418 Fault Finding of Control Head TM9100 Service Manual© Tait Electronics Limited August 200514.12 PTT FaultyThe PTT signal from the microphone connector is connected to the FPGAvia 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 servicingtasks by means of elimination test. For fault finding of the ??? cir-cuitry of the main board, refer to xxx on page yyy.If the PTT is faulty:1. With no PTT switch and hookswitch operated, check whether pin 4of 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 arepulled 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 ifpossible.Figure 14.17 Circuit diagram of the PTT circuitryJ106 pin 4: 4VR25: GNDJ106MicrophoneConnectorFPGA4FP PTT R25 MIC PTTC108 D106
