How To Repair Calculator Display?

© 1997 past Rick Furr
(email: rfurr@vt.edu)

Reprinted from the Spring 1997 issue of The International Calculator Collector.

One of the more than interesting things near old calculators is how they displayed their numbers. As piece of cake as information technology seems today, in the late 60s and early 70s information technology was quite difficult to devise a brandish system for a calculator, especially a portable i. This commodity will describe the construction and functioning of the major types of electronic displays both past and present.

CATHODE RAY TUBE

The Cathode Ray Tube (CRT) was adult for television set in the 40s. The CRT shoots a focused electron axle from the back of the tube to the front of the tube. The front of the tube is coated with phosphors that glow when they are struck past the electron axle. An image is created by moving the electron axle back and forth across the back of the screen. The beam moves in a pattern from left to right, peak to bottom and and then it repeats. Each time the beam makes a pass beyond the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. The intensity of the axle is modulated thus causing the screen phosphors to glow with unlike intensities or to fifty-fifty not glow at all. The desired images to be displayed are really retraced between 30 to 70 times each second. This keeps the images continually refreshed in the glowing screen phosphors without a flicker being perceivable to the middle.

The electron axle is generated from a filament and electrically charged cathode in the dorsum neck of the CRT. The electron axle is outset passed through a command grid. The control grid modulates the intensity of the electron beam. The higher the intensity the brighter the phosphor dot information technology strikes will glow. Adjacent the beam passes through an accelerating electrode, this will speed upward the electron beam. Then the axle passes through a focusing anode. This will focus or tighten the stream of electrons. All of these elements comprise the electron gun structure housed in the neck of the CRT.

The construction on the neck of the CRT is the yoke. The yoke contains iv electromagnets placed effectually the neck of the CRT in 90 degree increments. By varying the voltage of these four electromagnets, the electron beam can be deflected or bent to reach any location on the phosphor coated screen. A final stage of acceleration is achieved with the high voltage anode. The familiar suction loving cup wire that attaches to the side of the CRT is connected to this anode. This anode is oftentimes a metalized surface on the inside of the picture tube. Many thousands of volts are applied to the anode to pull the electrons towards the phosphor coated screen. Phosphors can be formulated to emit many colors though white and greenish are the nearly popular for monochrome screens. Additional circuitry in the calculator can create numbers, letters, and other symbols by using the control grid to turn the electron beam on and off, while simultaneously using the electromagnets to deflect the beam to the desired locations on the screen. Many early on desktop calculators like the Friden EC-130 and the Hewlett Packard 9100A used CRTs.

NIXIE TUBE Display

In a Nixie Tube display each numeral is a complete, lighted cathode in the shape of the numeral. The cathodes are stacked and so that dissimilar numerals appear at different depths, unlike a planar display in which all numerals are on the same aeroplane relative to the viewer. The anode is a transparent metallic mesh wrapped around the front of the display. The tube is filled with the inert gas neon with a pocket-size amount of mercury. When an electric potential of 180 to 200 volts DC is practical between the anode and any cathode, the gas nearly the cathode breaks down and glows. The digits glow with a orange-red color.

The proper noun Nixie came well-nigh accidentally. A [Burroughs] draftsman making drawings of the device labeled it NIX I, for Numeric Indicator eXperimental No. i. His colleagues began referring to it as "Nixie," and the proper noun stuck ( Scientific American , June '73, pp. 66).

Interestingly enough the Nixie design is considered "failsafe". If a filament (cathode) fails, the numeral is non illuminated. Whereas, in a vii-segment display if ane segment fails, a number other than the intended number may exist displayed. The Casio 121-A and the Sharp QT8-B used Nixie tubes.

INCANDESCENT FILAMENT DISPLAY

An Incandescent Filament display is usually housed in a vacuum tube like the either the Nixie tube or the early Vacuum Fluorescent tubes. This display is typically a 7 segment manner of brandish where each display segment is formed with a conductive anode tungsten filament. A modest voltage placed across a filament will crusade it to estrus to incandescence. They emit a yellowish -white light that can be filtered to any desired color. The filament voltage (three-5vdc) can as well be varied to change the brightness level of the display. The biggest problem with Incandescent displays is they accept a ho-hum response time and they eat a large amount of current. A pop version of this type of display was the RCA Numitron. Some early electronic kits used the Incandescent Filament brandish.

GAS Belch or PLASMA Display

A Planar Gas Belch or Plasma Display Panels (PDP) display utilizes the same principle the Nixie tube does. It'due south construction consists of sandwiching a hollow center layer filled with neon and a small amount of mercury between a drinking glass front and a ceramic back. A thick conductive paint forms the Cathodes on the inside of the ceramic back. The Cathodes class the segments of each digit. Each digit is covered by a split Anode that is deposited on the inside of the drinking glass front. The Anodes are formed from a thin transparent layer of tin oxide. When a sufficient voltage is applied between a cathode segment and information technology's anode, the gas around the cathode segment breaks down and begins to glow. Like the Nixie tube, the digits glow with a orangish-cherry color. Voltage requirements for these displays are typically 180-200 volts DC).

Burroughs manufactured a mutual brand of PDPs called Panaplex II. PDPs were used in many early calculators including CompuCorp's Scientist serial.

VACUUM FLUORESCENT DISPLAY MODULE

The Vacuum Fluorescent display (VFD) consists of a vacuum tube in which there are three basic types of electrodes, the filament (cathode), the anode (segment), and the grid. The VFD is substantially a minor Cathode Ray Tube. The filament (or filaments) is a very fine wire that is heated to a temperature but below incandescence. At that temperature it remains about invisible but it emits electrons. A transparent metal mesh grid covers each digit and controls the electrons emitted from the filament toward that digit. Seven phosphor coated anodes, arranged in the seven-segment configuration (that form a square eight), glow when struck by the electrons. When a positive voltage of 12 to 25 volts is practical to the filigree and the anodes, the electrons emitted past the cathode filament are accelerated and attracted to the positive anode segments which in-turn glow. If the grid has a negative potential then it will cake the electrons from passing regardless of the potential of the anodes under the grid.

VACUUM FLUORESCENT Display TUBE

VFDs were developed in Nippon in 1967. Early versions of VFDs were individual digits housed in vacuum tubes like the Nixie tube and Incandescent Filament displays. VFD Phosphors can be formulated to emit reddish, yellow, and green every bit well as the more common blue-light-green color. Later versions would firm all of the digits (and other graphics and indicators) in one large drinking glass assembly. Currently VCRs account for 30% of the VFD market place and Audio/Video products account for another 30%. Many early serial of calculators similar the Commodore 412F, Brother 310, and the MITS 816 used the individual digit VFD tubes. Later manufacturers such equally TI and Rockwell used the integrated multidigit VFDs in both handhelds and desktops.

ELECTROLUMINESCENT Brandish

Sparse-picture show Electroluminescent Displays (ELDs) utilize a thin film of phosphor (zinc sulfide (ZnS); ZnSe; ZnSMn or other fluorescent materials) sandwiched betwixt a dielectric layer that is sandwiched between ii drinking glass plates. Transparent electrodes (tin can-oxide) are deposited on the insides of the glass plates. When a sufficient Air-conditioning voltage (>100 volts) is applied to any of these electrodes the phosphors will be excited and will emit light. ELD phosphors tin can exist mixed with pigments to emit many colors of calorie-free including green, blueish-green, lemon-yellowish, orange, red as well as white light.

This type of solid state brandish can endure extreme conditions with exceptional tolerance to stupor, vibration, temperature, and humidity, while response times remain less than one millisecond. I have non seen ELDs used in calculators but they are used in some laptops, office machines and in the cockpit of the Spaceshuttle. They are also used to backlight LCD panels.

LED DISPLAYS

A Light Emitting Diode (LED) is an special type of diode that emits light when electricity applied to it's anode and cathode. A typical LED requires about 1 one/2 volts DC at 10 milliamps to begin emitting light. LEDs ordinarily produce red light only yellow, light-green and blue versions are too now available. The LED was starting time marketed by Texas Instruments around 1962. LED displays (seven or more individual LEDs) were introduced around 1967 but were very expensive. Calculators used LEDs that were arranged to form either a seven-segment brandish or a dot-matrix brandish.

Early vii segment displays formed each segment with many LEDs, later vii-segment displays would employ one LED per segment with a low-cal pipe to spread information technology's light across the segment. Also early LED displays were made small-scale in order to keep power consumption down. A clear plastic chimera lens was fabricated into the package to magnify the display for easier viewing.

NUMERIC REPRESENTATION

The dot-matrix style of brandish would class characters shaped similarly to that of a dot-matrix printer. A dot matrix of 4x7 or 5x7 is typically used. Notice how the 4x7 matrix makes upwards for the missing 5th column by slightly slanting the columns. LEDs require much more power than LCDs and are more expensive to industry. This is the uncomplicated reason for their demise from beingness used in calculators.

LIQUID CRYSTAL DISPLAY

The Liquid Crystal Display (LCD) was first developed at RCA effectually 1971. LCDs are optically passive displays (they do non produce calorie-free). As a result, LCDs require all nigh no power to operate. Many LCD calculators can operate from the power of a solar cell, others can operate for years from small push button cell batteries. LCDs work from the ability of liquid crystals (LC) to rotate polarized light relative to a pair of crossed polarizers laminated to the exterior of the display. There are two main types of LCD displays used for calculators today: Twisted nematic (TN) and supertwisted nematic (STN). TN displays twist polarized light to xc degrees and have a limited viewing bending. STN displays were adult to twist polarized light betwixt 180 to 260 degrees resulting in meliorate contrast and a wider viewing angle.

A LCD consists of two plates of glass, sealed around the perimeter, with a layer of liquid crystal fluid between them. Transparent, conductive electrodes are deposited on the inner surfaces of the drinking glass plates. The electrodes define the segments, pixels, or special symbols of the display. Next a thin polymer layer is applied on top of the electrodes. The polymer is etched with channels in order to align the twist orientation of the LC's helix shaped molecules. Finally, polarizing films are laminated to the outer surfaces of the glass plates at xc degree angles. Normally, 2 polarizing films at xc degrees should be dark, preventing whatever manual of light but due to the ability of LC to rotate polarized light the display appears clear. When Air-conditioning voltage is passed through the LC, the crystals within this field align then that the polarized light is not twisted. This allows the light to be blocked by the crossed polarizers thus making the activated segment or symbol to appear dark.

References:

Alan Sobel, "Electronic Numbers", Scientific American, pp. 64-73, June 1973.

"Note on the Liquid Crystal Display Industry, https://web.archive.org/web/20160418181108/http://www.auburn.edu/~boultwr/lcdnote.pdf (annal link).

"Brandish Technologies in Japan", http://world wide web.wtec.org/loyola/dsply_jp/toc.htm.

"Sharp - World of LCDs", https://web.archive.org/web/20000511090945/http://www.sharp-world.com/sc/library/lcd_e/indexe.htm (archive link).

© Text & photographs copyright Nigel Tout   2000-2022  except where noted otherwise.

Source: http://www.vintagecalculators.com/html/calculator_display_technology.html

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