Simple display driver for mechanical television

Requirements. To operate, my LED driver requires a mechanical scanner that you can synchronize with the program. Most mechanical TV (MTV) hobbyists use 32-line, 12.5 frames-per-second (fps) scanners. The scanner could be a disc or drum type. Enclose the LEDs in a lightbox with a frosted-glass, translucent window. Opposite the window and at least two inches back, mount the LED matrix. The amplifier circuit requires a power supply and a source of programs. I suggest the CDs on this page... NBTVA program discs.

The NBTVA discs are for use with a 32-line, 12.5 fps television scanner. Note: I haven't optimized this LED driver for the NBTVA discs. This driver design doesn't follow NBTVA specs, or specs of any other organization.

History

Daven was a Newark, New Jersey electronics manufacturer. This Daven advertisement has a photo of the Daven amplifier, plus many related Daven parts. Daven's amplifier was one of the early commercial TV amplifiers. In December 1928, Popular Mechanics ran a construction article about the amplifier. This article shows Daven's amplifier in use with the company's triple-spiral scanning disc. This disc could reproduce telecasts on the 24, 36 and 48-line television standards. A New York viewer of the day would need all three standards!

Better tubes. Just a few years later, screen grid tubes (tetrodes) began to appear in the input stages of TV preamplifiers. The newer tubes could develop far more voltage gain than could Daven's MU-20 triodes. In fact, the amplification factor for a Daven MU-20 is only 20. Note that Daven includes the amplification factor (mu) in the tube name. (Nice idea!) Amplification factor is an ideal, rather than a real-world gain figure. In typical circuits, voltage gain would be in the single digits. Later tubes could increase this gain by 10 times or more per stage. More capable tubes also drove the TV display. These superior tubes were power triodes or tetrodes. For example, tubes such as type 45, 50 or 10. These tubes could supply the load with higher voltage than could Daven's MU-6 triode.

New disc standards. Other changes were afoot, too. In 1928, the RMA approved new regional TV standards. Among these were the 45-line, triply-interlaced, 15-fps pictures that became the norm in the Midwest. Ulises Sanabria of Western Television invented this standard. On the East Coast, the standard soon became 60 lines sequential at 20 fps. Dr. Frank Conrad of Westinghouse designed this standard. RCA, Jenkins and CBS soon adopted Conrad's standard. Unfortunately for Daven, its tri-standard disc didn't support these new standards. A disc retrofit would have allowed Popular Mechanics' scanner to bring in 45-line pictures. Sixty-line reception was another matter. A 60-line aperture disc is some 31 inches across. It just wouldn't have fit into Popular Mechanics' cabinet. The cabinet design only allows for discs up to about 24 inches across. Just as today, obsolescence comes quickly for cutting-edge technology.

LED driver specs

My circuit provides the entire video electronics and the video display for a mechanical television receiver. See the block diagram above. Build my circuit, and you'll have the equivalent of diagram blocks 2, 3 and 4.

• My display consists of four series strings of three LEDs each.
  
  
• A high-gain, PNP preamplifier provides a substantial voltage boost.
  
  
• A Darlington power transistor drives the display.
  
  
• The circuit is entirely direct coupled. (Direct coupling is necessary to avoid phase shift, especially since I don't clamp the DC.)
  
  
• Total circuit current is in the neighborhood of 160 mA.
  
  
• Each LED string handles 40 mA peaks.

• The circuit input source is a CD containing narrowband video. The NBTVA sells such CDs to club members. See the sidebar for a link to the NBTVA.
  
  
• With this amplifier, a "Walkman" type CD player can drive the display. To reconstruct the picture, scan the display with a Nipkow disc.
  
  
• The gadget works like a champ. The contrast range is broad, and the pictures should be acceptable.
  
  
• As with the 1928 Daven circuit that inspired my design, picture sync is up to you. This circuit has no effect on sync.
  
  

My inspiration: 1928 video amp and driver by Daven

Where I part with Daven

My LED driver follows the Daven amplifier's use of three amplifying stages. Yet I've introduced several changes to the original design. Here's a summary of these changes...

• Transistors vs. tubes. For several reasons, I chose to use transistors. First, the Daven MU-20 and MU-6 tubes are unavailable today. The same goes for most radio batteries. Second, transistors are readily available. Third, transistors can directly drive the low-impedance load. Fourth, transistors offer a higher gain than do triode vacuum tubes. Fifth, transistors are durable, small, and much more efficient than tubes are. Transistor chassis and power requirements are much easier than equivalent tube requirements. Sixth, my junkbox included transistors, but no Daven triodes or kine tubes.

• Low-impedance output. My Darlington power amplifier drives LEDs instead of a neon tube. Unlike the unavailable neon kine tube, the LEDs are low-impedance devices. LEDs need low voltage at a high current. Neon tubes require high voltage and low current. The Darlington provides the appropriate current and voltage for the LEDs.

• Low-impedance input. Some of today's mechanical TV hobbyists are hams. They transmit mechanical video. Yet few programs go out over the air. (Of course, such transmissions aren't broadcasts. They're point-to-point communications or relays.) Most of today's mechanical TV hobbyists distribute programming on CDs. The CDs are my reason for changes to the preamplifier input. My transistor preamplifier accepts its input from the earphone jack of a CD player. If you want to use the line output jack instead, you'll need to add a higher-impedance preamplifier. This new preamp should produce current gain, but not voltage gain. I suggest an emitter follower before my first stage.

• Direct coupling. In my amplifier, I direct-couple the transistors. Daven resistance-coupled the three stages of its amplifier. Particularly in high-gain stages, resistance coupling offers the advantage of stability. Resistance coupled stages are also comparatively easy to design and maintain. But direct coupling preserves the DC content of the signal. In television, the DC content includes backgrounds, average scene brightness and slow-moving objects. Both resistance-coupled and direct-coupled amplifiers can produce a flat response across the band.

MECHANICAL TELEVISION LINKS

• Amplifier design
• Books
• Camera experiments
• Chicago television
• Color television
• Contact the curator
• Clubs
• Downloads
• FAQs
• Ham radio
• History
• Hobbyist sites
• How-To
• Light Valves
• Museums
• New! Mechanical television!
• Parts for hobbyists
• Recordings of mechanical telecasts
• Scanner emulator
• Technology
• Weird science

  
  

Copyright © 2002 by James T. Hawes. All rights reserved.

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