The Y-FM output signal from the FM modulator takes one of two paths depending on whether
the machine is in the monochrome or color mode of operation. In the monochrome mode, the luminance
signal is applied to pin 3 of IC-6. In the color mode, the signal then goes through a 1 MHz trap, C51 and
L12, which is used to provide space for the sidebands of the down-converted color signal. After going
through high pass filter C52 and R85, the signal is applied to pin 5 of IC-6.
IC-6, an electronic switch, will select either the pin 3 or pin 5 signal depending on the mode
selected (AUTO or B/W) and the type of input video signal being recorded (monochrome of full NTSC
color). The power supply for IC-6, at pin 4, is provided only in the RECORD or EE operating mode.
The output at pin 1, is therefore muted in all other operating modes. The output of IC-6 takes two paths
to the Y record amplifier on the RA-2 board and to the PS-ID board for Y demodulation feeding video
Learning Event 6:
FUNDAMENTALS OF Y RECORD AMPLIFIER CIRCUIT (RA-2 BOARD) AND CHROMA
SIGNAL RECORD SYSTEM
Refer to Figure 3-4 (fo) at end of lessons. The Y-FM signal from the output of the MD-4 board
is applied to the Y record amplifier circuit on the RA-2 board. The Y record amplifier is discrete,
consisting of transistors Q6 through Q12. Pot R22, a gain control at the Y record amplifier input,
adjusts the record current level. Transistor Q7 is a muting switch that is driven by the Blanking switcher
output. Pot R32 is used to adjust the frequency response of the Y record amplifier input, adjust the
record current level. Transistor Q7 is a muting switch that is driven by the Blanking switcher output.
There is only one Y record amplifier for both video heads. During recording, the two video heads are
series-connected to the secondary of transformer T1, the record amplifier output transformer. The
chroma record signal at 688 KHz is mixed with the Y-FM record signal in the primary of T1.
Almost all the chroma signal record circuitry is connected within IC-5 on the MD-4 board. The
full NTSC color video signal, selected by electronic input selector switches IC-2 and IC-1, is applied to
a 3.58 MHz bandpass filter. The filter consisting of L13, C58, and T1 has a 1 MHz wide passband
centered on 3.58 MHz. Only the chroma signal passes through the filter; the luminance signal
components are rejected.
After separation from the luminance signal, the chroma signal is applied at IC-5 at pin 13 where
it passes through the ACC amplifier. The ACC amplifier is an automatic gain-controlled amplifier
which performs the same function for the color signal as does the Y-AGC amplifier circuits as described
previously. The ACC amplifier is controlled by a DC control signal derived by sampling and detecting
the amplitude of the color burst in the output signal from the ACC amplifier.