tti Tech PMR-446, Service Manual, Page: 7 / 34

The Receive VCO ( C306 , L302 , R306 , C308 , C310 , Q305 , C311, L304 ) provides the LO signal. The VCO is running
at 21.7 MHz below the desired receive frequency and is applied to output Buffer Q301 . The output of the buffer through
the low-pass filter C407 , R406, L405 and applied to the mixer Q403 .
Mixer
The mixer is a FET type ( Q403 ). The mixer LO frequency is 21.7 MHz below the desired receiver frequency
When the receiver frequency is present, the mixer output will be a 21.7MHz signal. The mixer output is peaked for
21.7MHz at
L406, C409, C410 and the signal is filtered by crystal filter XF401 and XF402 and amplified by Q405
before being applied to the input of the IF IC Q503 .
Inside Q503 , the 21.7MHz IF signal becomes the input to a second mixer with a LO frequency of 21.258 MHz set by
X301 . The 455kHz ceramic filter F501 filters the second mixer’s output which is the second IF signal.
The mixer’s output is then fed to the internal limiting amplifier and then on to the FM decoder.
FM Detector and Squelch
The FM detector output is used for squelch, decoding tones and audio output.
The setting of the squelch adjustment the microprocessor set the input to the squelch amplifier.
The squelch amplifier is internal to
Q503 and its output is fed to an internal rectifier and squelch detector.
The audio is unmuted by the microprocessor Q106 Pin 13 switching to a high ( ~ 5V) thus biasing on Q506 .
The audio is then routed to the audio amplifier Q702 via the volume control SW201B .
Receiver Audio Circuit
The detector’s audio output also is fed to the tone (CTCSS and DCS) low-pass filter Q507A .
Then the output of the low-pass filter is routed to the second stage filter Q507B . The output of Q507B is applied to the
squaring circuit Q504D and finally to the microprocessor Q106 Pin11 for decoding.
Another branching of the detector output feeds the audio high-pass filter Q504A , Q504B and Q504C . The output of the
audio high-pass filter feeds the Volume Control SW201B (VOL). From the wiper arm on the Volume Control, the audio
is routed to Pin 4, the input to the audio power amplifier Q702 . The output of the audio power amplifier is routed through
the earphone jack J201 to the internal speaker E202
.
3-5 BATTERY SECTION
The battery connects to the contact pins ( CN203 and CN204 ) on the bottom end of the Radio. The positive terminal of
the battery connects to the ON/OFF Volume Control switch ( SW201A ) and the negative terminal connects to chassis
ground. Low battery sense R101 / R102 , voltage regulator Q208 and transmit power amplifier Q407 .
Battery voltage status is monitored by the microprocessor Q106 through R101 / R102 .
When the battery voltage is approximately 5.8V, the output is inputted into the Pin1 of the microprocessor Q101 .
The battery voltage is below the preset value.
When the Radio is on a channel with no tone programmed, the BATTERY SAVER Mode is enabled when programmed.
In the BATTERY SAVER Mode, the microprocessor
Q106 generates a square wave signal on Pin 21.
The signal's duty cycle varies according to what the POWER SAVE TIMER is set. When the microprocessor Q106 Pin21
goes high (approx. 5 V) Q205 is biased off, Q204B is cut off, Q203 is cut off, and Q204A is cut off, thus turning the
supply off to Q203.
Receiver Front End
In the receive mode, the RF signal enters thorough the antenna, then through the low-pass filter C427, C428, C430,
C432 and L412, L413, L414 . The diodes Q404 and Q406 are biased off so that the output of the low-pass filter is,
coupled ( C401 ) to the Front End RF overload protection diode pair Q401 and the first band-pass filter SAW401 .
The signal from the band-pass filter is applied to the input of the RF amplifier Q402 . The output of the RF amplifier
feeds the input to band-pass filter SAW402 . The output from the band-pass filter is applied to the mixer's Q403 .
Local Oscillator (LO)
3-4 RECEIVER SECTION
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