QUANTUN QP-750 Service Manual
Also see for QP-750: User manual
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Contents
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QP-750 Service Manual- 30 -6.3.2 Audio Power AmplifierThe audio power amplifier IC 2401 amplifies receiver audio from IC1408 pin 20 to a level sufficientto drive a loudspeaker. IC2401 is a bridge amplifier, which is sufficient to develop 500 milliwatts ofaudio power into the internal 16 ohm speaker or an external 16 ohm load. The audio poweramplifier is muted whenever speaker audio is not required to reduce current drain. The audioamplifier is muted when IC2401 pin 2 is low. When IC2401 pin 2 is high, Q2401B is pulled low byQ2405, enabling the audio amplifier.Because the power amplifier is a bridge-type, neither speaker terminal is grounded. Care should betaken that any test equipment used to measure the speaker audio voltage does not ground eitherspeaker output terminal, otherwise damage to the audio power amplifier IC may result. When a 16-ohm load resistor is used it should be connected between the tip and the sleeve of accessory jackJ1401, never to ground. External SPKR plug insertion mechanically disconnects the internalspeaker. Voltage measurements using test equipment that is not isolated from ground may bemade from one side of the speaker or load resistor (either the tip or the sleeve of J1401) to chassisground, in which case the voltage indicated will be one half of the voltage applied to the speaker orload resistor.6.3.3 Internal Microphone Audio Voice PathMicrophone audio from internal microphone is routed from J1402-1 via C2643, L2401, and IC2402to the IC1408 MIC audio input. During transmit Q2508 is turned on by a low at IC2506 pin 20,providing dc bias for the internal MIC via R2646. External MIC plug insertion mechanicallydisconnects the internal microphone. External MIC audio is coupled through L2401 and C2643 tothe MIC audio input.6.3.4 PTT CircuitsThe internal side-mounted PTT switch (K1507) is sensed directly by microprocessor port IC2506pin 84. External MIC PTT is sensed by measuring the current drawn through the accessoryconnector (J1402-4) by the MIC cartridge (which is in series with the accessory PTT switch). Thiscurrent is drawn through the base (pin 5) and emitter (pin 4) of a transistor in Q2508, causing itscollector (pin 3) to supply a logic-high to microprocessor port pin 47.6.3.5 VOX OperationVOX audio accessories do not have a PTT switch. Instead, the MIC cartridge is wired directlyfromJ1402-4 to ground. If the radio has been programmed for VOX operation and the VOXaccessory is plugged in prior to turning the radio on, the current drawn by the cartridge will turn onQ2508 (pins 3- 4-5) and a logic high will be seen at port P47 at turn-on. The microprocessor thenassumes VOX operation, with PTT controlled by the presence of audio at the MIC cartridge. A dcvoltage proportional to the audio level at the output of the D2406 is fed to an A/D input ofmicroprocessor IC2506 (pin 89).During VOX operation PTT is activated when the dc level exceedsa preset threshold. |
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