User's Manual

GMH Series VHF Radio Maintenance
BK RADIO Page 5-9
Connect a suitable 50~ power meter to the antenna output of the radio. Select "Transmitter
power" from the "Tune GMH" menu and press Enter. Select "Start Transmit Power Test" and
press Enter. "OK to Transmit?" appears in the lower left corner of the computer screen. Press Y
to transmit. You can only adjust transmit power settings while transmitting.
Press the Tab or arrow keys on the computer to move the cursor to the 2-character hexadecimal
value in the right column of the menu. Type in a different value, or press Page Up or Page Down to
change the value. For each of the three frequencies, adjust the hexadecimal control value to
obtain an output power of approximately 50 Watts.
To save the new settings in the radio, select "Store Transmit Power" and press Enter. "Write
power values to radio?" appears in the lower left corner of the computer screen. Press Y to save
the new values in the radio.
This setting is used to help maintain a constant transmitter output power of 50 Watts across
the RF frequency band. If components on the PA board are replaced, this adjustment may be
necessary.
Adjustment values at several RF frequencies are stored in non-volatile memory by
microprocessor Ii (system board). The power is set by the voltage from DIA network RN2
(system board) which feeds the power control circuit.
NOTE: To adjust the GMH transmit power to a setting below 50 Watts, select "RF Power
Code" from the "Edit Globals" menu.
5.5.7 FREQUENCY ADJUST
Connect a suitable attenuator and frequency counter to the antenna output of the radio. The
attenuator must be capable of handling the full power output of the radio and protecting the input
of the frequency counter. A 30dB attenuator capable of SOW is recommended.
In the "Tune GMH" menu, select "Freq Adjust" and press Enter. "Start Freq Adjust" appears in the
middle of the computer screen. When you press Enter, the message "Ok to transmit?" appears in
the lower left corner of the computer screen. When you press Y (for Yes) the radio begins
transmitting and the Control value is outlined. The message "Transmitting!! Press <ESC> to stop"
appears in the lower left corner of the computer screen.
With the cursor under the 2-character hexadecimal value in the right column of the menu, type in a
different value, or press Page Up or Page Down to change the value. Adjust the control value up
or down until the transmitter frequency is within +~00 Hz of the correct value. When the correct
frequency is obtained, press the Esc key on the computer.
The GMH radio's exact transmit and receive frequencies are derived from the reference
oscillator. The reference oscillator frequency may require adjustment due to crystal aging or if
components in the reference oscillator circuit are replaced. The above "Crystal Color Dot'
setting must be correct before performing this adjustment.
Changing the reference oscillator frequency setting causes the voltage applied to varactors
CR102 and CR103 (system board) to be varied, in turn altering the oscillation frequency of
reference crystal circuit slightly. the microprocessor Ii (system board) controls the voltage
using /A network RN1 (system board). The final setting is stored in non-volatile memory.