Onkyo TX-3000 - CIRCUIT DESCRIPTION; Touch Sensor Operation; Servo Locked Circuit Switching; Power;Signal Strength Meter Function

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CIRCUIT DESCRIPTION
1. Touch sensor
The purpose of this circuit is to operate both the power/signal strength meter and servo locked circuit switching
transistors.
1.1 Servo locked circuit switching circuit
In order to ensure accurate tuning, the servo locked circuit is turned off automatically once the tuning knob is
touched, and also when the muting circuit is switched off.
When a station is turned, Q702 will tum off and Q708 turn on (since Q705 will already be off and Q706 on), result-
ing in the TUNED lamp turning on. And since Q707 will turn off when Q708 turns on, the servo locked circuit will
also begin to operate.
When the tuning knob is touched, a certain amount of hum is induced.
This hum is amplified by Q701, rectified by D701 and D702 into a DC signal, and applied to Q702 is consequently
turned on, resulting in the servo locked circuit being switched off. If, however, the hum level is rather low, the tuned
lamp might not turn on even when the tuning knob is touched. If this happens, reset the back panel sensor switch
to either the Normal or High position.
1.2 Power/signal strength meter
When the tuning knob is not being operated, this meter displays the level of power applied to be right speaker
rystem. The instant the tuning knob is touched, the meter changes to display the signal strength of the radio
broadcast.
When the tuning knob is touched, Q702 tums on. And since Q703 and Q704 turn off and Q751 and Q752 turn on,
and power/signal strength meter is changed to signal strength meter from power meter.
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2. Protection circuit
The protection circuit is operated:
(1) When the B circuit is unstable when the power is turned ON (approximately 5 seconds)
(2) When the center voltage has increased because of trouble at the differential amplifier, etc.
When the voltage detector is operated by abnormal voltage Q904 is tumed ON by the voltage drop across R918. Q904,
Q903 constitute a digitalized, fast response Schmitt trigger circuit. When Q904 is turned ON, Q903 is tured OFF.
Q903 is a relay drive transistor. When it is turned OFF, the relay is also tumed OFF.
When the power switch is turned ON, charging current flows thru the loop R916 > C921 > R917 > R918 and Q904 is
tured ON by the voltage drop across R918. Consequently, Q903 and the relay are turned OFF until the charging
current drops below a certain value. When the power switch is turned OFF, the B voltage falls and C921 is quickly dis-
charged thru the loop R916 > C921 > D909. During normal operation, C921 is charged to almost the B voltage. But
since the saturation resistance of Q904 is sufficiently low, when Q904 is turned ON, C921 is quickly discharged thru
the loop C921 > R917 > Q904 and the relay is also turned OFF. The relay is not tumed ON again thereafter until C921
is charged, even if the set should return to normal and Q903 is turned OFF.
3. Muting Circuit
The Quadrature detector IC incorporates an IF level detector circuit (output at pin 12). If the IF signal level drops below
the muting level, pin 12 will be switched to high level, turning Q202 on. Consequently, the detector output signal will be
cut off before it can be applied to the multiplex IC. When, on the other hand, the IF signal level is higher than the
muting level, the Q102 pin 12 will be switched to low level, turning Q202, and Q705 off. Q706 wil! therefore tum on,
followed by the LOCKED lamp turning on.
4. Servo Locked Circuit ,
The DC potential difference across both ends of the tuning meter (corresponding to the DC portion of the ratio detector
output) is amplified by the Q701 operational amplifier in order to increase the AFC control capacity.