Barrett 2000 Series, Техническое руководство

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Main microprocessor faults
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Check that the reference oscillator (a DTCXO) Digitally Temperature Compensated XTAL Oscillator
is operating at TP5. This should be a CMOS level (3V P-P) signal with a frequency of 22.7275 MHz.
VCO faults
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The VCO should produce an output frequency at TP8 of The signal frequency plus 45MHz.
Problems which prevent this include:-
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Preset divider U42 may be dividing by the wrong ratio. It is normally fixed at divide by 10. This will
only not be true on some rare channels so as to avoid a self generated heterodyne on the channel.
42 output should be checked on TP20. The VCO will be stable but on the wrong frequency in this
case.
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Phase detector and charge pump faults will cause the VCO to freely run on the wrong frequency.
The phase detector is U56 and the charge pump is Q10, Q11, Q12, and Q15. See the table
following for waveform details.
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VCO failure will cause no output from TP8 at all. Check the FETS Q13 and 14 and for dry joints
around this circuit. Replace Q13 andQ14 if necessary.
Transmitter faulty
If the transceiver will not transmit, it is first necessary to determine which of the modules is at fault. First
disconnect the coaxial cable between the RF/Audio PCB output and the PA PCB input. Do this at the
RF/audio PCB end. Connect a coaxial cable from the RF/Audio PCB output to an oscilloscope. Terminate
the oscilloscope input in 50 ohms. Set the transceiver frequency to 10 MHz. Either inject a two-tone test
signal into the microphone socket or whistle into the microphone with the PTT activated. A signal of approx.
200mV P-P should be observed. If this signal is absent or very low then the fault is likely to lie in the
RF/Audio PCB, if not, re-connect the cable between the two PCBs.
Now check link LK2 on the PA. If it has blown, then investigate the possibility of PA transistor failure before
reconnecting the power. If it is not these transistors then a relay fault could exist in the output filter circuit
this causing high collector current in the PA transistors, due to the ALC being disabled in this condition, and
as a result of the load mismatch caused by the faulty relays.
If the fuse link is intact, measure the bias voltage at the bases of the PA transistors with PTT activated but
with no audio input. The voltage should be between 0.5V and 0.7V. Check the gate bias voltage on each
of the driver FET’s Q4 and Q5. It should be somewhere between 2V and 4V.If correct then check the DC
voltage on the driver and PA collectors. These should be between 12V and 14V. If these voltages are
within limits, then it is likely that the driver or PA transistors are faulty. Again inject a two tone signal or
whistle into the microphone with the PTT activated and observe the RF voltage on the driver FET drains
(Q4, Q5). The swing should be at around 14V P-P. If correct then the fault is likely to be in the final power
amplifier stage. If no voltage swing was observed on the driver drains then check the driver gate voltage
swing, this should be approximately 1V P-P using a whistle into the microphone. If this is not correct then
the fault could be the driver transistors, or in Q3, Q10, or Q1.