User Manual

SERVICING
5-3
June 1997
Part No. 001-3412-002
5.3.3 SECOND LO (Q401)
Verify the Second LO signal is present at U241,
pin 4. The Second LO should be at 52.50 MHz and
not less than 500 mV P-P.
5.3.4 AUDIO BUFFER AMP (U261)
The Data output on J201, pin 13 should be 280-
700 mV P-P, with the preceding injection signal. If
these levels are not correct, verify proper adjustment
of L253 (see Section 6.3). The gain of U261 is 2.5 for
25 kHz radios and 5 for 12.5 kHz radios.
5.3.5 IF AMP (U231)
The IF Amp input signal is 52.95 MHz. This
amp provides approximately 15 dB of gain to U241.
5.3.6 BUFFER AMP (Q211), CRYSTAL FILTERS
Q211 buffers the 52.95 MHz IF signal and pro-
vides matching to the crystal filters Z221/Z222.
5.3.7 MIXER (U211)
The mixer converts the RF signal (380-512 MHz)
to 52.95 MHz. The Local Oscillator is provided by
the VCO and Q301. The level of the LO should be
approximately +3 dBm.
5.3.8 LNA (Q202), HELICAL FILTERS
The LNA provides approximately 15 dB of gain
at 380-512 MHz. The helical filters Z201/Z202 have
approximately 2.5 dB of loss each. Q201 provides
active bias to Q201.
5.3.9 ANTENNA SWITCH
CR561, CR562, L561, C561 and C563 form a Pi
network antenna switch. CR561 and CR562 are
reversed biased in Receive Mode.
5.4 TRANSMITTER SERVICING
5.4.1 SUPPLY VOLTAGES AND CURRENT
Measure the supply voltages on the following
pins of interface connector J201:
Pin 2 - 13.3V DC nominal
Pin 3 - 3-16V DC
Pin 4 - 0.0V DC (while transmitting)
Pin 5 - 3-16V DC
Pin 6 - 2.5V DC ±1%/1.5V P-P max
Place a DC ammeter in the supply line to the
transceiver and the following maximum currents
should be measured:
Pin 2 - 2.5A maximum
Pin 3 - 150 µA
Pin 5 - 150 µA
5.4.2 VCO (A801)
1. Check VCO A801 for power output.
(Power output should be at least -10 dBm.)
2. Check 9V Transmit (Q141, emitter).
3. If 9V is not present check Q124, U111, Q121,
Q122, Q123, Q101 and Q102 (see Section 4.4.5).
4. Check voltages on Buffer Q501.
Input = 1.5V DC
Output = 3.5V DC
Power output should be at least 2 mW (+3 dBm) at
C504 (50 ohm point).
5.4.3 PRE-DRIVER (Q511)
1. Check voltages on Q511.
Collector = 8.6V DC
Base = 2.2V DC
Emitter = 1.6V DC w/o RF (2.2V DC with RF)
Power output should be at least 100 mW (+13 dBm)
at the junction of C517/L518 (50 ohm point).