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MX920 Base Station & Repeater
♦
If the RX centre frequency cannot be adjusted the TCXO may be faulty.
♦
Distortion should be less than 3%.
♦
If the distortion is high then FL3A, FL3B or FL4 may be faulty.
♦
If the sensitivity is still poor then return the Module to your Service Centre.
6.4 Exciter Module
6.4.1 VCO Locking
♦
Connect a DVM to the 'TX VCO' TP on the Microcontroller PCB and PTT,
check the Tx locking voltage is between 0.3 – 2.15 V.
♦
If the Tx locking voltage is out of spec then slowly adjust L48 on the TX VCO
and check for a change in the locking voltage.
♦
If there is a change in locking voltage, then realign the VCO voltage to 14V at
Upper frequency of the VCO frequency band.
♦
If there is no change in the TX VCO locking voltage then check the VCO supply
voltage TP4 is approx. 7-8V and with a CRO check the TCXO is oscillating on
pin 12 of IC14.
♦
If all the above tests didn't pass then the VCO or IC14 may be faulty. Return the
to your nearest Customer Service Centre.
6.4.2 RF Power
♦
Using in-built menu system increase the 'Transmit Power' control to maximum
and check that the output power is greater than +24 dBm.
♦
To insure power control is at max Short C208 together, (located under SKB)
♦
If the Exciter power is low, check for the +8 V supply voltage at the outputs of
TR27 and TR16.
♦
Lift one side of C216 which leads to R128 and solder a flying lead to that end of
the capacitor, check the output power of the VCO is between ≥ +7dBm.
♦
Use a RF probe to check for gain through TR27 and TR16.
♦
If all the above tests didn't pass, then return to your nearest Customer Service
Centre.
6.5 Power Amplifier
♦
First do a visual check of all the components on the PA looking for any damaged
components.
♦
Connect the input of the PA to signal generator with the RF output switched off.
♦
With PTT off measure the +13.8V supply at the Source of the driver and output
FET . If out of spec then check voltage at CN2-2, no volts then check supply
cable.
♦
With PTT ON measure the PA bias current at the Gates of the FET's. The Bias
current is band dependent. This is done by monitoring the current drain of the
whole PA with CN1 disconnected. Link the gate of TR2 to GND. Measure current
consumption (VHF High Band 200mA ,UHF 400mA.) This can be adjust
by RV2. Measure gate volts ≈ 3.4V
Technical Manual
2003 Revision
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