Power Indicator Circuit; Protection Circuit; Overload Detector; Dc Voltage Detector - Pioneer sx-d5000 Service Manual

4 . 1 0 P O W E R I N D I C A T O R C I R C U I T
The SX-D5000 output power indicators feature
fluorescent indicator tube (FL tube). In this tube,
thermionic emissions from the cathode are ac-
celerated into the fluorescent substance of the
segmental anodes, resulting in the emission of light.
This tube is used to indicate numerals, letters, and
other symbols.
An outline of the FL tube drive circuit is shown
in Fig. 4-22. The output circuit signal is applied to
pin no.6 (4) of the IC (TA7318P-A). The IC con-
tains a detector circuit, compressor (40dB), and
peak hold circuit for both left and right channels.
The dynamic range of the signal is thus contracted
by 40dB to obtain a "peak held" DC voltage.
The output power indicator segments of the FL
tube are driven by the HA12010 ICs (one for each
channel) equipped with L2 pairs of DC com-
parator. These amplifiers are biased at increasing
levels, so each amplifier will commence to operate
separately as the input level increases. And since
these amplifiers apply the voltages to the output
power indicator segments, each successive segment
will light up in turn as the input level rises.
4 . 1 1 P R O T E C T I O N C I R C U I T
The purpose of this circuit is to protect the
speakers and the power amplifiers. The relay in the
output circuit is automatically opened in any of
the following cases:
1. During the *transient operations" when the
power supply is turned on and off.
2. Upon detection of an overload, caused by a
short circuit in the load.
3. Upon detection of a DC voltage in the output
caused by component failure or accident.
Muting Operation when Power Supply is Turned
On and Off
With reference to Fig. 4-23 when the power
supply is turned on, QB remains off due to +82
(The time constant of the +82 circuit isvery small).
If there is no input (DC) on Q5 and Q6, they will
be off, and the timing capacitor C1 charges up
through R8 and R6, and thus Q4 turns on. When
Q4 conducts, the relay operates, and the output
muting on the power amplifier will be removed.
When the power supply is turned off, +B2 will
abruptly decay, and Q3 will conduct owing to the
residual component of +B1. As a result, C1 will
rapidly discharge, Q4 will cease to conduct, where-
upon the relay will become de-energized and
restore muting.
Overload Detector
The overload detector circuit incorporates the
load (RL) in one side of a Wheatstone bridge. The
base and emitter of a sensing transistor (Q1) are
connected to the opposite corners of the bridge,
so if RL decreases, Q1 will become forward biased.
If RL falls below a prescribed value, Q1 will turn
on, and Q3 will turn on, thereby C1 will rapidly
discharge. As consequence, Q4 will turn on and the
relay will become de-energized, thus causing the
output circuit to open.
DC Voltage Detector
The output circuit is connected to the Q6 emit-
ter and Q5 base via a low-pass filter (R9, C2).
Any DC voltages appearing the output circuit of
the power amplifier, it will be applied to the Q6
emitter and Qb base. If the voltage is positive, Q5
turns on. C1 will rapidly discharge. If the voltage
is negative, Q6 turns on. C1 will rapidly discharge.
As consequence, Q4 will turn on and the relay will
become de-energized, thus causing the output
circuit to open.
Protection Circuit
F i g . 4 - 2 2 P o w e r I n d i c a t o r C i r c u i t
Fis.4-23