Maxon SP-1102, Service Manual, Page: 24 / 35

Service Manual SP - 1402
Page 24 of 35
Second mixer, second IF amplifier and FM detector
The output of the first filter amplifier, Q34, is coupled, via C122 to the base of Q14. Q14 is the
second mixer .The second local oscillator signal from Q18 is 44.645MHz fed to the emitters of
Q14 and converted to 455KHz at the collector and then second IF amplified by Q16 from the
output of the second mixer. The mixer output is then routed to CF3 or CF4. These ceramic filters
provide the adjacent channel selectivity of 25 KHz or 12.5 KHz bandwidth then fed to pin25
IC2(IFIN). The receiver signal converted to the second IF signal frequency through the FC2, The
ceramic filter of 455KHz again. After limiting inside the IC2 and the FM demodulating by the
quardrature detector inside the IC2 the signal offers the output through the 29
th
pin of IC2 .
Mute (squelch) circuit
The mute circuit switches off the power amplifier when no audio signal is present. The squelch
circuit consists of IC6 and RV3, RV7 and their associated components. The noise signal form pin
30 of IC2 is amplified by internal amp of IC2and then fed into RV7(N Band) is used to adjust the
squelch circuit sensitivity and is normally adjusted to produce noise squelch opening sensitivity of
10dB to 12dB SINAD. By IC 6 this for level control SQ
Speaker audio amplifier
The audio signal which has been FM demodulate in the IC2 has a filter as de-emphasis and 300
HPF from pin29 signal is through pin 44 and filtered signal is out pin48 .From pin48 audio signal
is level controlled by internally Volume step .The controlled audio is into the pin53 and amplied
20dB by internal Audio Amp. Then, it turns up the speaker with the maximum output of 0.5W.
UHF PLL Synthesizer
The receiver is comprised of:
12.8MHz TCXO
PLL IC dual modules prescaler(AN29160AA)
Level shifter & charge pump
Reference frequency LPF
VCO
12.8MHz TCXO
The TCXO contains the 2-stage thermistor network compensation and crystal oscillator and
modulation ports. Compensation is +/-2.5 PPM or less from -30c to +60c.