Pioneer SX-1250 Service Manual page 18

Qb operates in the negative half cycle and Qa
becomes cut off. The signal flows is indicated by
the broken line arrows in the center of Fig. 20.
Q1 is biased by the potential difference between
point C and point E. If RL is extremely small, the
point C potential becomes considerably higher
than that of point E. Q1 tums ON and current
flows in D3.
If large current flows in Qa and Qb, Q1 becomes
ON due to the RE1 and RE2 voltage drops, and
current flows in D3. Cl prevents faulty operation
due to external noise.
lar
Fig. 20. Basic circuitrv for overload detector
3. Center Point Potential Detector Circuit
If a DC potential is produced at the junction point
of the power amplifier, a command is sent to the
relay drive circuit. Fig. 21 shows this operating
principle.
Q3 and Q4 compose a differential amplifier. When
the same input is applied to both input terminals
(Q3 and Q4 bases), no output is present. However,
if there is a difference between the terminal inputs,
the difference is amplified and becomes the output
between the two collectors. During normal opera-
tion, an AC signal only is present at the junction
point. As C2 reactance is sufficiently low, the same
signal is applied to Qg and Q4 bases, resulting in an
absence of output at the collector sides.
When a DC potential is produced at the junction
point, it becomes the input of Q3 only. If the
voltage is negative, Q3 collector current declines
And at Q4 the collector current increases and the
potential drops, causing current to flow through
D4,
If the DC voltage is positive, Q3 collector cunent
increases and the potential drops, while at e4 the
collector curent decreases and the potential rises.
Current therefore flows through D5.
Fig. 21. Basic circuitry for center point
potential detector
4 . 4 P O W E R S U P P L Y C I R C U T T
AII power supplied are regulated up to the power
amplifier predriver stage. Independent power trans-
former secondary windings are employed for left
and right channels in the power stage. Each channel
is also supplied from an independent bridge recti-
fier and two 22,OOOpF capacitors. Position lamps
are lighted by DC from a regulated power supply
for stable illumination.
Inrush Current Suppressor Circuit
Ordinarily, power transformer inrush cunent when
the power supply is tumed on is opposed by DC
resistance of the winding and instantaneous induct-
ance of the air core. In the toroidal power trans
former used in the SX-1250, both these factors are
very low. Combined with the high charging cunent
of the four 22,OOO1F capacitors in the secondary,
inrush current can reach several hundred amperes.
A resistors is therefore provided in series with the
primary coil to suppress ttris inrush current to
sevetal tens of amperes. This resistor becomes
shorted after power tlansformer excitation.
Ihe inrush suppressor circuit is shown in Fig. 22.
51 is the POWER switch and S2 a relay contact.
The Microtemp is a temperature responding fuse.
52 is open before power is turned on, connecting
the power transformer primary coil in series with
R1 and the Microtemp. After power has been
turned on, the relay operates when DC current is
obtained from the rectifier, closing 52. If 32 fails
to close, due to power supply or relay malfunction,
R1 heating opens the Microtemp (109'C operating
temperature), thus opening the primary circuit.
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