Onkyo TX-2500 Service Manual page 4

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2. Quartz Locked Circuit
The IF component of the quadrature detector output of Q 102 and the quartz oscillation signal at 10.7 MHz of
Q 119 are detected with the cycle of 19 KHz, and the variance of the detected output is amplified with the DC
voltage being imparted to the variable capacitance diode at the front end. In this way, the local oscillation frequency
is regulated.
(Circuit Performance)
Q 123 is set to ON-OFF motions with the rectangular waves at 19 KHz inducted from No. 10 terminal of Q 103
of MPX IC at the cycle of 19 KHz.
In case Q 123 is set to ON position, Q 118 and 124 are turned OFF, and the IF signal is amplified by Q 116, detected
at T 104 and 105 and added to No. 2 terminal of Q 117.
Q 122, on the other hand, is turned OFF as Q 123 is set to ON position, with Q 121 and 125 being also turned
ON to cause the quartz oscillation signal (10.7 MHz) to precipitate to the earth. Conversely, when the Q 123 is
turned OFF, the switching transistors (Q 118, 121 — 125) perform functions in complete reversal to the above,
with the quartz oscillation signal amplified by Q 116, detected in turn at T 104 and 105, added to No. 3 terminal
of Q 117, resulting in the fall of the IF component to the earth.
The preceding performance repeats itself at 19 KHz, amplifying the input variance (between the IF signal and the
quartz oscillation signal) with the OP amplifier of Q 117, which is then caused to pass the low-pass filter, to be
imparted to the variable capacitance diode. In this manner, the local oscillation frequency is regulated.
The deviation from center of the detector transformers T 104 and T 105 will be registered alike with both transformers
and will have no relationship whatsoever with the AFC input.
fig—3
3. Tuning Meter Circuit
The IF signal and the quartz signal detected at the 19 KHz cycle are added to the bases | and 2 of Q 127. Since the
voltage at the bases | and 2 of Q 127 are equal to each other at the time of tuning, the voltage at the bases of Q 128
and 129 (OV) will also be equivalent to each other, thereby causing the T-meter to point to the center.
In the event the IF frequency is on a higher level than the quartz frequency, the T-meter is caused to swing to the
left as the voltage at the base 1 of Q 127 is higher in this case than that at the base 2, and at the same time, same at
the base of Q 128 is registered at a lower level than at the base of Q 129. Incidentally, R 257 and R 254 are the
semi-fixed volume designed to regulate the T-meter center and the breadth of the same, respectively. In addition,
Q 130 is designed to avert any error in performance by shortcircuiting the T-meter at the time of detuning.
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