The processed high and low noise signal is out of phase with noise in the unprocessed signal so they
cancel each other out. Thus, if this part of the circuit is misaligned or has a problem, the picture will
lose resolution and appear to be smeared.
3.
The output of Q5 is buffered by Q6 and this drives a complimentary pair of amplifiers which
feed the luminance signal to pin 2 of the 8-pin TV connector and the video out connector.
Learning Event 6:
FUNDAMENTALS OF THE CHROMA DEMODULATOR (PS-10)
1.
Refer to Figure 4-5 (fo) at end of lessons. The RF signal is picked up as the video heads scan the
tape field to field, and is amplified on the PS-10 board by Q1, Q2, and IC-1, respectively. R21 across
the output of IC-1 controls the field to field chroma balance. The chroma output of IC-1 passes through
L6 and C18, low-pass filter. Adjusting R26 sets the overall playback level of the chroma signal. The
RF signal from R26 is applied across a second trap formed by C20 and L7. This is an audio bias trap
designed to prevent the audio bias signal from interfering with the 688 KHz chroma signal in the Audio
Insert mode. The chroma playback signal is applied to pin 3 of IC-3 on the PS-10 board, a type CX130
electronic switch. The other input to IC-3 at pin 5 is the E to E CHROMA signal supplied from the MD-
4 board. IC-3 is switched by the video REC/EE 12V line and supplies playback RF signal in the
playback mode and E-E chroma at 688 KHz in the Record and E to E modes. The output signal is
supplied to the DM-7 board.
2.
The output signal from the PS-10 board is supplied to a low pass LC filter that removes all
frequencies above the chroma signal frequency. The signal thus filtered is buffered by Q11 and splits
two ways; one path feeds the time code track detector circuit and the other feeds chroma amplifier Q12.
3.
As stated in the description of the luminance demodulator, the video heads will see the time code
track, if present on the tape, as 200 KHz information. This signal must be removed from the video
circuits because it may create instabilities in the Automatic Color Control and Automatic Phase Control
circuits, as well as produce video hash and chroma flicker during vertical blanking. Thus, a 200 KHz
low-pass filter at the output of Q11, draws the 200 KHz time code signal away from the 688 KHz
amplifier Q12. This signal is then amplified (Q41 and Q42), shaped (Q43 and Q44), and inverted
(Q45), before being fed to IC-6. IC-6 contains two flip-flops whose time delays differ from each other.
Also, feeding IC-6 at pin 1 is a Vertical Blanking pulse from the servo section. The flip-flops in IC-6
are tied in such a way that pin 13, at the output, will go high during vertical blanking whenever time
code is present. This high is seen at the base of Q15 which turns it on and the output of the chroma
amplifier, Q12, will go to ground. Q10 is also turned on taking the luminance signal to ground. There
should be no chroma flicker during vertical blanking whenever a time code is present.
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