Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Color Wheel FAQs


QUESTION. Would a Col-R-Tel adapter work with today's TV sets?

ANSWER. With analog TV, probably not. With digital TV, definitely not. The rest of this answer refers to analog TV. Today, the receiver IF bandwidth probably isn't broad enough to drive Col-R-Tel. Some connections that appear on the schematics would be inside of ICs today. Also, many TVs use neckboards instead of plugs with leads. Neckboards make connections difficult. Cliff Benham also warns that many, if not most TVs use a hot chassis. Adding circuitry to them without using an isolation transformer is hazardous.

QUESTION. How can I adapt Col-R-Tel circuits to today's television?

ANSWER. To restore the IF bandwidth sufficiently to drive a color adapter, you need gain. The gain comes from a stage or more of chroma amplification. You'll have to add these gain stages. Circuits appear in old TV trade school textbooks. Most such books bear a copyright date between 1973 and 1979. Before 1973, color TVs still used tubes. After 1979, color TV circuits became more and more integrated. Newer texts responded by replacing the detailed circuit analysis with block diagrams and cursory discussions. (I wonder how anybody learns from these presentations? If you need to design an amplifier, a block diagram isn't much help.)

Anyway, if you add the chroma bandpass amplifiers, then you probably need a delay line. Now the circuit isn't so simple anymore. Those bandpass amps require tuning, peaking coils and traps. The delay line must be the right one, or the colors appear in the wrong place. At least we don't have to build our delay lines today. The old timers did, though. Want to know how they did it? Here's a link... How to Wind a Delay Line.

If I were building a Col-R-Tel, I'd consider standardizing the 3.58 MHz tuned circuits. Col-R-Tel uses different LC values for different tuned circuits. Who needs so many values? A quantity of the same value would be cheaper to buy, support and stock!

Also, Col-R-Tel uses tuned coils and fixed capacitors. These days, those tuned coils are tough to find, and they're expensive. At one time, little inductor gnomes merrily wound coils in an old shoe. Where have those guys gone? I think the surface mount companies offered more money. Speaking of money, I'd drop the tunable coils. To save a wad of cash, I'd switch to trimmer capacitors and fixed coils. Typical modern values for a 3.58 MHz tuned circuit are...

  • Coil: Fixed, 100 uH
  • Capacitor: Trimmer, 6.5 to 30 pF
  • Bottom Frequency: 2902 kHz
  • Top Frequency: 6236 kHz
  • Required Frequency: 3580 kHz
  • Available from: Mouser

• Before building, know your risks! Read our disclaimer. Other risks might also apply.

QUESTION. Col-R-Tel uses two RF transformers, T2 and L2. What should I use for these parts?

ANSWER. For the transformers, particularly T2, I'd consider switching to 3.58 MHz ceramic resonators. Abracon, ECS and Murata make such resonators.

One resonator produces about a 30 dB dip or boost. Three-terminal resonators include two of the necessary capacitors, reducing your parts count. (In the illustration, the coupling resonator would use these capacitors. For simplicity, the drawing omits the capacitors.) Resonators are low-impedance devices that typically require capacitive coupling. For more about using crystals or resonators for coupling, see W1FB's Design Notebook by Doug DeMaw.

The circuit at right provides examples of both coupling and decoupling with resonators. COUPLING: X1 offers its least resistance at the resonant frequency. DECOUPLING: X2 boosts stage gain at the resonant frequency. Resonators might require series capacitors.

If you want to use a tunable coil, try a tunable AM or FM oscillator coil. If the transformer winding measures 240 uH, the capacitor value should be 8.2 pF. Assuming enough range in the coil tuning, anything close to 8.2 pF should tune 3.58 MHz. Here are some typical tuning coil and capacitor values...

Schematic: Coupling & decoupling with
       ceramic filters
Coupling and decoupling with ceramic resonators
4.5 14:1 15K:300 438 470 Orn 421F124 Yes
360 10:1 Osc 5.48 5.6 Red 421F300 Yes
360 13:1 Osc 5.48 5.6 Red 421F100 Yes
360 35:1 Osc 5.48 5.6 Red 421F110 Yes
680 22:1 50K:500 2.9 3.0 Yel 421F104 No
680 6:1 20K:5K 2.9 3.0 Blk 421F106 No

With any coil and capacitor decision, you'll need to test your result. I haven't figured tuning sharpness. I have no Q data from Col-R-Tel to compare with modern coil Q. I suspect that combining large coils and small capacitors might yield unsatisfactory results. Also, the coil resistance might cause your circuit to tune too broadly. For these reasons, I advise against combining the 680 uH coil and 3 pF capacitor.

If your circuit tunes too broadly, then tuned circuit Q is too low. You can increase Q and decrease bandwidth by adding a second filter. Some transformers provide a tap. You can increase Q by connecting to the tap instead of to one transformer end. Either approach requires experiments. Tightly tuned transformers might require neutralization.

If your circuit tunes too narrowly, the tuned circuit Q is too high. You can reduce Q and increase bandwidth by adding a parallel resistor across the coil. In a transformer circuit, use a resistor across both the primary and secondary. You'll need to experiment with resistor values. The cost of the added resistor is a reduction in filter effectiveness.

For comparison purposes, I've estimated some of the Col-R-Tel transformer specifications. Transformers T2 and L2 resonate at 3.58 MHz. The table below provides my calculations...

T2 Primary 27 1:1.7? Osc 80 ?
T2 Secondary 47 1:1.7? Osc 47 ?
L2 Primary 27 ? Osc 75 ?
L2 Secondary ? ? Osc Untuned ?

• Before building, know your risks! Read our disclaimer. Other risks might also apply.

QUESTION. Where do I get parts?

ANSWER. Believe it or not, most of the parts are still out there. Unfortunately, parts aren't as cheap as in 1955 to 1971, the day of color adapters. Try Mouser and Digi-Key for transistors, resistors, diodes, capacitors and coils, perfboard, etc. See my Mechanical TV Links page for surplus vendors for wire, wall wart power supplies, PC board stock, chassis, cabinets, connectors, etc. If you insist on building with tubes, try Antique Electronic Supply. You're on your own for the disc and disc cabinet. Kodak sells Wratten filter material, but it's very expensive.

• Before building, know your risks! Read our disclaimer. Other risks might also apply.

QUESTION. Could I use some surplus circuit?

ANSWER. Yes. Look for a junker, tube color TV set. At least these parts must be intact... Chroma, burst, demodulator and CRT driver sections. Some early transistor sets might have a useable crystal and some usable coils. For solid-state sets, the best vintage is about 1975 to 1979 or so. The junker must have identifiable, discrete parts. You must be able to remove and rewire the parts. Please don't destroy a classic TV set!

Word to the wise: Unless you know what you're doing, don't mix transistor and tube parts. If you'll use your Col-R-Tel with a tube TV, salvage parts from another tube TV. If you'll use your Col-R-Tel with a transistor TV, transistor TV parts are okay.

Salvaging offers you a couple of options...

  • If the color section doesn't work, you might be able to salvage the coils and maybe the capacitors. Since Col-R-Tel tuned circuits are both rare and desirable, you'll be ahead of the game. Try to get the delay line and 3.58 MHz crystal, too.

  • If the color section works or nearly works, you're in better shape yet. A Col-R-Tel demodulator is basically a stripped-down NTSC demodulator. You can salvage the entire circuit. You might even use part of the chassis. If the set has printed circuits, don't remove anything yet. First, be sure of the interboard connections. Also, note down the power requirements. You'll have to provide the salvaged circuit with the proper voltage and current. Most TVs use a scan-derived power supply. When you disconnect the board from the TV, the board loses this power source.

Start your modifications with these circuits...

  • Phase-delay network. Most TV chroma boards only develop two CW, hue references phases. (Typical phases are B-Y and R-Y, or X and Z). For Col-R-Tel, you need three references: Red, blue and green. A switch connects one phase at a time to the demodulator: Any two phases should be about 120 degrees apart. Start with the NTSC red phase, which is about 77 degrees from the color burst. This phase should come off the limiter. A hue (tint) control must allow the viewer to adjust the red position. The range for this control should be about 180 degrees. In the factory Col-R-Tel, the hue control is in the ringer amplifier stage.

  • Color demodulator. Unlike standard color demodulators, Col-R-Tel's demod develops significant voltage gain. Also, Col-R-Tel's demod has the color saturation control in its plate circuit. This control is another departure from conventional designs. In regular TVs, the saturation control is usually in the chroma amplifier (bandpass amplifier). Col-R-Tel has no bandpass amplifier.

Then, you'll eliminate several parts...

  • One demodulator. Keep the one without the phase shifter on the input. The phase-shift network could be either in the chroma or subcarrier reference line.

  • The chroma amplifier (all stages). Col-R-Tel doesn't use a chroma amplifier. All chroma gain comes from the demodulator stage.

  • Two color difference amplifiers (CRT drivers). Col-R-Tel only uses one color difference amplifier. The monochrome CRT only has one grid (or cathode) to drive.

  • The matrix circuit. A standard NTSC matrix requires three color difference amplifiers. Col-R-Tel only uses one color difference amplifier.

  • The luminance circuit. Col-R-Tel uses the luminance circuit in the host TV.

  • The reactance tube. To assure phase lock, Col-R-Tel uses a crystal ringer. The ringer requires no reactance tube.

  • The burst phase detector. Col-R-Tel's crystal ringer requires no burst phase detector.

  • The color killer. When a monochrome show comes in, the color killer cuts off the chroma amplifier (bandpass amplifier.) Col-R-Tel has no chroma amplifier.

  • Optional: The automatic color control (ACC). Maybe you'll keep it. The ACC usually alters chroma amp gain. Since you're disabling the chroma amp, you'll automatically dispense with the ACC. If you want ACC, alter it so that it affects the demodulator output signal or gain.

Conversion Notes

  • Instead of eliminating parts, you might just choose to disconnect them. Then the unused parts become onboard spares. Not a bad deal!

  • Your hue (tint) control could be in part of the circuit that you're not using. If so, then move this control somewhere else. Without a functional hue control, Col-R-Tel won't work properly. Typically the control is in the subcarrier reference circuit, after the burst gate. Somewhere before the demodulator, the control shifts the reference signal. (The Col-R-Tel chassis labels this control "Color Lock.")

  • After the conversion, you might need to trim the delay line. If you have an analog, passive, wirewound delay line, trimming might be possible. If your delay line is either active or encapsulated, look for a new line. See our question on delay lines.

You'll still have to add...

  • The color switching circuit

  • The motor control circuit

  • The interface to the TV set

• Before building, know your risks! Read our disclaimer. Other risks might also apply.

QUESTION. Where do I get a delay line?

ANSWER. There are three ways to get a delay line...

  • Find a surplus coil. See: Surplus delay line. Before you junk that old color set, salvage the delay line! The link lists the other must-save parts. These are getting very rare, and will go up in value. One delay line might not be enough. Not all lines delay the luminance by the same amount. Every type of color circuit delays the color difference signals by a different period. The line must match the luminance delay to your circuit's color difference signal delay.

  • Wind the line yourself. See: Wind a delay line for TV.

  • Buy a new, passive delay line. Something with about a 1-μS delay should do. For example, check out this vendor's 1μS line: Polara® delay lines.

Lo-Z Coils. There's a fly in the ointment. A recently manufactured line will probably have a much lower impedance than the factory Col-R-Tel line had. For example, a tube delay line might have input and output impedances of 2,000 ohms. Your new line might have input and output impedances of only 100 ohms.

Remedy. Impedance-matching amplifiers at the input and output should fix this problem. They made transistors for such applications. One transistor at each end of your coil should do the trick. Each circuit must have a reliable current gain of 20. The device beta should be about 200. The bandwidth must allow video to pass without distortion. Fortunately, modern transistors can easily handle these requirements. I believe that common, 2N2222A devices should do. If not, then try something more posh, such as RF transistors. Keep the leads short! This is no place for sloppiness. Neat construction pays off! On the other hand, you're still not out of the woods. The transistors might add too much internal capacitance. If so, trim the delay coil or add some form of compensation.

Here are the matching amplifier types you need...

  • At the delay-line input: A cathode or emitter-follower amplifier.

  • At the delay-line output: A common-grid or common-base amplifier.

• Before building, know your risks! Read our disclaimer. Other risks might also apply.

QUESTION. What Col-R-Tel parts can I salvage from a PC monitor?

ANSWER. The neckboard is the main item. A monitor's cathode or grid drive circuits resemble the last stage in Col-R-Tel. Plus, a good neckboard provides you with three such circuits.You'll only need one. In most monitors, the three circuits are identical. Each circuit includes a TO-220 driver transistor, various adjustment pots, and passive components. Sometimes you'll also find a preamplifier or blanker for each color channel.

Cascode circuit. If your monitor is the SVGA or multisync type, you probably have cascode drivers up on that neckboard. Cascode drivers are very desirable, because they're very broadbanded. Actually, they're probably better than you need for NTSC. But maybe you have a higher-definition project in mind. Either way, cascode drivers will serve you well. For each color circuit, cascode drivers include two transistors. These two-transistor combinations are slightly more complicated than what you'll find in a typical TV. But the complexity level is quite manageable and well worth the improved performance.

If your monitor is an older, VGA, EGA or CGA type, don't fret. You'll have a neckboard with one power transistor per channel. This one-transistor circuit is quite capable of doing what you need it to do. Plus, you don't need to hassle with the extra complexity of cascodes.

Copy the circuit. The neckboard fits on the CRT. The CRT socket will probably be useless to you. With the neckboard still connected, though, check the operating voltages of neckboard transistors. Then power down the circuit and draw a schematic of the circuit that you want. You can then remove parts and copy the circuit to your own PC board.

Now, for what you won't find in that monitor. Of course, monitors don't decode composite video. For that reason, monitors don't have decoder circuits. Among these circuits are the...

• Chroma demodulator

• Phase selector

• Phase shifter

• Limiter

• Color crystal

• Oscillator or ringer amp

• Hue control

• Burst gate

QUESTION. Where can I get a color wheel?

ANSWER. You probably need to build the wheel. For optimum performance, you should use the proper filter material. The CRT phosphor is also important. According to expert Cliff Benham, Col-R-Tel designed its filters for CRTs with P4 phosphors.

Wratten filters. Builders buy the Wratten filter material. This is a Kodak product...

Wratten Filter Numbers
Red Green Blue
# 26 # 58 # 47

Roscolux filters. Save a fistful of dollars! Cliff Benham writes that Rosco makes excellent filter material. According to Cliff, Rosco filters aren't gel like the Wratten type. Instead, Rosco makes a "sandwich" type filter. The new material is fade resistant, too. You can buy Rosco filters online, or through a theatrical supply shop. See... Roscolux.

Certain Rosco colors very closely duplicate Col-R-Tel hue and saturation values. In side-by-side tests, Cliff has proved this fact. He projected various light sources through Roscolux filters and Col-R-Tel filters. For all the tests, Cliff used a matte white screen. The matching process was painstaking, because Cliff wanted an exact match. According to Cliff, Col-R-Tel filters are by far the best color converter filters. Here's what Cliff says about Col-R-Tel...

    I can tell you one thing about color wheels: The very best ones ever made bar none are the Col-R-Tel wheels. They are now all 50 plus years old and I have yet to see one that has faded any at all or one on which the plastic tape has gone bad.

Cliff found that for all light sources, certain filters matched more closely than the others. Here are the Roscolux filters that Cliff recommends...

Roscolux Filter Numbers
Red Green Blue
# 25 # 90 # 76

The next step is trimming the filters to form color wedges. Then builders mount the color wedges between two plexiglass discs. Or, they just mount the wedges on top of one disc. The second disc is merely for protection. Filters cost a lot of money, so most people want to protect them.

A far more economical answer is printing a disc on film. I have a link to the site with free printing software. You can also Google "print your own color wheel" or something similar. The link will pop right up. I don't think that the resulting disc is very big, though.

QUESTION. How good could a printed color wheel be? My printer doesn't have RGB. My printer only has cyan, magenta, and yellow (CMY).

ANSWER. As you know, commercial printing uses combinations of CMYK to make a broad gamut of colors. Cyan, magenta and yellow are the complements of red, green and blue. The "K" stands for black. For example, TVs reproduce cyan as approximately "negative red." Magenta is negative green. Yellow is negative blue. Here's something that may come as a surprise: The TV station doesn't transmit red, green or blue. Like a printer, your TV manufactures all three colors.

Now back to printing and color wheels. If you've ever seen a National Geographic, you know that print color reproduction can be excellent. Also, some printers now use six or seven colors instead of four. The dots are small. From normal viewing distances, you might not notice them. That depends on what "normal" is, and how critical your eyes are.

You'd need to try a printed disc and see if it meets your purposes. Cost is an important factor. A printed disc can be very economical. A disc that includes Wratten filter material can be very expensive. You must decide about quality vs. cost. How much will you pay for a certain level of quality? How much quality is enough?

By the way, some commercial TV manufacturers also use inkjet printers to make their filters. See... FILTERS

QUESTION. I already sync my color wheel to my tristable switch. Why should I bother to sync each color wedge to vertical blanking?

ANSWER. I guess that your question is a matter of taste. Do you mind watching colored stripes on your TV? If not, then switch sync is all that matters.

If you mind the stripes, then the wheel wedge must also sync with vertical blanking. At the start of the video field, the wedge should begin descending down the tube. I reread Stanley's ideas on the subject. He tried randomly syncing the wedge and the TV display. That method didn't work for him. (Stanley's thorough discussion of color wheel adapters appeared in Radio-Television News. The articles include both circuits and theory.)

QUESTION. Isn't there some way to shrink down the disc size?

ANSWER. No and yes. That sounds noncommittal, but it's really my way of launching off onto a two part answer.

Part One: Each disc wedge must be wide enough to cover the entire face of your TV picture. Assume that you have a picture with a diagonal measurement of 12 inches (30 cm). (We use diagonal picture width, because that's the largest screen dimension.) Let's say that your disc has six wedges. (That's the layout that CBS and Col-R-Tel followed.) Here's the math...

  • (6 wedges * 12 inches) = 72 in. [183 cm] circumference
  • Disc diameter = (72 / pi) [or 183 / pi]
  • Minimum diameter = 22.9 inches [58.17 cm].
  • Add 15% for clearance of the inside screen edge (3.45" or 8.73 cm).
  • Round up.
  • You need a 27-inch [67cm] diameter disc!

Here are some figures for larger screens...

Screen Diagonal (in) 14 15 20 25 30 40 50 60 80 100
Screen Diagonal (cm) 36 38 51 64 76 102 127 152 203 254
Disc Diameter (in) 31 33 44 55 66 88 110 132 176 220
Disc Diameter (cm) 79 84 112 140 168 224 279 335 447 558

Part Two: You have an option. Forget about scanning the picture tube and scan the eyes instead. My friend Adam Ross has worked with head-mounted sequential scanners. A disc wedge is only the size of the eyeglass frame. Of course, you need two discs. But the smaller disc size might bring down filter and motor costs.

QUESTION. How do I get the disc to sync with the TV picture?

ANSWER. Vertical sync is the basis for disc speed. You must pick up and amplify the TV vertical oscillator signal. You can apply this signal in either of two ways...

  • Operate your scanning motor from the signal.

  • Correct motor speed with the signal.

The comparison signal might come off a disc sensor. For example, you could glue a small magnet to the disc. Then detect the magnet with a Hall effect sensor.

Disc speed is vertical sync speed divided by the number of color wedges.

• With Col-R-Tel, that's: (60 * 60) = 3,600 max. rpm / 6 wedges = 600 rpm
• With CBS, that's: (144 * 60) = 8,640 max. rpm / 6 wedges = 1,440 rpm

     Key of knowledge unlocks doors. Likewise, sync unlocks TV imagery.

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Before building any television project, be aware of the risks. Heed warning labels, especially those that concern fire and shock hazards.

Electrical equipment can produce lethal voltages and currents. Even when a device is off, charged parts can shock or burn you.

Wear protective ear gear. Picture tubes can implode, throwing glass shards eight feet.

If you take apart the family TV, your dad will yell at you. Pursue your experiments at your own risk. I take no responsibility for your results. Expenses, losses, injuries or damages that you incur are your responsibility. I offer no guarantee as to the accuracy of the information on this page. Sometimes I slip on my calculator.

Copyright © 2006 by James T. Hawes. All rights reserved.

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