HM135 Service Manual
Preliminary version Page 9 of 13
The microprocessor manages the analogue switches for the scrambler as well, which is base-band-inversion type.
2.7.b CTCSS (Continuous Tone Coded Squelch System)/DCS (Digital Coded Squelch)
CTCSS signals and DCS signals are synthesized by the microprocessor DD5 (pin 39 - line
MCU_CTCSS_DCS_PWM) and appear as PWM, then smoothed by the CTCSS/DCS 300 Hz low pass filter which
consists of DA9:A and related circuitry to produce an acceptable sine wave output. The output of the filter is fed in
the pin 2 of the IC DA10 which adjusts the level and outputs the regulated CTCSS/DCS at its pin 12. This signal is
then applied to the second input of DA9:C in order to be summed with the speech/Selcall signal.
The CTCSS/CTS decoding is provided by the microprocessor DD5 (pin 60) which gets the proper signal from the
detector as explained in par. “CTCSS/DCS signal routing”.
2.7.c Selective call (Selcall) encoder
Similarly to CTCSS/DCS, Selcall signals are also generated and decoded by the microprocessor DD5. Selcall
encoding audio PWM signaling coming from the pin 37 of the microprocessor (line MCU_SELCALL_PWM) is fed
to the 3 KHz low pass filter which consists of DA9:B and related circuitry. Its output is then fed to the input of the
DA11:B, so routed the same way of the speech audio.
The speech/Selcall signal is applied to the input of DA11:B which limits the peak-to-peak output, then is fed to the
pin 3 of IC DA10 which is a double digital audio regulator. After the level regulation, the speech/Selcall is output at
pin 11 and sent to the first input (pin 9) of DA9:C in order to be summed with the CTCSS/DCS signal.
The Selcall decoding is provided by the microprocessor DD5 (pin 59) which gets the proper signal from the
detector as explained in par. “Selcall signal routing”.
2.8 ON/OFF switch, rear connector and internal connectors
2.8.a ON/OFF switch
The line ON/OFF_SWITCH is normally pulled up by the resistor R182. When the front ON/OFF switch is switched
on, this line becomes low, so the zener VD32 can bias the transistor VT35 which activates the main electronic
power ON/OFF switch VT33 which feeds the main voltage to the regulators DA7 (+8V) and DA8 (+5V). The diode
VD37 acts as a typical protection against polarity inversion.
2.8.b Rear connector
The rear connector XT3 accomplishes a variety of connections and functions allowing to connect the radio to
many kinds of devices. For example:
Pins 3 and 16 (lines EXT_SPEAKER- and EXT_SPEAKER+) can be connected to an appropriate external speaker
The line AUX_OUT_FROM_MPU coming from the pin 16 of the main microprocessor DD5 drives the transistor VT33 which
can switch ON/OFF by software a 5,6 V voltage at pin 1 (line AUX_OUT) of XT3 is an auxiliary output programmable by
firmware.
The pin 13 of XT3 duplicates the hang up function normally provided by the microphone hang up: grounding or not the line
HUNG_UP reflect a status change in the line HANG_UP_TO_MPU via the zener VD27 and the transistor VT29
Pin 9 duplicates the PTT connection normally provided by the microphone connector in the front panel. Its line
EXTERNAL_PTT drives the zener VD30 and the transistor VT32 which reflect a status change in the line PTT_TO_MPU
2.8.c Internal connectors (accessory board)
The internal connectors XP1 and XP2 are used to internally fit a variety of option boards, such as scrambler
modules, audio processing modules etc. For this reason there are many contacts in parallel with XT3, e.g. XP1
has AUX_OUT at pin 13 and EXTERNAL_PTT at pin 1. Moreover, the two connectors have other specific lines in
order to handle a large number of internal signals, e.g. flat unsquelched RX audio at pin 14 pf XP1 and
microphone input/output at pins 1 and 2 of XP2.
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