Midland GXT720 Series Bedienungsanleitung

TRANSMITTER ALIGNMENT METHOD

GXT720/775

1. Frequency Setting

A. Connect a frequency counter or communications service monitor capable of at

least five watt RF.

B. Press the PTT switch.

C. Adjust CT201 trimmer capacitor such that the output frequency is equal to the

channel frequency with a maximum error of +/- 200 Hz. CT201 is located the

right side of VCO Shield can.

2. Output Power Alignment

A. Connect a communications service monitor or a wattmeter and dummy load to

the antenna connector.

B. Press the PTT switch.

C. Adjust air coil (L6) by spreading or de-spreading, to achieve maximum power at

6V Vcc.

Note: Normally, it is not necessary.

D. Adjust semi-resistor(RV1) by rotate, to achieve 0.5W at 6Vcc

3. Deviation Adjustment

A. Connect an audio generator with 10uF coupling capacitor to the microphone jack.

The audio frequency should be set at 1KHz with a level of 200mV.

B. Connect an FM deviation meter or communications service monitor to the

antenna connector. Set the monitor to read peak deviation.

C. Press the PTT switch.

D. Adjust RV2 for +/- 2.0KHz deviation. RV2 is located the bottom right side of PCB.

LABORATORY TESTING PROCEDURES

GXT720/775

Page 1

UNIT TEST - (UNIT ASSEMBLED)

TEST PREPARATION

1) Install Alkaline battery (observe polarity markings).

2) Turn on unit by pressing the power button.

SYSTEM TEST

1) Radiated Transmit and Receive performance may be observed.

2) Audio out & Audio in are available at the Headset jack.

LABORATORY TEST - (UNIT UN-ASSEMBLED)

TEST PREPARATION

1) Disassemble unit (5 screws – 2 behind batteries). Remove the PCB from the

cabinet.

2) Remove the antenna with soldering iron and install a 50 ohm coax cable in its place.

3) Either clip alligator leads or solder test leads to the power supply connections. The

negative terminal is the lower right PCB area. The positive terminal is the lower left

PCB area.

4) Connect 6VDC power source to the terminals, observing correct polarity.

5) Connect an 12-ohm load through the Headset jack

6) Connect a audio generator with 10uF coupling capacitor through the Headset jack

7) Select desired channel 1-36 using CH up/down keypad switch. The rubber keypad

may be removed from the front cabinet and used directly on the PCB.

LABORATORY TESTING PROCEDURES

GXT720/775

Page 2

SPECIFIC TEST METHODS AND GUIDANCE

Modulation Characteristics – (paragraph 2.1047(a) of the Rules)

FOR TX AUDIO FILTER RESPONSE

1. Connect audio generator with 10uF coupling capacitor to microphone input jack.

Press PTT button.

2. Connect RF output with modulation meter. (Filters of modulation meter should be

set to a 25Hz to 15KHz.)

3. Adjust audio generator 6-7mVrms of 1KHz audio (approx. 0.75KHz modulation).

4. While transmitting, sweep generator and note measurement.

5. Results should be multiplied by a two times because of compressor.

6. Please compensate the back-ground noise level.

Modulation Characteristics – (paragraph 2.1047(b) of the Rules

FOR TX AUDIO LOW PASS FILTER RESPONSE.

1. Connect audio generator with 10uF coupling capacitor to microphone input jack.

Press PTT button.

2. Connect AC voltmeter or other test equipment via jumper wire with junction (TP9) of

C53, R82.

3. Adjust audio generator for 200mV.

4. While transmitting, sweep generator and note measurement.

Occupied Bandwidth – (paragraph 2.1049(c) of the Rules)

1. Connect an audio frequency sweep generator with 10uF coupling capacitor to

microphone input jack.

2. Adjust audio generator to a frequency of 2500Hz and a level of 51mV rms (+16dB

above 8mV per FCC).

3. With a spectrum analyzer, transmit the radio and monitor the transmitter though an

antenna.

4. Note required measurements per FCC.

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