Learning Event 7:
FUNDAMENTALS OF PLAYBACK ACC AND ACK CIRCUITS
In the chroma demodulation process, once the 688 KHz chroma signal passes through the low
pass filter at the base of Q11, the signal is amplified by Q12. Transistor Q14 controls the level of
chrominance signal during slow, (reverse and pinch roller) off modes of operation. The gain of the
chrominance signal is increased as the SLOW/REV/PINCH OFF 5 VDC is seen at the base of Q14.
When the collector of Q14 goes to ground, the gain of transistor Q12 increases. This ensures that the
amplitude of the chroma signal remains essentially the same as the machine changes speed through
FORWARD, 1/20th speed, REVERSE X1, and REVERSE 1/20th speed. Also when the machine is in
the PAUSE mode, and the pinch roller is off the tape, the track angle will change. The amplitude of the
recovered 688 KHz signal will decrease and the gain at TP-2 will increase. This will happen in all
modes except FORWARD X1. Q14 offsets the effect of this change.
Q12 amplified the 688 KHz signal which is then shaped by a phase equalizer, C30, L8, C31, L9,
and C32. This filter removes amplitude and phase errors that may still be present in the signal. Q13
buffers the signal and is then further shaped by transformer T2, a tank circuit tuned by C10 and C34.
IC-2 is the playback Automatic Chroma Control amplifier. It controls the gain of the chroma line by
line and field by field. This circuit takes care of head-to-tape contact problems caused as the tape moves
around the video head drum. These errors occur at the horizontal rate, thus line-by-line correction is
necessary. Field-by-field amplitude errors occur when the record amplifiers do not have the same gain
or when the playback head balance is not matched. Remember, the chroma information is amplitude
modulation and is recorded directly on the tape at 688 KHz. Therefore, if any field-to-field or head-to-
head differences did exist, they would be seen on a color monitor as flicker and would disappear when
the chroma was turned off.
IC-2 has six sections: (1) the Playback Automatic Chroma Control amplifier, (2) a Frequency
Converter, (3) a Chroma Amplifier, (4) the Automatic Chroma Control, (5) Automatic Chroma Control
Detector, and (6) the ACC DC Amplifier.
After the signal has been phase-equalized by T2, section 1 of IC-2 amplifies the signal. After the
signal leaves the ACC amplifier, if the machine is in the Dub mode, there will be no need for a 3.58
MHz subcarrier output and the signal will pass directly to Q21. R96 adjusts the level of the 688 KHz.
The second section of IC-2 contains a Frequency Converter. In this section, the 688 KHz signal is
heterodyned with 4.27 MHz. Band pass filter FL3 allows the passage of only the difference frequency.
The third section of IC-2, a chroma amplifier, tow amplifies the 3.58 MHz chroma signal. R86
has been labeled the playback color killer because it determines the output level of the chroma amplifier.
Section four of IC-2 contains a chroma amplifier and burst gate. The 3.58 MHz chroma signal is
amplified again. The burst gate extracts only the burst portion of the 3.58 MHz chroma signal. This
chroma information will then ring transformer T3 which is tuned to 3.58 MHz. The level of the signal,
field to field or head