An AC hysteresis motor powers the revolving head drum. An application of a brake on the head
drum rotor affects the speed of the head drum. The servo PG coil, inside the revolving head drum,
provides the servo PG pulse. The reference signal automatic selector circuit selects the reference 30 Hz
to phase-compare to the servo PG pulse. To drive the brake coil, this phase-comparator output is
amplified. Servo control is effective in both the record and playback modes.
An AC hysteresis motor generates the head drum rotational torque and transmits it to the head
drum by a rubber belt. The servo PG output and the 30 Hz reference are phase-compared and the phase
error, a DC signal, controls the braking of the rotary head drum. The same servo operation occurs in
both the record and the playback modes. The rotary head drum has two PG coils, the SERVO PG coil
and the EDIT PG coil. The servo PG pulse sets the flip-flop while the edit PG pulse resets the flip-flop.
The flip-flop output is integrated and produces the 30 Hz ramp waveform. The 30 Hz reference signal
samples the positive slope of the ramp waveform. The output of the sampling circuit is converted to a
DC voltage. The DC voltage is held by the hold circuit which is sent to the brake coil to regulate the
head drum rotation. When the FWD button is off, the brake current mutes so that the head drum spins
with no braking.
Learning Event 2:
FUNDAMENTALS OF THE CAPSTAN SERVO AND CAPSTAN SPEED SERVO CIRCUITS
The capstan servo system controls the capstan DC motor. This system consists of two servo
loops: (1) The capstan speed servo loop, and (2) the capstan phase servo loop. A frequency generator
(FG) inside the DC motor supplies the FG signal. The FG signal of 360 Hz, varying with the speed
variation, goes to the capstan speed servo which functions to maintain constant capstan speed. The
capstan phase servo operates in the record mode and in the playback mode. In the record mode, a 30 Hz
derived from the 360 Hz signal is at the same time recorded onto the video tape as the CTL signal. In
the playback mode, the PB CTL signal is phase-compared with the reference 30 Hz. The capstan speed
servo output and the phase-compared output go to dual inputs of a differential operational amplifier.
This output goes to the capstan DC motor.
The servo system goes into the RECORD operating mode during both the normal record mode
and ASSEMBLE editing mode. The servo systems are put into playback mode during both normal
playback mode and the insert editing mode.
The FG output is amplified and shaped into a series of positive-going pulses. Q4 and Q3, C1
convert the FG pulse to a sawtooth waveform, having a positive-going slope. The same FG pulse
samples the positive peak of the saw-tooth signal. The output converts to a DC signal by a HOLD
capacitor, whose stored charge is amplified to drive the capstan motor. When the capstan motor speed
decreases, the time period of one cycle of the FG signal becomes longer. This produces a higher peak
saw-tooth signal voltage and a higher DC voltage from the sampling output. This increases the voltage
across the capstan motor and increases its speed.