Pioneer SA-708 Service Manual page 8

Muting Operation When Power Supply is Turned
On and Off
With reference to Fig. 4-2 when the power sup-
ply is turned on, Q, turns off due to +B, and +B,.
If there is no input (DC) on Q, and Q,, they will
be off, and the timing capacitor C, charges up
through R3; and D3; and thus Q; turns on. When
Q3 conducts, the relay operates, and the output
muting on the power amplifier will be removed.
When the power supply is turned off, +B, will
abruptly decay, and Q, will conduct owing to the
residual component of +B,. As a result, C, will
rapidly discharge, Q3 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 (see
Fig. 4-3). The base and emitter of a sensing transis-
tor (Q, ) are connected to the opposite comers of
the bridge, so if RL decreases, Q, will become
forward biased. If RL falls below a prescribed
value, Q, will turn on, thereby passing a current
through R, and D,. Due to the voltage difference
generated across R3;, Q, will become forward
biased, and consequently turn on. C, will rapidly
discharge. As consequence, Q; 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 Q; emit-
ter and Q, base via a low-pass filter (R,, C3).
Any DC voltages appearing at the output circuit of
the power amplifier, will be applied to the Q,
emitter and the Q, base. If the voltage is negative,
Q, turns on. Due to the voltage difference generat-
ed across R3, Q, will become forward biased, and
consequently turn on. C, will rapidly discharge.
If the voltage is positive, Q, and Q, turns on. C,
will rapidly discharge. As consequence, Q;3 will
turn on and the relay will become de-energized,
thus causing the output circuit to open.
® Overload detector ®DC voltage detector
©Power OFF detector
Fig. 4-2 Protection circuit
Fig. 4-3 Overload detector