Johnson DM3474, Руководство по обслуживанию

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SERVICING
5-2
Part No. 001-3474-002
NOTE: The user-supplied circuitry must disable the transmitter and receiver when an out-of-lock condition is
indicated.
When the VCO is unlocked, the
f
R
and
f
V
inputs to the phase detector are usually not in phase (see Section
4.1.2). The phase detector in U801 then causes the VCO control voltage to go to the high or low end of its
operating range. This in turn causes the VCO to oscillate at the high or low end of its frequency range.
As shown in Figure 4-1, a loop is formed by VCO Q850, amplifier Q851/Q852, and the RF IN of U801.
Therefore, if any of these components begin to malfunction, improper signals appear throughout the loop.
However, correct operation of the counters can still be verified by measuring the input and output frequencies to
check the divide number.
Proceed as follows to check the synthesizer I/O signals to determine if it is operating properly.
5.2.2 REFERENCE OSCILLATOR
Check the signal at U801, pin 8. It should be 17.5 MHz at a level of approximately 1.5V P-P. If the TCXO
module is defective, it is not serviceable and must be replaced with a new module as described in Section 5.1.5.
5.2.3 VCO
Output Level
The output level of Q853 can be measured with an RF voltmeter or some other type of high impedance meter.
The minimum level after a power splitter at R851 should be -10 dBm.
Control Voltage
Check the DC voltage at C815 with a channel near the center of the band. If the VCO is locked on frequency,
this should be a steady DC voltage near 3V. If it is not locked on frequency, it should be near the lower or upper
end of its range (0V or 5.5V).
Output Frequency
Check the VCO frequency at R851. If the VCO is locked on frequency, it should be stable on the transmit
channel frequency. If the VCO is not locked on frequency, the VCO control voltage is probably near 0V or 5.5V.
5.2.4 SYNTHESIZER (U801)
Lock Detector
When the VCO is locked on frequency, the lock detect output on J201, pin 7 should be high.