Maintenance GPH-CMD VHF Radio
Page 5-8 BK RADIO
D. VCO Modulation Sensitivity
This adjustment controls the maximum analog and digital mode FM deviation of the
transmitter. If components in the VCO or Transmit Audio amplifier or filter are changed this
adjustment may be necessary to maintain an FM deviation below 5 kHz (2.5 kHz for
narrowband or digital mode).
The level of the Transmit Modulation is controlled using a digital potentiometer U18
(Systems board).
Connect a suitable attenuator and service monitor to the antenna output of the radio.
Follow the instructions for TX Modulation in the GPH-CMD Editor Software.
E. Receiver IF Alignment
Coupling transformers in the IF section are manually tuned to optimize receiver distortion. If
components in the mixer or IF circuitry are replaced this adjustment may be necessary to
maintain specified receiver distortion.
As the transformers are manually tuned, the IF filter passband shape is affected.
To perform the adjustment:
1. Program 136 MHz receive frequency, wideband (25/30 kHz) mode. Turn the
monitor/squelch control fully clockwise. Adjust the volume control to mid range.
2. Connect an RF signal generator tuned to 136 MHz to the antenna jack of the radio.
Modulate the generator with a 1 kHz tone at 3 kHz deviation. Set the output RF level to
1 mVrms.
3. Connect a distortion analyzer to the radio speaker output.
4. Adjust T2 and T3 for the lowest audio distortion. Preset the cores to the top of the coil
before making the adjustment. When adjusted for the lowest distortion, the top of the
core should be in the upper half of the coil to prevent coil breakage.
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