Col-R-Tel Motor-Control Section
Syncs display & wheel. The color wheel must remain in sync with the 59.94
Hz, TV vertical signal. Decoded video must also match the color wedge that
is before the CRT. Col-R-Tel requires both types of synchronization.
The wheel commutator is the device that achieves them. The commutator is a
two-level wafer switch. Each switch level deals with one type of sync. Brief
switching periods for speed syncronization take place between long switching
periods for color selection.
The first switch level has six contacts in a circular array.
(Top-left switch level on the drawing, right.) The six contacts
include two in parallel for each TV primary color. Why two per color?
So that the disc can spin at half the speed. Whenever a wedge
slides before the CRT, level one closes a circuit. Insulating material
separates the contacts. Now imagine Col-R-Tel operating in sync with the TV
station. Each video line period brings one of the six contacts into play.
This contact and its partner across the disc connect to one color-select
line. The circuit has three color-select lines, one for each additive
primary color. In one disc rotation, each color-select line activates twice.
When the active contact closes, it grounds one select line. The grounded
circuit switches one of three color subcarrier phases to the demodulator.
Each phase directs the chroma detector to demodulate one color signal
(red, blue or green).
The second switch level includes another six contacts. (Bottom-left
switch level on the drawing, right.) Each contact applies a wedge-change
signal to the motor control amplifier. This wedge-change signal is a
59.94-Hz sawtooth wave. An RC, low-pass network derives this wave from the
vertical sync signal. During installation, you connect a blue wire to the
TV's vertical output tube plate. This wire carries the vertical signal to the
Col-R-Tel shaping network. With proper disc speed, all six
wedge-change signals are the same. With improper disc speed, at least one
signal may vary from the others. Speed correction is a frequent event. The
reason for this situation is that the 60-Hz line drives the motor. Yet the
motor must sync to the 59.94-Hz vertical signal.
Connections to two-level Col-R-Tel commutator (not showing
speed control or color phase selector)
Motor Control Amplifier
Rotates wheel. The motor control amplifier accepts an input signal from
the TV's vertical output plate. On the way to the motor control amplifier, the
signal encounters a low-pass, R/C filter. (See the diagram, right.) The
filter converts the signal to sawtooth waves. Next, the wheel commutator gates
the signal and passes it to an error preamplifier. At the preamplifier, the
signal drives the two halves of the motor control amplifier. Each plate of this
amplifier connects to one end of a transformer primary. The secondary couples
60 Hz AC into the two plates. The plates are 180 degrees out of phase with one
another. The transformer primary center tap provides B+ power to the amplifier
plates and cathode. The transformer secondary is in series with the motor speed
control pot, motor field and AC line. With no error signal, the coil poses almost
zero voltage drop. With an error signal, the secondary voltage aids or opposes
the AC voltage across the motor field.
Col-R-Tel RC shaping network between vertical sync and motor control
The motor drives the Col-R-Tel disc with a belt. The motor
is small, but operates at a comparatively high rpm. Color converter expert
Cliff Benham owns a Col-R-Tel motor. He provides these motor
specifications from the motor nameplate...
Col-R-Tel Motor Specs
115 volts AC at 1.5 ampere
1/25 horsepower, four-pole, shaded induction motor
Speed converter. Through the laws of unequal pulleys, the motor
develops enough torque to spin the disc. The wheel pulley (2-7/8 inches ID)
is much larger than the motor pulley (1 inch ID). According to Cliff, the
motor-to-wheel pulley ratio isn't quite 1:2.875. Cliff estimates that the
design allows about 8 rpm for slippage. To develop the normal disc speed of
600 rpm, the motor must turn at 1725 rpm. That is...
(600 * 2.875) = 1725
[ (1750 - 1725) / 2.875 ] = 8.7 Hz (rounded)
Composite. The sensor on the block diagram is actually the commutator.
As we said above, this commutator is a composite. It includes the 12 switch
contacts inside the commutator. six contacts sync the disc to the TV station's
vertical signal. The remaining six contacts each select one of three color
subcarrier phases: Red, blue or green. The picture tube illuminates one
disc (wheel) wedge at a time. The commutator mounts mechanically to the color
wheel. For this reason, the commutator always selects the phase that
corresponds to the illuminated disc wedge.
The color wheel. The Col-R-Tel disc (right) looks a lot like the CBS
disc. Both discs have six transparent color wedges. The wedges follow the
order R-B-G-R-B-G. On both discs, the bottoms of the color wedges twist 30
degrees (one clock number). Discs with the characteristic twist can scan
larger picture tubes than can discs with no twist. From the viewer's position,
both discs rotate clockwise.
Rotation. The Col-R-Tel disc rotates at 600 rpm. The CBS system used
1,440 rpm, because CBS broadcast 144 fields per second. On the other hand,
our NTSC TV system broadcasts 60 fields per second. Col-R-Tel has to maintain
compatibility with the NTSC field rate. Col-R-Tel's 600 rpm disc speed is
approximate. Your set's 59.94 Hz vertical frequency determines the exact disc
speed. This speed is somewhere around 599.4 rpm. Col-R-Tel's disc measures
31 inches across. The disc can convert pictures up to 14 inches across. If your
picture is larger, you reduce it with Col-R-Tel's size control.
Col-R-Tel & CBS color wheel
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