to head, is adjusted by a DC level generated by the PB ACC Field Memory. A field pulse from the
servo section is inverted by transistor Q16; the same pulse is also seen at the base of Q18. Inverter Q16
drives Q17 so that when Q18 is off, Q17 is on, and vice versa. Capacitors C38 and C39 will alternately
charge and discharge accordingly. The DC level produced should be continuous and equal field to field.
The ACC Detector, section 5 of IC-2, detects the presence of chroma and when there is no chroma
present, takes the output of Q18 and Q17 to ground. When a chroma signal is present, a DC bias is
maintained by the detector. In the pause mode, Q18 and Q17 remain locked to the state which existed
before movement of the tape path is stopped. The last field pulse will set the field memory until another
field pulse changes the state of this transistor flip-flop. The chroma signal response of the coil will
remain as it was when the pause mode was initiated. The output of T3 is detected by transistor Q19, an
emitter follower. The DC output of T3, when chroma is present, will be 5.5 VDC. Q19 will be turned
on and 5.5 V will be seen at TP-3. The color switch in section 6 of IC-2 will be switched away from
ground. Thus, when chroma is present, TP-4 will be a plus 4 VDC and the collector of Q23, the 3.58
MHz chroma out buffer will not be grounded. In the B&W mode, the switch will be closed and 0 VDC
will be seen at TP-4.
6.
The ACC DC AMP amplified the DC output of Q19. This DC correction voltage is then fed
back to the PB ACC amplifier. The DC reference level of the incoming chroma is set by the level of the
previous field.
7.
To convert the 688 KHz color under subcarrier to 3.58 MHz cleanly and without jitter, the
demodulation system must be able to compensate for temperature changes and differences in chroma
signal produced when the linear tape speed varies. In addition, frequency phase errors that occur as a
function of head-to-tape contact and raw time base error must be removed. As in the modulator, the
demodulator will use the difference of two frequencies to produce the demodulated signal. As
mentioned previously, this will occur in section two of IC-2 and a 3.58 MHz signal will result when 4.27
MHz and 688 KHz chroma signal from the tape are heterodyned. The frequency and phase error in the
688 KHz incoming signal will not be present in the 3.58 MHz output of the demodulator. This is done
by giving identical phase variations to the 4.27 MHz signal as are present in the 688 KHz signal itself.
8.
After the 4.27 MHz signal is heterodyned with the 688 KHz recorded color signal, band pass
filter FL3 selects the 3.58 MHz difference frequency for amplification by the chroma amplifier in
section 3 of IC-2. This signal is again amplified in section 4 of IC-2. The output of this amplifier will
go to the burst gate described previously to generate the DC correction voltage and to pin 21 of IC-3.
When the 3.58 MHz signal arrives with the burst flag pulse generated by IC-7, the APC burst gate
allows the chroma signal to pass to the phase detector. The phase detector compares the converted 3.58
MHz signal with the output of a 3.58 MHz reference oscillator. The DC voltage generated by the phase
error, if any, speeds up or slows down a 688 KHz voltage controlled oscillator (VCO), causing the
output of the 688 KHz signal. This 688 KHz will then be heterodyned with the output of the 3.58 MHz
reference oscillator. The additive result of this mix will leave the frequency converter on IC-5, pass
through the ceramic filter, and be fed to the 3.58 converter on section 2 of IC-2. Thus, the 4.27 MHz
signal will have phase coincidence
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