Pioneer tuning fork Service Manual page 33

1.2 Drum (cylinder) S c a n n e r
The Scanner consists of a stationary Lower-drum, a rotary
Upper-drum equipped with two REC/PB-video-heads
diametrically opposite each other, a Drum-motor for turn­
ing the Upper-drum, and a Rotary-transformer for elec­
trically coupling the heads with signal processor. In /5
V C R s , Upper-drum is also stationary and the heads are
mounted on a Rotary-disc inserted between the drums. In
8 m m VCR, Upper-drum rotates. The rotary heads and
d r u m driving mechanism are the most important for pick­
ing up recorded information correctly. The Upper- and -
Upper-cylinder
Shaft
Video-head
Rotary
transformer •
FG-pattern-
Magnet'
Fig. 2 Drum (cylinder)
1 . 3 Video Head
1.3.1 Material
The requirements to video head material are the same as
those to audio head material. Mn-Zn ferrite among mag­
netic oxides, and permalloy and sendust among nonoxides
are suitable for the head. H o m e VCRs employ hard-
wearing and highly permeable Mn-Zn ferrite. Sendust and
amorphous heads of high saturation flux density will be
employed in the near future. The characteristics of sen­
dust and ferrite heads obtainable when a metal tape is
played back are shown in Fig. 4.
1.3.2 Structure of video-head
The structure of Video head is simple. Its coil has tens of
turns. The narrow portion of the head-top is protected by
glass as seen in Fig. 5. The head is fixed on a head-base
and is mounted on the Upper-drum or Rotary-disc in be­
tween the drums. The head deals with the signals of short
wavelength having a very narrow gap. The length and
width of the head-gap determine the recording/playback
performance.
Lower-drums of aluminum alloy are processed in a weather
chamber in the accuracy of submicron order. The Lower-
drum has a rail to guide the tape helically. The angle and
linearity of the rail determines the tape track pattern. The
heads are mounted on the d r u m or disc so that their faces
protrude out of the drum face by 30/im to 50/mi. The align­
ment of the heads have precisely been made in factory.
Driving-motor (generally 3-phase) and Rotation-senser (FG
& P G ) are also very accurate t o minimize jitter or time-
base error. Video signal is highly affected by jitter. Rotary
head is superior than stationary head in minimizing jitter.
Grounding-brush
- Lower-
cylinder
Hall
element
PG magnet
Coil
Fig. 3 Rotary Transformer (Electric signal coupler)
Fig. 4 Playback Characteristics of Sendust and Ferrite
Core
B-ch coil
Gap
Metal tape
5.8m/s
35/zm w i d t h
Head
Gap
Glass
Coil
(a) Audio head
Fig. 5 Audio and Video Heads
Core
A-ch coil
Coil
Rotary core
Stationary
core
Sendust head
Ferrite head
(MHz)
Gap
Coil
Core
(b) Video head
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