HM135 Service Manual
Preliminary version Page 12 of 13
7) Release the PTT.
8) Power Off the radio. Disconnect the short from TP1. Keep the capacitor (step 2 above) connected.
3.7 Setting the frequency and VCO’s tuning.
1) Switch the radio ON
2) Go to CH3 (173.9 MHz)
3) Connect an oscilloscope between TP1 , located in the center of VCO shield, and ground. Turn CV1, RX VCO ADJ, slowly
until the voltage at TP1 is between 6.5 and 7.1VDC, without notable ripple.
4) Press the PTT. Turn CV2 TX VCO ADJ slowly until the voltage at TP1 is between 6.5 and 7.1 VDC, without notable ripple.
Release the PTT
5) Switch to CH1 (135.1 MHz). The voltage at TP1 must drop to 1.8 to 3V. Press the PTT. The voltage must be within 2 and
3.1V.
6) Repeat all above, if necessary.
7) Remove the 300pF capacitor (step 2 of the paragraph “Setting the Power Amplifier Bias” above).
8) Switch to CH2 (155 MHz).
9) Go to transmit (PTT) and turn slowly RP7, maximum TX Power until the output power is reached approx 5 to 8W.
10) Rotate RP2, FREQUENCY ADJUST until the frequency is within 155.000 MHz plus/minus 200Hz. Release the PTT.
11) Change the channel to CH1, check the frequency, it must be 135.000 MHz plus/minus 200Hz.
12) Go to CH3, 173.900 MHz, check the frequency, it must be 173.900 MHz plus/minus 280Hz. Realign the RP2 to tune the
frequency as near as possible to 173.9 MHz.
3.8 Setting the RF POWER.
1) Switch to CH2, 155.1 MHz. Set the power to High (20W).
2) Press the PTT. Turn RP7, Maximum Power ADJ, until the power is set to 20 plus/minus 1W.
3) Check the power on CH1 and CH3 . It must be between 18.5 and 20W.
4) Change the power on CH1 to CH3 to LOW. Check the power, it must be between 9 and 10 W.
3.9 Setting of modulation
1) VERY IMPORTANT! Switch off all the modulator filters in the deviation meter (communication tester) before this
measurement. Radio’s modulation input used for this setting is (i.e. without pre-emphasis).
2) Disconnect the microphone from the radio. Set the radio to CH2 155.1 MHz
3) Connect an AF Generator between the pin 7 of rear DB25 connector (TX AF IN) and Ground (pin 14).
4) Short pin 9 (External PTT) of rear DB25 Connector to the ground. It is equivalent to pressing the microphone PTT.
5) Set the AF Generator to 2000 plus/minus 50Hz and output level of 1V.
6) Adjust the RP4, maximum deviation control until the deviation is set to 4.6 kHz ((peak to peak)/2)
7) Decrease the output of the AF generator until the deviation is set to 2000 plus minus 50Hz.
8) Keeping the level of the AF generator the same (it must be approx 100mV) decrease the generator frequency to 100Hz.
9) Turn RP3, deviation balance until the deviation is set to 2000 Hz. Observe the demodulated signal, it must be clean , without
notable higher (reference) frequency ripple. Presence of ripple means that balance is not set properly. Near the balance the
ripple disappear.
10) Check the deviation at 300 Hz, 500 Hz, 1 KHz, 2 KHz, 2.7 KHz. It must be within 1.8 and 2.3 KHz. Repeat steps 5 to 9, to
adjust the deviation as near as possible to 2kHz if necessary.
11) Increase the level of the AF Generator to 1V. Check the deviation at 1kHz, it must be approx. 4.6 kHz.
12) Remove the short at pin 9. Disconnect the AF Generator.
13) Switch to CH7. Short pin 9 (Ext. PTT) to the ground. Check the deviation. It most be 500 plus/minus 120 Hz.
14) Go to RX, change the channel to CH8. Check the deviation, it must be between 400 and 650Hz.
15) Change the channel to CH9. Press CALL1. Note the maximum measured deviation. It must be between 2.55 and 2.9 KHz.
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