Simrad RD68 Service Manual page 27

The audio level required for alarms is controlled by IC1d. For normal key
' beeps' the microprocessor, IC4, generates a square wave which is filtered and
reduced in level by R15, R75, C14 and C105 and then fed into the audio
amplifier via PLG1. For alarm generation IC1d short circuits R75 to increase the
level of signal being fed to the audio amplifier.
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