No Reception - Codan X-2 Service Manual

Maintenance 5-6
5.3.3

No Reception

104-02907 sheet 1
For a receive failure:
1.
2.
3.
4.
5.
6.
7.
Check the microphone plug is fitted. The link in the plug completes the
audio path to the internal loudspeaker.
Check the Mute gate is operating and closed.
Check the supply voltages are correct as detailed in Section 5.3.2
Using a Signal Generator and a series capacitor (about 100 nF) apply a
signal to the test points shown on the circuit diagrams. The receive
levels and frequencies, shown on the RF and IF circuit diagrams,
should cause the AGC to fall by about 0.5 V from its no signal level of
approximately 5.2 V (under no fault conditions).
Start at the 455 kHz IF amplifier and proceed back through the
receive path towards the antenna. When the injected signal is no longer
heard or the AGC no longer falls by 0.5 V under application of levels
shown on the circuit diagram, a close examination of the satisfactory
and failed check points should indicate where the fault lies.
In the case of an audio fault, apply a signal from the Signal Generator
to the receiver input at a frequency set to approximately 1 kHz above
the channel frequency (1 kHz below for LSB). With the aid of an
oscilloscope, trace the audio signal path from the demodulator output
to the loudspeaker.
Note: If the AGC is operating satisfactorily then audio signal should
be present at the output of pre-amplifier IC303/A, as this point
also feeds the AGC detector circuit.
If the receive fault shows as an oscillation or instability, check that
neither of the transmit mixers, IC2
( < 0.5 V). Check the modulator has no DC at pin 5. If DC is measured
at any of the points indicated, check the following:
•
Check the base/emitter voltage to switching transistors V2
V9. If it measures 0.5 V or more, check NAND gates IC7/C
and IC7/D output voltages are correct as shown on the circuit
diagram.
•
If DC is measured on pin 4 and 11 of mixer IC2, unsolder and lift
one side of resistor R7
collector of V2, suspect collector/emitter leakage of transistor and
replace. If the voltage still remains on pins 4 and 11 of the mixer,
replace IC2.
X-2 Technical Service Manual
and IC9, have DC on pin 4 or 11
[D3]
. If the voltage still appears at the
[C4]
[B4] and
[A6]
Publication No: 15-02047