GTX 327 MAINTENANCE MANUAL
Page 3-5
190-00187-05
Rev. B
If the KEY LED DIMMING fails, verify the following:
7.
Q501-4 has a 4 kHz PWM similar to the traces shown in figure 3-1.
8.
Verify that as the display dims, the PWM duty cycle decreases.
After performing the steps listed, replace the front display assembly if:
•
An error is found using the matrix test in Table 3-2.
•
One or more keyboard tests locates a fault that is not related to the main board assembly.
•
There are other front panel display problems that are not caused by the main board assembly.
3.8
RECEIVER AND TRANSMITTER ASSEMBLIES
Apply a standard test signal to the GTX
327. With the transponder in either ON or ALT mode, verify on the
ATC-1400A that the unit is replying. If the ATC-1400A indicates that the unit is not transmitting but the
GTX
327 front panel display indicates that the unit is replying, the problem may be in either the diplexer cable or
the transmitter.
1.
Using the oscilloscope, check for P
1
– P
3
pulses at TP-2 on the receiver board. Verify that the P
1
– P
3
pulses arrive at I204 pin 64 on the main board.
2.
On the main board, check for modulator pulses at I204 pins 58, 59 and 60.
3.
If there are pulses, lift and rotate the receiver over to access the transmitter. Referring to the P501
signal levels in Figure 3-2 verify that all power supply voltages are correct in the transmitter.
Verify that the modulation pulses just checked on the main board are also seen at TP500, TP527 and TP518 on
the transmitter. These pulses drive the modulator and enable the transmitter. If these pulses are correct at the
transmitter but there is no RF output, it indicates either a transmitter problem or a fault in the RF path between
the transmitter and the antenna connector. Also inspect the diplexer coax cables and in particular, the center pins
of each connector on the transmitter and receiver boards.
Replace the receiver assembly if, while the unit is receiving valid interrogations:
1.
There are no P
1
– P
3
pulses at receiver TP-2.
2.
Receiver voltages given in Figure 3-4 are measured and found to be incorrect.
3.
Unit does not operate correctly over RF dynamic range from approximately -74
dBm to -20
dBm.*
4.
Side Lobe Suppression does not function correctly over the receiver dynamic range.
*
Units P/N 011-00490-00 prior to S/N 83707331, P/N 011-00490-01 prior to S/N 83925021 and
P/N 011-00490-10 prior to S/N 84000116 have an MTL of -72 dBm.
The document reference is online, please check the correspondence between the online documentation and the printed version.