LESSON 4
FUNDAMENTALS OF THE VCR PLAYBACK CIRCUITS
TASK
Describe the fundamentals of the VCR playback circuits.
CONDITIONS
Given information and illustrations about terms relating to the fundamentals of the radio frequency
playback (RF) amplifier.
STANDARDS
Demonstrates competency of task skills and knowledge by correctly responding to 80 percent of the
multiple-choice test questions, covering terms relating to the fundamentals of the VCR playback
circuits.
REFERENCES
None
Learning Event 1:
FUNDAMENTALS OF RF PLAYBACK AMPLIFIER (PS-10)
1.
Refer to Figure 3-4 (fo) at end of lessons. The signals from the A and B rotary video heads are
coupled through the rotary transformers in the head drum assembly to relays RY-1 and RY-2. In the
playback mode, RY-2 connects head A to the primary of transformer T2 and RY-1 connects head B to
T1. The playback RF signal from each head is coupled through the transformers to FET Q1 and Q2.
Due to the low signal levels involved, Q1 and Q2 are connected to IC-1 as cascade amplifiers because of
their low noise characteristics. The input circuits of the two FETs are tuned by C2-R3 and C3-R4,
playback amplifiers are inside IC-1, except for tuned circuits connected at pins 20 and 21 which adjust
the mid-band (4.7 MHz) frequency response equalization. The A and B preamplifiers inside IC-1 are
divided into two sections, a head amplifier stage and a RF OUT stage.
2.
Between the two stages, there is an electronic switch that is actuated by a field pulse supplied
from the servo circuits. The switch alternately switches the two preamp channels on and off, so that
only the channel corresponding to the head in contact with the tape is turned on. Switched RF output
from the two channels is supplied from pins 16 and 17 of IC-1. Pots R20 and R21, connected across
pins 16 and 17, are used to balance the Y and chroma RF signals between and A and B channels.
Transistor Q3 is a power
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