Pioneer SX-3700 Service Manual page 10

The DC voltage on pin no.4 of PA300?-A de-
scribes an S curve when tuning to and away from
a particular broadcasting frequency, the voltage on
pin no.2 serving as the reference level. This DC
voltage is amplified by the differential amplifier
(Q19 & Q20) and then applied to a poladty detec-
tors switch circuit (Q16 & Q26).
SCHMiTT
Fig. 4-7 TUNING indicator driver circuit
When tuning to a frequency from the high fre-
quency side (or tuning away from the frequency
to a higher frequency), the voltage on pin no.4 will
be higher than that on pin no.2. The Q20 collector
voltage will thus be lowered and the Q19 collector
voltage raised, resulting in Q26 being turned on,
and the higher frequency (right hand side) detun-
ing direction indicator also being turned on. When,
on the other hand, the broadcasting frequency is
approached from the low frequency side (or when
tuning away to a lower frequency) the pin no.4
voltage will be lower, resulting in Q16 being turned
on to light up the lower frequency (left hand side)
detuning direction indicator. When either Q16 or
Q26 is on, the Q14 base voltage will be high,
resulting in Q14 being turned on and Q18 turned
off, which means that the center tuning indicator
will not be lit up.
Once the broadcasting frequency has been tuned
properly, the voltages on pin nos.2 & 4 will be
equal. Consequently, Q16 and Q26 will both be
turned off, which means that neither of the detun-
ing direction indicators will be on in this case. And
since Q14 is turned off because of the decreased
base voltage, Q18 will be turned on, and the center
tuning indicator light up.
Furthermore, C16 is
charged up via R84, resulting in Q23 being tumed
on, thereby lighting up the Quartz Locked indi-
cator LED.
SIGNAL Indicator Circuit
The SX-3700 SIGNAL indicator consists of an
FL tube 5-point indicator display. The signal meter
drive signal obtained from the FM IF system IC
(P43007-A) and AM tuner is first amplified and
then applied to the indicator drive IC (HA12010).
This IC contains 12 pairs of voltage comparators
similar to those employed in the power indicator
circuit, 5 of these pairs being used to drive the
SIGNAL indicator.
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