Pioneer tuning fork Service Manual page 44

9. Servo S y s t e m ( N T S C )
Precise servo system for capstan and d r u m m o t o r is re­
quired to draw a specified pattern on the tape in record­
ing and t o trace it correctly in playback.
The revolution of the D r u m or Head-disc should always
be at 1800rpm and that of Capstan should be at a speci­
fied value depending on the tape velocity m o d e . A 30Hz
control (CTL) signal is recorded as a reference for play­
back. The Capstan, therefore, should move the tape at a
velocity to keep the reproduced control signal at 30Hz at
any tape m o d e . A-head should meet the beginning of A-
track when C T L level rises. Fig. 28 and 29 are simplified
diagrams of Drum and Capstan servo blocks.
Drum FG, PG
Video Input
3.58MHz
X'tal OSC
Capstan servo system:
In recording, the capstan-FG(CFG) signal generated in
Capstan-motor is speed-detected and is NFBed to the
Capstan-motor. The C F G signal is compared with the V-
sync/2 reference signal, and the error signal is NFBed to
the motor. Some models use the reference signal made
from the 3.58MHz signal instead of the V-sync/2. The dot­
ted block of Fig. 27 are peculiar to the types indicated.
42
Speed Det.
SWP
generator
V sync.
Sep.
REC
PB
Divider
Fig. 28 Block Diagram of Drum Servo Circuit
Drum servo system:
In recording, the signals generated by Drum-frequency-
generator (DFG) is speed-detected, and the error signal,
or difference signal, is negatively fed back(NFB) to the
D r u m - m o t o r . The signal generated by
generator (DPG) is compared with a reference signal (V-
sync/2) made from the video input signal, and the error
signal is also NFBed to the motor.
In playback, the internally oscillated 3.58MHz signal is
used instead of the video input signal for making the
reference.
RF switching Pulse
Phase
Comp
In playback, the reproduced CTL signal is phase-compared
with the reference signal made from the 3.58MHz signal,
and the error signal is NFBed to the motor. The C F G sig­
nal is speed-detected and NFBed as above. As a result, the
F G and C T L signals are phase-compared with the help of
the reference signal made from the 3.58MHz signal, and
the error signal controls the Capstan-motor to make the
movement of Heads and tracks inphase.
Drum-pulse-
Motor
Drive
Drum Motor