GPH-CMD VHF Radio Maintenance
BK RADIO Page 5-7
A. VCO Adjustment
The frequency range of the VCO must be tuned. If components in the VCO are replaced
this adjustment may be necessary.
As the inductance value of the VCO resonator is manually tuned, the synthesizer U12
(Systems board) changes the voltage applied to varactor CR4 (Systems board) to maintain
the programmed frequency.
Connect a suitable 50Ω RF load to the antenna output of the radio.
Follow the instructions for VCO Tuning Voltage Adjustment in the GPH-CMD Editor
Software. This will allow you to adjust the Tuning voltage through an interactive, partially
automated process.
B. Transmit Power
The high and low transmit power settings control the RF output of the transmitter. If
components on the RT board are replaced this adjustment may be necessary.
Microprocessor U1 (Systems board) sets the transmitter power control voltage using DA
converter U2 (Systems board). High and low power settings are stored in non-volatile
memory.
Connect a suitable 50Ω power meter to the antenna output of the radio.
Follow the instructions for Transmit Power Adjustment in the GPH-CMD Editor Software.
Check the power at low (136 MHz), mid (155 MHz), and high (174 MHz) frequencies. If
any high power reading is below 5 Watts, readjust to obtain 5 Watts. If any low power
reading is below 2 Watts, readjust to obtain 2 Watts.
C. Reference Oscillator Frequency
The GPH-CMD radio's transmit and receive frequencies are derived from the Reference
oscillator. The Reference oscillator may require adjustment due to crystal aging or if
components in the Reference oscillator circuit are replaced.
Compensation for error in the Reference oscillator is provided by a divide value in the
synthesizer. The final setting is stored in non-volatile memory.
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 30 dB attenuator capable of 10 Watts is
recommended.
Follow the instructions for TX Frequency in the GPH-CMD Editor Software.
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