
volume. R10 determines the audio alert volume level. R34 provides an alternate audio
signal path in the event of an open volume control potentiometer.
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.
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