Anti-Birdy Filter; Fm Multiplex Decoder; Fm Control Circuit; Af Amplifier And Muting - Pioneer TX-9500II Service Manual

Anti-birdy Filter
The function of this filter is to remove any com-
ponent signals belonging to
adjacent stations
within 200kHz of the tuned station which are
still contained in the signal after detection. If it
were not for this filter, beating in resonance with
the 10th and 11th harmonics of the 19kHz pilot
signal generated in the MPX IC of the final stage
would occur, and appear as noise in the reproduced
signal.
FM Multiplex Decoder
This section includes another IC recently devel-
oped by Pioneer, the PA1001. See Fig. 5 for the
block diagram.
This IC, an improvement over a previous IC
(HA1196),
incorporates a pilot
auto-canceller
circuit. This circuit filters out the leak canier
(19kHz) contained in the demodulated signal
without any loss in the frequency characteristics
of the audio signal. This filtering action is a con-
siderable improvement over the previous low pass
filter method. Other features include a wider
input dynamic range, and the ability to perform
well even when a 3O0Vo modulated input signal is
applied to the IC. S/N ratio and distortion factor
have also been improved by the incorporation
of a NFB circuit in the decoder amplifier. Distor-
tion has also been improved in the case of MONO
signals by connecting together the output circuits
of the L and R channels, and thus cancelling the
out-of-phase distortion components contained in
the L and R channel audio signals.
Fig. 5 Block diagram of PA1001
AF Amplifier and Muting
This circuit employs the Pioneer developed IC
(PA1002). The block diagram is shown in Fig. 6.
This AF amplifier is a direct coupled amplifier
used as a differential NFB amplifier providing
TX-s'E'clOtr
outstanding dynamic range, S/N ratio, and distor-
tion factor. De-emphasis is provided by inserting
a time constant circuit in the NFB loop of the AF
amplifier. The elements in this time constant
circuit include a metal film resistor (lVo tolerance)
and a polystyrene film capacitor (27o tolerance)
which have made it possible to reduce de-emphasis
deviation to within +0.2d8 to -0.5dB in the 20Hz
to 15kHz range.
The switching circuit for the muting function is
activated when a DC voltage from the power
ON/OFF detector section or from terminal no.8,
is received. The signal circuit is then connected to
gtound, thus preventing the output of any signals.
This power ON/OFF detector circuit detects the
instantaneous change when the POWER switch
is turned ON or OFF, sending a trigger signal to
the switching circuit, and thus activating the
muting function. As a result, the popping noise
usually generated when the POWER switch is
turned ON or OFF, plus the residual noise, is
eliminated.
F i g . 6 B l o c k d i a g r a m o f P A 1 0 0 2
FM Control Circuit
The FM control circuit used to control the meter
drive and FM muting circuits, is independent of the
audio signal circuitry. This circuit, too, incorpo-
rates an IC (PA3001) recently developed by
Pioneer. The block diagram is shown in Fig. 8.
The PA3001 is very similar to the previous IC
(HA1137) in function and operation. It differs in
that the FM muting level can be controlled extemal
to the IC. The input signal is supplied by the IF
amplifier in NARROW band (See Fig. 7).
The FM muting function in the TX-9500II works
in 5 different ways:
r
-_l
+ l
our
R
T O S T E R E O i n d
1 3