Sony AV-3420CE Service Manual page 44

The Head Drum has a built-in dc motor that
provides torque to rotate both the Video Head and
the Capstan. The Video Head assembly is connected
to the shaft of the dc motor. Torque is transmitted
to the Capstan (M) via Drum Pulley (T) and the
Capstan Belt (V). Torque is also used to drive the
Take-up Reel Table (O) via the Drive Pulley (R) and
the Drive Shaft (S). The Sub-motor (X) and the
Sub-Motor Belt (W) are used in the playback mode
for the capstan servo system.
The Head Drum also has a built-in 13,125 Hz
frequency generator coil, 25PG coil, and 50PG coil.
The relative position of the video heads and the PG
coils is shown in Fig. 9-2. Two output pulses
from the 50PG coil, 1 pulse from the 25PG coil,
and 625 cycles from the 13,125 Hz FG coil are
obtained for each revolution of the video heads;
50 Hz, 25 Hz and 13,125 Hz are obtained at 25
revolutions per second.
brushes
PG coils
video head
D-c motor with
13,125 Hz FG
Note: FG: frequency generator
PG: pulse generator
Fig. 9-2. FG and PGs
9-2
described in four sections: Brake System (See 9-2-1),
Pinch Roller Mechanism (See 9-2-2), Tape Take-Up
System (See 9-2-3) and Tension Control System
(See 9-2-4).
9-2-1. Brake System
9-2-2. Pinch Roller Mechanism
Pinch Roller pressure is applied against the
Capstan when the Solenoid
is energized.
Refer to
Fig. 9-3. When power is applied to the Solenoid,
the plunger moves
in the direction indicated by the
arrow,
rotating the
Pinch Roller
Pressure
Plate
clockwise and thus compressing the Pressure Spring.
This causes Pinch Roller Lever (A) to rotate clock-
wise, forcing the Pinch
Roller against the Capstan.
9-2-3. Tape Take-Up System
Drive is transmitted from the Head Motor to
the Take-Up Reel as described below. Refer to
Fig. 9-1. The Sub-Belt transfers drive from the
Drum Pulley on the Head Motor to a Drive Shaft
Pulley on the Drive Shaft. The Take-Up Belt and
Take-Up Pulley couple drive torque to the Take-Up
Reel.