416 CODEC and Audio Fault Finding TM8100/TM8200 Service Manual© Tait Electronics Limited November 200713.5 Faulty ModulationIntroduction This section covers the case where the radio transmits the correct amount ofRF power, but there is either no modulation or the modulation is distorted.There are three tasks:■ Task 12: initial checks■ Task 13: check 2.3V DC supply■ Task 14: check bias networkThe initial checks will determine whether the frequency synthesizer, the2.3V supply, or the bias network is at fault.Task 12 —Initial ChecksCarry out the following checks to isolate the part of the circuitry that isfaulty.1. Apply a 1kHz audio signal of 20 mV pp at the microphone input onthe control head.2. Enter the CCTM command 33 to place the radio in transmit mode.(The frequency is that of channel 1.)3. Check that the 1kHz signal appears at the TP 503 test point(see Figure 13.8).4. Enter the CCTM command 32 to place the radio in receive mode.5. If the 1kHz signal is present, go to “Frequency Synthesizer FaultFinding” on page 185. If it is not, go to Step 6.6. With no microphone connected, check the voltage at the junction ofC708 and C732 (CH MIC AUD) (see Figure 13.8):7. If the above voltage is correct, go to Task 14; the bias network issuspect. If it is not, go to Task 13; the 2.3V supply is suspect.TP503 test point: 1kHz signaljunction of C708 and C732: approximately 3V
