Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Adapt Your TV for the CBS Color System

Color Wheel Starts Rolling

It's June 1951, and color TV is here! Everybody wants it, but few can afford it. Then ingenuity comes to the rescue. Equipped with an electronics magazine, a soldering iron and that old seven-inch black-and-white TV set, you homebrew your own color TV. Riffling through the well-stocked junkbox, you probably find all the necessary electronic parts...

  • Two resistors

  • Two potentiometers

  • A four, five or six-pole switch (depending on your TV model)

That's it! The color wheel and motor control are up to you. Yes, this is field-sequential color, not the "compatible color" that came along a few years later. After we detail the wiring changes, we'll tell you more about the wheel. Anyway, even without the wheel, you'll be able to see CBS shows in black-and-white.

Art: Summary of wiring
             changes for CBS color reception on monochrome sets.
Typical wiring changes. Some sets require more poles on the switch.

New Frequencies

The basics. All TV sets have timebases. Inside each TV timebase is a horizontal and a vertical oscillator. These oscillators output frequencies that control picture scanning. Picture scanning is what forms pictures on the screen. In the CBS color system, both oscillators operate at higher frequencies than in the regular American TV system (NTSC).

CBS frequencies. For the horizontal frequency, the CBS system uses 29,160 instead of 15,750 Hz. For the vertical frequency, the CBS system uses 144 instead of 60 Hz. Fixed resistors in series with the TV's front-panel “hold” controls set these timebase frequencies. You'll add a multipoint switch that selects the monochrome or color frequency. The switch connects either the old horizontal and vertical resistors or new ones. The resistors that you need to add are all fixed resistors. The new parts connect in series with the horizontal and vertical hold potentiometers. A typical value for the new resistors is 20K. In some models, a 10K resistor does the trick.

Extra size controls. The switch also selects a second horizontal width and vertical height potentiometer. These parts are identical to those in a stock set. The new pots keep the color picture the same size as the monochrome picture. These pots are definitely worthwhile! Without them, whenever you switch to color, you must increase picture size. All the size adjustments could get to be a major pain in the neck. With the second set of pots, you adjust the color picture size just once. And then you're done.

Circuit Details

The table below summarizes the wiring changes for several popular TV models. The changes allow your set's horizontal and vertical oscillators to operate at higher frequencies. If you know your thumb from a soldering iron, you can't go wrong. Peter Goldmark and his CBS engineers worked everything out.

About the table. The table below lists five TVs with similar circuits. All these sets use a 12SN7 or 6SN7 tube for the horizontal and vertical oscillator. Another common point among the sets is that they use electrostatic deflection. The sets also have scope-like power supplies. That is, the high voltage doesn't derive from a flyback power source. The added fixed resistors control oscillator frequencies. Each resistor usually runs between ground and the grid of a triode. The triode could be part of a multivibrator. Or it could be a blocking oscillator. In the table, the (B&W) value indicates the stock resistor in the monochrome circuit. The (C) value is the resistor value that you must add.

Circuits Allow Conversion to CBS Color Standard

TV Model Horizontal hold resistor, R1 Vertical hold resistor, R1 Add: Horizontal
width pot VR1
Add: Vertical
height pot VR1
V & H tubes Switch See Schematic
Switch Pole 1 2 3 - 4 4 - 6 NA NA NA
Admiral 24D1-E1-F1-G1 270K(B&W) → 20K(C) 560K(B&W) → 20K(C) 2.5M 2.5M V: 12SN7
H: 12SN7
5PDT #1
Belmont 22A21, 22AX21, 22AX22 22K(B&W) → 2K(C) 220K(B&W) → 10K(C) 500K 2M V: 6SN7
H: 6SN7
5PDT H: #1
V: #2
Hallicrafters T54 270K(B&W) → 20K(C) 560K(B&W) → 20K(C) 2.5M 2M V: 12SN7
H: 12SN7
6PDT H: #4
V: #2
Motorola H71 22K(B&W) → 2K(C) 470K(B&W) → 20K(C) 20K 5M V: 12SN7
H: 12SN7
4PDT H: #3
V: #1
Teletone TV149 220K(B&W) → 10K(C) 470K(B&W) → 10K(C) 5M 10M V: 12SN7
H: 12SN7
4PDT #5

Generic schematics. Below the table are five generic schematics. Many seven-inch TV sets of the late Forties and early Fifties use circuits very close to these. You'll find such circuits in both the horizontal (H) and vertical (V) deflection timebases. Of course, exact wiring tends to differ slightly between models. Use our schematics for a guide, but always refer to your set's exact schematic.

RCA horizontal circuits are a marked departure from these schematics. In its seven-inch sets, RCA adopted automatic phase correction for horizontal oscillators. Converting such sets for the CBS system requires switchable modifications to the AFC circuit. You need at least a six-pole switch. Regardless of manufacturer, typical sets with larger picture tubes use automatic phase-correction circuits. For conversion, the two most likely RCA candidates are models 630TS and 8TS 30.

♦ WARNING: Before modifying your set. Keep safe! Know what you're getting into and why. Before proceeding with changes, read our disclaimer. (At the bottom of this page, see WARNING.) Word to the wise: Without extra equipment, these changes won't allow you to receive color broadcasts on today's TV sets.

Generic schematic of horizontal or vertical, cathode-coupled multivibrator 
(mechanisches Farbfernsehen, frei schwingender Multivibrator)

Schematic #1

Multivibrator, cathode-coupled (Admiral; Belmont horizontal, Motorola vertical)
Add the switch poles & "Color" resistors...

• Admiral sets. Follow Schematic 1 for both V & H oscillators.

• Belmont. Use Schematic 1 for H circuit. For V circuit, see Schematic 2.

• Motorola. Use Schematic 1 for V circuit. For H circuit, see Schematic 3.

Generic schematic of vertical, cathode-coupled multivibrator for Belmont & Hallicrafters models (mechanisches Farbfernsehen, frei schwingender Multivibrator)

Schematic #2

Vertical multivibrator, cathode-coupled (Belmont & Hallicrafters only)
Add the switch poles & "Color" resistors to the vertical oscillator.

• Admiral or Belmont horizontal circuit. See Schematic 1.

• Hallicrafters horizontal circuit. See Schematic 4.

Generic schematic of horizontal, cathode-coupled multivibrator for Motorola models (mechanisches Farbfernsehen, Sperroszillator)

Schematic #3

Horizontal blocking oscillator, plate-coupled (Motorola)
Add the switch poles & "Color" resistors to the horizontal oscillator.

• In some Philco sets, a similar circuit works well.

• Motorola vertical circuit. See Schematic 1.

Generic schematic of horizontal or vertical, cathode-coupled multivibrator 
(mechanisches Farbfernsehen, frei schwingender Multivibrator)

Schematic #4

Horizontal multivibrator, cathode-coupled (Hallicrafters)
Add the switch poles & "Color" resistors to the horizontal oscillator.

• Hallicrafters vertical circuit. See Schematic 2.

Generic schematic of horizontal or vertical, cathode-coupled multivibrator 
(mechanisches Farbfernsehen, frei schwingender Multivibrator)

Schematic #5

Multivibrator, cathode-coupled (Teletone & National)
Add the switch poles & "Color" resistors to both the horizontal & vertical oscillators.

• In some National Radio sets, a similar circuit works well.

Automatic Size Control

The extra size controls certainly add complexity and tedium to the conversion process. If only the TV could adjust its own size! Actually, an automatic size control circuit might be possible.

Circuit description. Imagine a limiting amplifier with an adjustable output. You'd insert this amplifier between the sync multivibrator and the sweep capacitor. The limiter amplifies, inverts and clips sync pulses. Regardless of input frequency (NTSC or CBS), the limiter outputs pulses of the same height. Since the limiter inverts, you'd connect it to the negative sync side of a multivibrator. A blocking oscillator circuit would probably require an extra inverter stage.

"Bare-bones" automatic size control. A passive automatic size control might be possible. The passive circuit wouldn't invert. It would connect between the sync oscillator and the sweep capacitor. The bare-bones size control consists of a diode and a resistor. You determine resistor value by experimentation. The resistor sets the diode's clip voltage. Instead of a fixed resistor, you might prefer a calibration pot. You'd only need to set this pot once. Unlike the duplicate size controls, this pot would mount right at the circuit.

Setup. Here's a rough idea of how to adjust the new automatic size control...

  1. Switch the H and V multivibrators to the "CBS" position.

  2. Switch on TV power.

  3. Set the original size controls for the CBS system.

  4. Switch off TV power.

  5. Connect the automatic size controls.

  6. Switch on TV power.

  7. Set the automatic size controls for the best picture.

  8. Switch off TV power.

  9. Switch the H and V multivibrators to the "NTSC" position.

  10. Switch on TV power.

  11. Check to see that the picture remains the right size.

Click and go...





National Radio  




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Copyright © 2008 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/mtv_cbssys/cbssysadapt1.htm
Webmaster: James T. Hawes
•Acknowledgments: Circuits by the late Normal L. Chalfin K6PGX.
For research assistance, thanks to Cliff Benham.
•Revision—November, 2011  •Page design tools: HTML, Notepad & Explorer

WARNING. If you build a circuit from this page, you do so at your own risk. I take no responsibility for your success or failure. Further, I neither make nor imply any warranty or guarantee as to the accuracy or effectiveness of these methods. I have not tested these circuits. I provide them merely as theoretical illustrations. A television set contains potentially lethal voltages. These voltages persist for a long time, even after you shut off the set. Only qualified service personnel should attempt repairs or modifications. Proceed at your own risk.

CBS-system broadcasts ceased in October 1951. Installing CBS timebase circuits won't necessarily allow you to receive CBS-system broadcasts. To reproduce CBS-standard programming, you need...

  • A CBS timebase

  • A synchronized color wheel

  • A scan converter

The scan converter must be able to convert NTSC, ASTC or other-type programs to CBS standards.