Hawes Mechanical Television Archive by James T. Hawes, AA9DT
How Col-R-Tel Works, Part 3

Col-R-Tel Burst Section

Block diagram of Col-R-Tel burst section.

Color burst amplifier. The station suppresses the 3.58 MHz color subcarrier. Without subcarrier phase lock, Col-R-Tel can't properly demodulate chroma signals. For this reason, Col-R-Tel regenerates the subcarrier. Regular color TV sets do, too. Of course, the regenerated carrier must be in phase with the suppressed subcarrier. The trick is to pick up and use a mysterious signal that we call the "color burst."

The color burst is a fleeting reference signal that only pops up between video lines. To transmit the burst, the station blanks out the TV picture for just an instant. The viewer never notices. During this time, the electron beam flies back to the left side of the screen. The pulse that blanks out the picture carries the color burst. This color burst is an unmodulated sample of the 3.58 MHz color subcarrier. The station transmits the signal at zero phase. (That's zero degrees on some vectorscopes, but 180 degrees on others).

As I mentioned, Col-R-Tel regenerates the subcarrier. To be of any use, Col-R-Tel's subcarrier must match the station's. To achieve this match, Col-R-Tel's subcarrier must sync and phase lock to the burst. The burst is part of the horizontal pulse. For that reason, burst phase locking occurs during horizontal blanking. Eight to 11 burst cycles appear on the horizontal pedestal's back porch. The rest of the time, the burst disappears to make way for the color signal.

Crystal ringer. To generate the reference subcarrier, most TV receivers use a CW (continuous wave) oscillator. Not Col-R-Tel. Instead, the burst signal, multiplied at Col-R-Tel's burst amplifier, shock excites a 3.58 MHz crystal.

During installation, you tie the converter burst amplifier to the horizontal blanking signal. The orange wire makes this connection. When horizontal blanking occurs, it gates on the burst amplifier. The rest of the time, the burst amplifier remains off. (In the Col-R-Tel burst gate circuit, "off" actually means saturated.) When the burst amplifier gates off, the crystal keeps ringing. In fact, the crystal continues to ring over the entire next line period.

• See the schematic of the burst amplifier:
• See a transistor version of the burst amplifier:

The subcarrier amplifier multiplies and limits the ringing crystal frequency. The crystal puts out a damped wave, but the detector needs a constant-amplitude signal. The second subcarrier amplifier stage achieves such a signal. This stage, a pentode with lots of gain, limits the crystal signal. The limited signal remains strong throughout the video line period. At the amplifier output, a transformer produces two phases of the 3.58-MHz subcarrier.

At the beginning of a colorcast, you adjust Col-R-Tel's "Color Lock" control. This control is a tunable coil in the subcarrier amplifier plate circuit. Really, the coil is a hue control. The object of the adjustment is to shift the subcarrier frequency by about negative 90 degrees. Since you're adjusting while watching the Col-R-Tel screen, you're not thinking about phase shifts. You're thinking about natural fleshtones. Either way, the result is the same. This adjustment produces the negative red subcarrier or reference signal. This is the signal that passes on to the limiter and phase shifter.

The phase shifter accepts an in-phase and an out-of-phase, 3.58 MHz subcarrier. These continuous wave signals originate at the subcarrier amplifier. The phase shifter passes these signals through R/C networks. At the phase shifter output are three subcarrier phases. Each phase matches the phase of one color signal, the red, green or blue. Any two Col-R-Tel phases are some 120 degrees apart.

The table, right, indicates the three color vector positions. Thanks to Cliff Benham for measuring these vectors on a working Col-R-Tel unit.

About the readings. You'll notice that there are two measurements for each color. That's because some vectorscopes read counterclockwise, while others read clockwise.

\ CCW Scope CW Scope
R-Y 104° 76°
B-Y 347° 193°
G-Y 236° 304°

Cliff made the readings in the left column. He used his counterclockwise-reading vectorscope. In the right column, I've converted Cliff's readings to what you'd see on a clockwise-reading vectorscope. All three readings depend on the Color Lock control setting. Using a color bar generator, Cliff set his control to provide NTSC-standard red. Col-R-Tel derives the other two color references from the viewer's red setting.

The color phase selector is the next stage. Three switching diodes make up this stage. Each diode switches independently. The phase selector accepts a color wedge signal from the wheel commutator. This "one of three" control signal is a ground on one color-select line. Each color select line connects to one diode cathode. The grounded select line causes one of three diodes to conduct. As the diode conducts, it selects one color subcarrier phase. The selected R, G or B subcarrier phase then passes to the chroma demodulator.




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