4.
The separated 688 KHz chroma signal component is passed through an ACC circuit which
removes any signal amplitude variation between the two channels. The signal is then frequency
converted from 688 KHz back to 3.58 MHz. In the frequency conversion process, the phase instabilities
in the playback chroma signal caused by mechanical jitter are corrected by an APC loop. The playback
and chrominance are then mixed to provide a National Television Standards Committee (NTSC) output
signal.
Learning Event 2:
FUNDAMENTALS OF THE LUMINANCE SIGNAL RECORD SYSTEM (MD-4 BOARD)
1.
Refer to Figure 3-2 (fo) at end of lessons. The VCR has three video inputs. One input is a BNC
connector, another input is part of the 8-pin connector and the third input is part of the 7-pin DUB IN
connector. Selection between the three inputs is controlled by the INPUT SELECTOR switch located
on the control panel. The INPUT SELECTOR controls the switching of electronic switches, IC-2 and
IC-1, located on the MD-4 board. IC-2 has two inputs, the VIDEO IN signal from the BNC connector is
applied to pin 3 and the TV IN signal from the 8-pin connector goes to pin 5. The selected output is
supplied from IC-2 at pin 1. IC-2 selects between the two inputs by means of a DC control voltage
applied to pin 6. The DC control voltage is switched by the INPUT SELECTOR switch on the control
panel.
2.
IC-1 also has two inputs; the selected output of IC-2, either LINE or TV, is applied to pin 5, and
the DUB IN signal from the 7-pin connector goes to pin 2. The DUB IN signal will be luminance only
video signal from another VCR equipped with DUB IN/OUT connectors. The selected output is
supplied from IC-1 at pin 1. IC-1 selects between its two inputs again by means of a DC control voltage
applied at pin 6. The DC control voltage is switched by transistor gate Q201. Two conditions must
exist before IC-1 will select the DUB signal; the INPUT SELECTOR switch must be in the DUB
position, and there must be a DUB Y signal present at pin 1 of the 7-pin DUB IN connector.
3.
The output signal from IC-1 is supplied to three circuits: the Y-FM modulator and AGC circuit
on the MD-4 board, the chroma signal record process circuit on the MD-4 board, and the SYNC
SELECT circuit on the CS-8 board. Under certain conditions, the servo system will lock to the IC-1, pin
1, signal in both the record and playback modes.
Learning Event 3:
DESCRIBE THE FUNCTION OF Y-AGC CIRCUIT (MD-4 BOARD, RS-2 BOARD)
1.
Refer to Figures 3-2 (fo) and 3-3 (fo) at the end of lessons. All active elements of the Y-AGC
system are contained within IC-4 on the MD-4 board. The input video signal, selected by IC-1, is
applied to the AGC amplifier in IC-4 (1/4) at pin 13. The gain of the AGC amplifier is controlled by a
DC control voltage (also developed within (IC-4) so that its output level is constant). The AGC
amplifier's output signal at pin 15 of IC-4 (1/4) takes one of the two possible paths depending upon
whether the VCR is in the color or monochrome mode of operation. In monochrome it goes through a
low pass filter consisting
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