Hytera MD78X, Service Manual

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Circuit Description
VHF (136MHz
–
174MHz)
22
(RMS should be 80 mV when frequency is 1 kHz and deviation is 3 kHz). Then the codec (U231)
converts PCM data into analog audio data via DAC.
U231 provides two outputs: SPK1 and SPK2. SPK1 is amplified by U238 and then is fed to the PA
U201, to derive two outputs of received audio, which will be applied to the front panel speaker and
further development interface respectively. SPK2 is amplified and divided into the four paths of signal
Handset_audio
,
RX_audio
,
PUB_Address1 and PUB_Address2.The first two paths are amplified by
U235, and the last two paths are amplified by U236. Output of Handset_audio
,
PUB_Address1 and
PUB_Address2 are controlled by Q231, Q235 and Q233 respectively.

TX Audio Path
The audio processor U231 has two MIC inputs: the internal MIC and external MIC. The internal MIC
is connected to the MICIN_HND of U231, and is combined with AUX1 to provide differential input.
After 9.3V is divided by R255 and R264, a bias voltage of about 7V is provided to the front panel MIC.
The external MIC is connected to the MIC_HED of the codec (U231) as a uni-directional input.
Both MICs are controlled by Q232. When the internal MIC is active, EXT_MIC_EN is at low level,
bias voltage of external MIC is disconnected, and the ADC in U231 samples voltage at the
MICIN_HND. When the external MIC is active, EXT_MIC_EN is at high level, Q232 is turned on, bias
voltage is connected, and the ADC in U231 samples only audio data at MICIN_HED.
Audio signals input from the MMC interface and the accessory connector are fed to the Codec (U231)
and then are converted by the ADC into 16-bit PCM digital audio signals, which will be fed to U302
via the SSI.
When the mobile radio is set to Public Address mode, the audio signal will be generated by the
internal MIC, and then is buffered by U231, and finally is output via SPK2. The output audio signal is
amplified, and then is output as the two paths of audio PUB_Address1 and PUB_Address2.
5.3
RF Section
5.3.1
Transmitter Circuit
The transmitter circuit is composed of power amplifier circuit and power control circuit. The former is
used to amplify the voltage control oscillator (VCO) signal to the appropriate output power level, while
the latter can keep the output power at the appropriate level, so as to protect the power amplifier from
damage caused by over heat, antenna mismatch, and out-of-range voltage (over voltage or under
voltage may result in damage to Q801, Q805, Q804, Q802 and Q803).