Codan NGT 2010 RF Technical & Service Manual page 733

3012—Fault finding
Removing the rear panel assembly
To remove the rear panel assembly:
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Fault finding the power supply in the data modem
If a data modem is faulty, the first fault finding step is to test all the power supply rails.
Power supply rails need to be checked on the User Connector PCB first, then on the DSP
Modem PCB.
Testing voltages on the User Connector PCB
To test the voltages on the User Connector PCB:
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Remove the top cover as described above.
Unscrew the retaining screw in the centre of the bottom cover.
Unplug the three wiring looms from the rear panel assembly.
Unscrew the four countersunk retaining screws from the rear panel and withdraw the
rear panel assembly.
Switch off power to the data modem (via the transceiver).
Remove the top cover of the data modem (see
modem).
Unplug the 4-way loom that connects to header P2.
This isolates the User Connector PCB from the DSP Modem PCB.
Switch on power to the data modem (via the transceiver).
Measure the voltage across the GND and
(Test point A is located near header P2.)
The voltage should nominally measure +13.5 V (+10.5 to +15 V is allowable). This
confirms that the data modem is receiving supply from the transceiver.
If the voltage is not within these limits, check the cabling to the transceiver and
check the transceiver power supply. Also, check D1 and L1 on the User
Connector PCB.
Measure the voltage across the GND and +5 V test points on the User
Connector PCB.
The voltage should be +5.0 V ±0.1 V.
If the voltage is not within these limits, check the IC2 chip and its associated
components (C36 and C37).
Check the power supply of the isolated serial port by measuring across both the
0 V –ISOL and +5 V –ISOL test points, to the +10 V –ISOL test point.
These voltages should be +5.0 V ±0.5 V and 10 V ±1 V respectively.
If the voltages are not within these limits, check the isolated DC−DC converter chip
IC1 and its associated components.
page 310, Dismantling the data
test points on the User Connector PCB.
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NGT Transceiver System Technical Service Manual