4.
The burst signal is separated from the rest of the chroma signal by a burst gate also located inside
IC-5. The burst gate is a switching circuit that is actuated by a burst flag pulse supplied from the
luminance channel. The burst flag pulses, which are applied to the burst gate at pin 10 of IC-5, unlock
the burst gate only during the duration of the pulse. Only the burst portion of the color signal is
therefore supplied as output from the burst gate at pin 8 of IC-5.
5.
The gated burst signal is supplied to a crystal ringing filter consisting of transformer T2 and
crystal X1. T2 adjusts the filter so that it is resonant at the 3.58 MHz color frequency. The successive
string of burst signals occurring every 63.5 microseconds causes the circuit to ring and oscillation is
sustained between the bursts due to the circuit resonance. The output signal from the crystal filter circuit
is therefore a continuous wave 3.58 MHz signal, the amplitude of which follows the amplitude of the
burst signal.
6.
The 3.58 MHz continuous wave (CW) signal, with amplitude proportional to burst amplitude, is
fed back into IC5 at pin 6. This signal is amplified in a 3.58 MHz CW amplifier and supplied to a
detector. The detector produces a DC output signal proportional to burst amplitude and it is filtered by
C67 and supplied to a DC amplifier inside the IC. The gain of the DC amplifier is adjusted by R97
which controls the ACC settings. The output from this DC amplifier is the control signal that controls
the gain of the ACC amplifier.
7.
The DC amplifier output signal also controls the Automatic Color Killer operation, which is an
electronic switch inside IC-5. Whenever the incoming burst signal drops more than 23 db below its
normal level, which the killer actuates, it places the system in the monochrome mode. When the
automatic color killer actuates, the frequency converter which down-converts the chroma signal to 688
KHz is turned off. The electronic switch inside IC-4, which selects the Y signal filter path, is also
switched to the monochrome mode.
Learning Event 7:
CHROMA SIGNAL FREQUENCY DOWN-CONVERTER CIRCUIT AND CHROMA RECORD
AMPLIFIER CIRCUIT
1.
The color signal amplitude regulated by the ACC amplifier in IC-5, is down-converted to 688
KHz from 3.58 MHz. The frequency down converter consists of a balanced modulator and a low pass
filter. The balance modulator, also located inside IC-5, modulates a 4.27 MHz CW signal with the 3.58
MHz color signal. The 4.27 MHz signal is supplied from a crystal-controlled local oscillator. The
output signal from the balanced modulator, at 4.27 MHz 3.58 MHz, is supplied to the low pass filter,
consisting of C77-80, and L15 and L16 from IC-5 at pin 15. Only the low frequency components, 4.27
MHz minus 3.58 MHz = 688 KHz, pass through the low pass filter. The down converter chroma signal
at 688 KHz is supplied to delay line DL2 which maintains the proper timing relationship between the
chroma and luminance signals.
2.
The delayed chroma signal from DL2 is fed to pin 3 of IC-7, an electronic switch. IC-7 will
select either the line/TV 688 KHz chroma at pin 3 or the
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