Tait TM8100 mobiles, Service Manual

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444 Fault Finding of Control Head with Graphical Display TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
14.12 PTT Faulty
The PTT signal from the microphone connector is connected to the FPGA
via a resistor (R25) and relayed to the radio as a digital command.
Note
This section only describes faults to the PTT caused by the con-
trol head, which has been established during the initial servicing
tasks by means of elimination test.
If the PTT is faulty:
1. With no PTT switch and hookswitch operated, check whether pin 4
of J106 is 4V.
If the signal is correct, continue with Step 2 .
If the signal is incorrect, inspect R25 for open or shorted contacts.
Repair if necessary. Repeat
Step 1 .
2. With the PTT switch operated, check whether the same 4V are
pulled to ground on the other side of R25.
If the signal is correct, continue with Step 3 .
If the signal is incorrect, inspect D106 and C108 for short-circuits.
Repair if necessary.
3. Verify continuity between R25 and the FPGA. Repair PCB track if
possible.
Figure 14.17 Circuit diagram of the PTT circuitry
J106 pin 4: 4V
R25: GND
J106
Microphone
Connector
FPGA
4
FP PTT
R25
MIC PTT
C108 D106