Microcontroller Section Of Pcb; Receiver Section; Vco; Rx Front End - RELM BK RADIO SDMX920 Technical Manual

Technical Manual

6.2 Microcontroller Section of PCB

Check fuse. If blown, replace with a 5x20 mm 3A fast blow fuse.
Check jumper is in the correct position for operation.

6.3 Receiver Section

6.3.1 VCO Locking
♦
Using the in-built menu, check all Hardware settings are correct.
♦
Connect a DVM (digital voltmeter) to the RX VCO TP14 and check the RX
locking voltage is between 0.3 – 2.15 V. This point is located in the
Microcontroller section of the PCB.
♦
Check to see that the Rx VCO is in lock ,TP13 is at a logic high +5V.
♦
If the RX locking voltage is out of spec then slowly adjust L27 on the RX VCO
and check for a change in the locking voltage.
♦
If there is a change in the locking voltage then realign the VCO voltage to 14V at
the upper frequency of the receiver band.
♦
If there is no change in the RX VCO locking voltage, then check that the VCO
supply voltage at TP4 is approx. 7-8V and with a CRO check the TCXO is
oscillating on pin 12 of IC12.
♦
If all the above tests didn't pass then the VCO or IC9 may be faulty. Return the
to your Service Centre.

6.3.2 RX Front End

♦
Solder a 2-way Berg header onto a flying coax lead. Remove \ cut link I near the
mixer (M1) and then fit solder link J. Inject a RF signal into CN5. Check
the sensitivity is better than –105 dBm.
♦
If the sensitivity is OK past this point, then check that the supply voltage (TP4) to
the collector TR24 is approx. 8V . If the supply is OK, then replace TR24. If
this does not repair the receiver then other components on the front end may
be faulty.

6.3.3 IF Section

♦ If the Receiver has low sensitivity past this point then increase the RF level to –
60 dBm and check the RX DISC voltage is set to 2.50VDC (TP1).
♦ Connect a coax lead with a pickup loop around the end from the antenna input on
the HP8920A to the case of X6. Set the HP8920 to TX test, change the Tune
Mode to Manual and change the centre frequency of the comms test set to
89.545MHz .
♦ If CT1 cannot be adjusted to match the above frequencies then CT1 or X6 may
be faulty.
♦
Set the centre frequency of the HP8920A to the RX freq.-/+ 90MHz . To find out
injection check using the built in menu system hardware setting
configuration.
♦
Adjust the TCXO to within +/- 20Hz.
© SPECTRA ENGINEERING 2003 Revision 1.1.4
Installation and Operation
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