3012—Fault finding
NGT Transceiver System Technical Service Manual 313
1 Switch off power to the data modem.
1 Reconnect the 4-way loom to header P2.
Testing voltages on the DSP Modem PCB
To test the voltages on the DSP Modem PCB:
1 Remove the screen covering the DSP Modem PCB (see page 310, Dismantling the
data modem).
1 Re-measure the voltage across the GND and test points on the User
Connector PCB.
The voltage should nominally measure +13.5 V (+10.5 to +15 V is allowable).
If the voltage is not within these limits, but was within these limits when the User
Connector PCB was tested in isolation, the DSP Modem PCB has a fault that is
overloading the power source. In this case, the DSP Modem PCB must be repaired
or replaced.
1 Measure the voltage across the GND and +5 V points on the DSP Modem PCB.
The voltage should be +5.0 V ±0.1 V.
If this voltage is not within the specified limits, a component such as the power
supply on the DSP Modem PCB has failed and the PCB must be repaired or
replaced.
1 If the PCB is issue 06 and earlier, measure the voltage across the GND and –5 V
points on the DSP Modem PCB.
The voltage should be –5.0 V ±0.1 V.
If this voltage is not within specified limits, the analog negative supply rail has
failed and the DSP Modem PCB must be repaired or replaced.
Testing the HF subsystem
To confirm the correct operation of the data modem audio output path:
1 Switch off power to the data modem (via the transceiver).
1 Remove the top cover (see page 310, Dismantling the data modem).
1 Remove the screen covering the DSP Modem PCB.
1 Turn the digi switch on the DSP Modem PCB to position E.
The DSP Modem PCB is now in service test mode.
1 Switch on power to the data modem (via the transceiver).
1 Turn the digi switch to position 0.
If the PCB is issue 06 and earlier, the DSP Modem PCB generates a 1 kHz tone of
approximately 6 V p–p in the transmit audio path when measured at the AF_OUT
test point on the DSP Modem PCB.
If a tone is not present, the DSP Modem PCB may be faulty.
A
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