Circuit Descriptions; Fm Front End; T U N E R S E C T I O N - Pioneer SX-1280 Service Manual

6 . C I R C U I T D E S C R I P T I O N S
Refer to the block diagram on page 11 and the
schematic on page 43 for the circuit composition
of this unit.
6 . 1 T U N E R S E C T I O N
AM Tuner
The AM tuner employs a 2-gangvariable capaci-
tor, one IC (HA1197) and one AM ceramic filter.
Its block diagram is given in Fig. 1.
HA1L97 is an IC containing an RF amplifier,
converter, 2-stage IF amplifier, detector, and AGC
circuit, and features excellent AF frequency re-
sponse and distortion. Q1 of the output circuit is a
special AM muting circuit. This circuit is operated
until the AM tuner stabilizes immediately after the
FUNCTION AM switch has been set to ON. The
instant the AM switch is set to ON, +3 is supplied
to Rr, thru Ce, and the base of Q, is forward
biased.
Consequently, Q, is tumed ON, ancl the AM
output signal is shorted to ground during the time
constant of Ce, R6.
FM Front End
The FM front end employs a 4-gang variable
capacitor in the tuning circuit, dual gate MOS
FETs at the RF amplifier and mixer, and a local
osoillator with J-FET buffer.
The FM front-end input is 75St unbalanced
single-tuned circuit. The RF amplifier is a dual
gate MOS FET. An M-coupled double-tuned cir-
cuit is inserted between stages. The dual gate FET
is an amplifying element suitable for RF circuits,
and features extremely stable amplification
be-
cause of its low NF (Noise Figure), high PG (Power
Gain) and low feedback capacitance.
The mixer is also a dual gate MOS FET. The
reception signal is applied to gate 1 and the loeal
oscillator signal is applied to gate 2. This circuit
C e r o m i c
f r l t e r
ANT
has low local oscillator signal injection power and
the strong reception signal has little effect on the
local oscillator.
The local oscillator is a modified Clapp circuit.
Its output signal is injected into the mixer thru a
J-FET buffer.
lF Amplifier
Four dual element ceramic filters are used as the
selection elements, and one IC (TA7302P) contain-
ing two differential amplifiers and one FM IF IC
(PA3001-4) are used as the amplification elements.
The TA7302P compensates for the filter inser-
tion loss, and also limits the amplitude of the l'M
signal.
The PA3001-A performs
IF amplification,
amplitude limiting and FM detection. It also
drives the TUNING and SIGNAL meters and con-
trols muting.
The block diagram of the PA3001-A is given in
Fig. 2.
FM Multiplex Decoder
The FM multiplex decoder is a PLL (Phase-
Locked-Loop) MPX IC (PA1001-A). Its block
diagram is given in Fig. 3.
F U N C T I O N
( A M s w i t c h )
+ B
O U T
S I G N A L m e t e r
Fig. 1 Block dragram of AM tuner
Fig. 2
Block diagram of pA3001-A
H A l l 9 7
M i x e r
D e l e c l o r
A G C l i n e
A G C A m p
1 3