MT-4E VHF & UHF Receiver & Transmitter Instruction Manual
IM08-MT-4E-TXRX-P1162
VHF & UHF Hardware Tuning and Troubleshooting
17
Synthesizer and udB Module
The Synthesizer Module and UDB are bound together through their communication link. At this level
of fault diagnosis it is not feasible to isolate the problem between the two units.
If the receiver front panel ON/OFF switch is set from the OFF position to the NORM position and the two
front panel LEDs stay on, check that the synthesizer and the UDB are properly seated on the receiver
mainboard and check the following test points on the UDB. To access the UDB, the shield lid covering
the UDB will need to be removed.
Test Point
Signal Monitored
Typical voltage
TP1
Ground
0 VDC
TP2
Logic power
+3.3 VDC ± 5%
TP3
DSP core supply
+1.2 VDC ± 5%
TP4
CPLD core supply
+1.8 VDC ± 5%
If the UDB voltages are not within the specified tolerances, it is most likely that the receiver
mainboard (which supplies these voltages to the UDB) is faulty or there is a short within the UDB.
If the UDB voltages are within the specified tolerances, inject a -70 dBm carrier on the receiver
frequency into the RF input and check the RSSI meter reading in RSS Service mode. If the RSSI
meter reading is low (< 30), perform the following test to verify that the synthesizer is locked on
frequency:
Disconnect the LO output SMB connector from the synthesizer to the RF Preselector and connect the
synthesizer LO output to a radio communication test set. Confirm that the synthesizer LO frequency
matches the Target Synthesizer RF OUT found in the “Reference Oscillator” field of RSS Service
mode. The LO output should be approxi7 dBm.
The frequency of the synthesizer LO output will be above or below the receiver frequency by the
IF frequency (see the VHF and UHF Receiver Performance Specifications data sheet). Check that
this output is within the tolerance for the local oscillator frequency stability (see the VHF and UHF
Receiver Performance Specifications data sheet). Also ensure that the RF frequency will change with
programmed channel changes in the RSS.
Receiver RF Preselector
Check the +9.5 VDC power supply wire to the RF Preselector. The supply voltage should be
+9.5 VDC ± 5 %. Inject a -30 dBm carrier on the receiver frequency into the RF Input and check that
the IF Output of the RF Preselector is equal to the specified IF frequency (see the VHF and UHF
Receiver Performance Specifications data sheet) and the output can be demodulated through the
communication test set. If this is not the case, the synthesizer or the RF Preselector, or both, are
faulty.
To further isolate the fault, the LO output can be disconnected from the synthesizer and an LO input
can be applied from an external source to the RF Preselector. The frequency of the external LO input
should be the Target Synthesizer RF OUT found in the “Reference Oscillator” field of RSS Service
mode. Check the 21.4 MHz output of the module using a communication test set.
