14: Maintenance
14-20
PRC1099A-MS
The frequency should be the operating frequency of 76.6 to 105 MHz
depending on the channel frequency. For example, if the channel
frequency is 5.123 MHz, the local oscillator output should be
80.123 MHz in USB mode or 78.473 MHz in LSB mode.
3. Connect the frequency counter to 2nd LO output (J3).
The frequency should be approximately 73.35 MHz.
4. Connect a frequency counter to BFO output J1 pin 12 (R65 solder pad
closest to J1 connector pin 7).
The frequency should be 1650 kHz in USB mode, or 1647 kHz in LSB
mode.
Mixer Board
If the radio works correctly in either the transmit or receive mode, the Mixer
board is operating normally. To verify signal levels, make the following
measurements:
1. Connect an oscilloscope to
TX OUT
and the signal generator to
1650 kHz I/O
(refer to “Mixer Board Signal Connections and
Adjustment Points” on page 14-26).
2. Set the generator frequency to 1650 kHz and the output level to
-10 dBm. Key the radio without modulation input.
The output measured on the oscilloscope at
TX OUT
should be
approximately 0.5 Vpp at the channel frequency.
3. Connect the oscilloscope to
1650 kHz I/O
and the signal generator to
RX IN
.
4. Set the signal generator to the transceiver operating frequency (USB
+1 kHz) and the output level to -60 dBm.
The output measured on the oscilloscope should be approximately
20 mVpp at 1650 kHz.
Note:
If the 75 MHz IF amplifier 1 (Q7) is not operating, check the
AGC voltage (refer to Figure 14-9 on page 14-26). It should be
approximately 4V. The AGC voltage is supplied by the
Audio/Filter board, and a fault in this board can prevent the
Mixer board from operating correctly in receive mode.
1650 kHz IF
Board
In transmit mode, a low level sample is routed through the receive amplifier
stages on the 1650 kHz IF board to produce a sidetone signal that is applied to
the Audio/Filter board and to the front panel Audio connectors. The presence
of a sidetone at the handset or headset verifies that the 1650 kHz IF board is
operating correctly in receive mode as well.
