X-2 Technical Service Manual Maintenance 5-7
Publication No: 15-02047
• If DC is measured on pin 4 and 11 of mixer IC9, unsolder and lift
one side of resistor R35
[C8]
. Check if the DC is still on the mixer
pins, replace IC2.
If the voltage is only on the collector of V9 and R35 is lifted,
apply a short circuit between pin 5 of IC301
[D3]
and ground
[04-02907 sheet 2]. If the voltage goes from the collector of V9,
replace modulator IC301. If the voltage remains on the collector
with pin 5 shorted, replace V9 (collector/emitter leakage).
5.3.4 Transmitter Failure
For a transmit fault proceed as follows:
1. Check the supply voltages are correct as detailed in Section 5.3.2.
2. Apply an audio signal of about 1 kHz at a level of 10 mV RMS to the
microphone input. Select transmit mode (PTT) and with the aid of an
oscilloscope trace the audio, IF and RF signals through the transmit
path. Check the measured levels correspond approximately to the
levels shown on the circuit diagrams. A reduction of signal or complete
failure at any of the check points should indicate the approximate
location of the fault.
5.3.5 Unlocked Synthesizer
104-02907 sheet 1
Loss of lock on VCO1 or VCO2 is indicated by:
• a warning tone in the loudspeaker
• illumination of H1 and/or H2 on the Rx/Exciter & Control PCB.
Should the above warnings occur, proceed as follows:
1. Check the 18 V supply is present and connected to:
• oscillator VCO1
[E2]
(V3 and V4)
• control amplifier V6
[G3]
and V7.
If VCO2 has failed, check it is present at the control amplifier
IC12/A
[H7]
.
2. Check VCO1 and VCO2 are oscillating. Check that the outputs at
TP4
[E4]
(VCO1) and TP1
[A2]
(VCO2) are approximately the level shown
on the circuit diagram. Frequency should be within the range indicated.
3. Check the output levels at pin 3 of the prescalers IC4
[E5]
and IC10
[F7]
.
Check the frequency is approximately the VCO frequency divided by
64.
4. Check the VCO control voltages on TP3
[G1]
and TP13
[F6]
are within the
limits indicated on the circuit diagram.
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