LESSON 5
FUNDAMENTALS OF THE AUDIO AND SERVO SYSTEMS OF A HELICAL SCAN VCR
TASK
Describe the operation and fundamentals of the audio and servo circuits of a helical scan VCR.
CONDITIONS
Given information and illustrations on the fundamentals and circuits of the audio and servo systems of a
helical scan VCR.
STANDARDS
Demonstrate competency of the task skill and knowledge by correctly responding to 80 percent of the
multiple-choice test questions covering the audio and servo systems of a helical scan VCR.
REFERENCE
None.
Learning Event 1:
FUNDAMENTALS OF THE SERVO SYSTEM AND SYNC SELECTOR CIRCUITS (DC-4, GH-1,
DS-6, PS-10, ED-7, and CS-8)
1.
Refer to Figures 3-1, 4-5, and 5-1 (fo) at end of lessons. The servo system consists of three
major circuits: (1) the 30 Hz reference signal generating circuit, (2) the drum servo circuit, and (3) the
capstan servo circuit.
2.
The sync selector on the CS-8 board selects the external reference, video, or external sync, from
which vertical drive will evolve. The source control for the electronic switch, IC-2, is the SYNC
SELECT on the front control panel of the machine. In the VIDEO position, a +12 VDC signal disables
transistor switch, Q16 and Q17. With no control voltage on pin 6, IC-2 remains in the NORM position
allowing Reference Video at pin 3 to pass through. When the switch is placed in the external position,
the +12 VDC signal is removed, allowing operation of transistor switch Q16 and Q17. The incoming
sync is applied to IC-2 pin 5 and transistor switch Q16 and Q17. The transistor switch turns on and
supplies control voltage to IC-2 pin 4, opening the switch. The external sync now passes through the
switch. The output of IC-2 is either external sync or reference video as determined by the SYNC
SELECT switch.
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