supply regulator which reduces the -12V supply voltage to -6V for IC-1. Transistor Q4 provides muting
for the preamp during the REC/EE mode to prevent distortion from entering the playback circuits.
The RF signal from the preamplifier is applied across Y-RF playback level Pot R20. The output
from R27 is in turn applied to pin 3 of IC-2 across a trap formed by C24 and L8. The trap removes
audio bias interference during the AUDIO INSERT mode. Electronic switch IC-2 selects between
playback RF and RECORD/E-E RF, supplying the selected signal to the succeeding stages. IC-2 is
controlled by the VIDEO REC/EE 12V line, which goes high in the RECORD/E-E mode. When the
line is high, IC-2 supplies the RECORD/E-E RF signals as output.
Learning Event 2:
FUNDAMENTALS OF THE TRACKING METER AND DROPOUT COMPENSATOR CIRCUITS
Refer to Figure 3-4 (fo). The PB RF signal at TP-6 is supplied to PB Y-RF AMF Q10 and buffer
Q11. Q10 and Q11 provide an off-tape RF output that can be used by a Time Base Corrector for
dropout compensation. The signal is also supplied to another PB Y-RF AMP, Q5, and to the tracking
meter driver, Q6. The RF signal is than rectified by diodes D3 and D4 so that the tracking meter reads
the average luminance level.
In the color mode, the Y-RF signal at TP-6 passes through a high pass filter consisting of C29,
C30, and L10. The high pass filter eliminates the 688 KHz chroma signal to prevent interference with
the luminance. Phase lag introduced by this filter is compensated by a phase equalizer circuit, C28 and
L9. In monochrome operation, this filter is bypassed for improved picture resolution. The signal path
selection is made by an electronic switch inside IC-1.
The Y-RF signal selected by the color monochrome RF switch, passes through the RF amplifier
stage and is applied to the inputs of two limiter stages, all within IC-1. The limiter in the main route
provides about 20db of limiting to remove AM components from the RF signal. The output of this
limiter goes to the normally closed input of an electronic switch. The output of the other limiter is
applied to a dropout detector stage. The dropout detector has two outputs, one of which actuates the
dropout compensator electronic switch, and the other actuates the switching transistor Q8. The
sensitivity of the dropout detector is controlled by the setting of pot R56. When a dropout is detected,
Q8 is switched by the output signal from the dropout detector. This keeps the dropout detector switches
on until the dropout ends.
Normally, when there is no dropout, the Y-RF signal passes through the normally closed side of
the dropout compensator electronic switch. This signal goes through an RF output amplifier and passes
out IC-1 at pin 4. This signal is buffered by Y-RF OUT BUFFER Q7, which supplies the signal as Y-
RF OUT. The output from Q7 also feeds the input for DL1, a 1H delay line. DL1 delays the Y-RF
signal by an amount equal to the period of one horizontal line (63.5 microseconds). The delayed Y-RF
signal is applied to the normally open side of the dropout compensator electronic switch at pin 6 of IC-1.