with the off tape 688 KHz signal. R184 sets the level of the 4.27 MHz carrier. A differential amplifier,
Q28 and Q27, filters and tunes the 4.27 MHz. T4 should be tuned to the maximum output from the
amplifier at 4.27 MHz.
9.
The amplitude of the chroma subcarrier during burst is one half of the chroma level during the
video portion of the recorded signal. Thus, the 688 KHz color under subcarriers amplitude will drop
during the burst period. The amplitude of the 4.27 MHz signal should also drop to ensure field to field
uniformity of the 3.58 MHz chroma subcarrier. The 4,27 MHz carrier gate, Q53 and the voltage divider
capacitors, C170 and C79, operate to prevent the difference in amplitude of the 4.27 MHz signal and the
688 KHz signal during burst. A pulse from sync inverter, Q49, turns Q53 on during the sync interval.
C170, part of a 50-50 voltage divider (C170 and C79), discharges to ground through Q53. At this point,
one-half of the 4.27 MHz signal goes through C170 to ground and the other half arrives coincident with
the incoming 688 KHz signal at the frequency converter in IC-2. Correct phase and amplitude
relationship will be maintained during this interval. R80, at the input to the frequency converter, may be
adjusted to maintain the correct level of 688 KHz chroma. Noise is kept to a minimum by the voltage
divider gate and the adjustment of R80 if the incoming burst varies greatly in level.
10.
Burst samples do not exist for nine lines during the vertical blanking. The phase detector will
have no reference during this interval and the 688 KHz VCO will try to move off frequency. To prevent
this from happening, Q30 and Q31 are turned on during the vertical blanking interval. Capacitors C69
and C71 are charged through the transistor to ground. The time constants of these two RC networks at
TP-16 and TP-18 are such that the networks will remember the last charge throughout the blanking
interval. During vertical sync, the difference between TP-18 and TP-16 will remain stable and the VCO
will remain on frequency. At line 10 of the video signal, Q30 and Q31 will be turned off and once again
the phase difference between the 3.58 MHz reference oscillator and the generated 3.58 MHz signal will
drive the 688 KHz VCO.
11.
Another means of controlling this oscillator is to turn the color lock control at the rear panel of
the machine. This varies the DC bias reference level to the 688 KHz voltage-controlled oscillator.
R126 in series with the color lock control is a calibration potentiometer. It should be set when the color
lock control is in its center detent position.
Learning Event 8:
FUNCTIONS OF THE AUTOMATIC FREQUENCY CONTROL CIRCUIT
1.
The Automatic Frequency Control (AFC) circuit allows the APC circuit to track during the
forward 1/20th, stop, and reverse 1/20th and reverse X1 modes. The sync separator triggers a
multivibrator in IC-7 which produces a square wave having the same period as horizontal sync. This is
the square wave gates transistor Q50. The output of this transistor is tuned by T8 and produces a sine
wave at the horizontal rate of TP-25. This sine wave rings a high Q phase center tank, T9, C161, and
C162. The phase difference between the incoming generated horizontal sync and the frequency of the
tuner, T9, is seen as a DC level at the input to the AFC error buffer IC-7, pin 1. This
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