Pioneer TX-608 - FM Tuner Circuitry

4.2 FM TUNER
Front-End
A frequency linear 3-gang variable capacitor is
used with a single stage FET RF amplifier.
The FET possesses high input impedance com-
pared with a transistor, and allows simple coupling
with the input tuning circuit, plus a significant ad-
vantage in terms of noise.
The local oscillator, Q3 employs a modified
Clapp circuit, thereby reducing the amount of
drift caused by changes in time, ambient tempera-
ture, and power supply voltage. Also since the
oscillator signal is obtained from the tuning circuit,
there are far fewer higher harmonics, resulting in
a much cleaner waveform with less spurious inter-
ference.
The oscillator signal is applied to the base of
the mixer transistor Q2.via low capacity capacitor.
When the FUNCTION switch (S3) is in the AM
position, +B is applied to the cathode of D1 via
R34. D1 will be cut-off and Q3 will turn off.
Q1 RF AMP. Q2 MIXER IFT
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Fig. 4-2 FM front end
IF Amplifier and Detector
This stage includes 2 dual-element ceramic fil-
ters, a transistor and an IC (PA3001-A) with a high
SN ratio and very little distortion. The transistor
inserted between the two ceramic filters is employ-
ed for impedance matching purposes as well as to
increase the gain.
PA3001-A includes the IF limiter amplifier, FM
detector (quadrature detector), AF amplifier,
muting circuit and the meter. drive circuit.
The muting circuit is turned on when pins 5 and
12 are connected by means of the FM MUTING
switch. If the dial pointer is moved approx.
+70kHz away from a station, and the input level is
very low (equivalent antenna input less than 4uV),
a 5V DC signal is generated at pin 12, and applied
to pin 5, thereby activating the muting circuit
within the IC.
FM MPX Stereo Demodulator
The IC (PA1001-A) employed in the FM multi-
plex stereo demodulator stage also features a high
SN: ratio and reduced distortion. Due to the in-
corporation of a pilot auto-canceller circuit, very
good frequency characteristics are obtained. Un-
like the more conventional pilot signal (19kHz)
canceller circuits, which fail to completely remove
the pilot signal if it is not at standard level,
PA1001-A pilot auto-canceller circuit also includes
a pilot signal level detector circuit. Changes in
pilot signal level are consequently responded to
immediately, resulting in very effective suppression
of the pilot signal leak level.
Filter Circuit
Besides eliminating the sub-carrier signals (more
than 23kHz), this PNP transistor 18dB/oct. active
filter also serves as an amplifier for the low-pass
region, and as a crosstalk canceller.