Motorola Commercial Series Información de servicio - Página 13

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Controller Circuits

3.3.2

Audio Power Amplifier

The audio power amplifier IC U490 amplifies receiver audio from U451 pin 41 to a level sufficient to

drive a loudspeaker. U490 is a bridge amplifier delivering 3.46 volts rms between pins 5 and 8

without distortion, which is sufficient to develop 500 milliwatts of audio power into the internal 24

ohm speaker or an external 24 ohm load. The audio power amplifier is muted whenever speaker

audio is not required to reduce current drain. The audio amp is muted when U451 pin 14 is low.

When U451 pin 14 is high, U490 pin 1 is pulled low by Q490, enabling the audio amplifier.

Because the power amplifier is a bridge-type, neither speaker terminal is grounded. Care should be

taken that any test equipment used to measure the speaker audio voltage does not ground either

speaker output terminal, otherwise damage to the audio power amplifier IC may result. When a 24-

ohm load resistor is used it should be connected between the tip and the sleeve of accessory jack

J471 (3.5mm port), never to ground. External SPKR plug insertion mechanically disconnects the

internal speaker. Voltage measurements using test equipment that is not isolated from ground may

be made from one side of the speaker or load resistor (either the tip or the sleeve of J471) to chassis

ground, in which case the voltage indicated will be one half of the voltage applied to the speaker or

load resistor. The Motorola RLN4460 Portable Test Set and PMKN4004 Programming Test Cable

provide the proper interface between the radio's ungrounded audio output and ground-referenced

test equipment.

3.3.3

Internal Microphone Audio Voice Path

Microphone audio from internal microphone is routed from J470-1 via C475, L471, and C470 to the

ASFIC_CMP mic audio input (MICINT, U451 pin 46). During transmit, Q470 is turned on by a low at

U451 pin 35, providing dc bias for the internal MIC via R478. External MIC plug insertion

mechanically disconnects the internal microphone. External MIC audio is coupled through L471 and

C470 to the mic audio input. An input level of 10 mV at J471 pin 4 produces 200 mV at the output of

U451 pin 40, which corresponds to 60% deviation.

3.3.4

PTT Circuits

The internal side-mounted PTT switch (S441) is sensed directly by microprocessor port PJ0 (U401

pin 71). External mic PTT is sensed by measuring the current drawn through the accessory

connector (J471-4) by the mic cartridge (which is in series with the accessory PTT switch). This

current is drawn through the base (pin 5) and emitter (pin 4) of a transistor in Q470, causing its

collector (pin 3) to supply a logic-high to microprocessor port PJ1 (pin 72).

3.3.5

VOX Operation

VOX audio accessories do not have a PTT switch. Instead, the mic cartridge is wired directly from

J471-4 to ground. If the radio has been programmed for VOX operation and the VOX accessory is

plugged in prior to turning the radio on, the current drawn by the cartridge will turn on Q470 (pins 3-

4-5) and a logic high will be seen at port PJ1 at turn-on. The microprocessor then assumes VOX

operation, with PTT controlled by the presence of audio at the mic cartridge. A dc voltage

proportional to the audio level at the input of the ASFIC_CMP (U451 pin 46) is fed to an A/D input of

microprocessor U401 (pin 62). During VOX operation, PTT is activated when the dc level exceeds a

preset threshold.

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