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Photo 4. Electrolytic Capacitors

Photo 6. Diodes

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Photo 5. Light Emitting Diodes

Photo 7. Transistors lum, ceramic, polycarbonate, polystyrene, bead tantalum, ceramic disk, silver mica and paper. The general physical construction is similar—layers of foil are separated by non-conducting material (dielectric). The material used, its thickness, and the number of layers determine the amount of current the capacitor can store, and the amount of voltage it can stand before the dielectric shorts across.

In general, ceramic disk capacitors are used wherever only ball-park capacitance is needed. These caps are temperature sensitive and have a tolerance of ± 20 percent or worse. Paper capacitors are good for higher voltages, and dipped paper caps are fairly stable. Polycarbonates and polystyrenes are used in audio circuits because of their cleanness and low noise. Silver mica is the choice for close tolerances (within five percent or better), and bead tantalums work well wherever precision, durable high-capacity parts are needed. The TRS-80 uses ceramic disk capacitors throughout, except in its power supply section.

Capacitor values are marked in fractions of a farad. Now, a farad is one heck of a lot of capacitance; look at Photo 4. These are electrolytic capacitors, where the current capacity is increased by using a chemical dielectric. The one on the far right weighs in at half a pound and is less than one-twelfth of a farad! The smallest capactor in Photo 4 has a value of five microfarads, and the smallest capacitor in Photo 3 has a value of 10 picofarads. Capacitor values are abbreviated mf, uf, or fif for microfarads, and pf for picofarads. Sometimes the f is capitalized (mF, pF), and older texts occasionally use the term micromicrofarad in reference to picofarad.

Numerical abbreviations in picofarads are found on capacitors like those for ohms on resistors—474 will mean 470,000 pico farads (0.47 microfarads), and so forth. Again, British standards are somewhat different (and even more sane): Since the most-used capacitor values are midway between the picofarad range and the microfarad range, they will write these values in nanofarads. So don't be thrown if some peripheral devices specifies a "22 nF" capacitor—it means .022 microfarads.

The symbol for a capacitor shows its function well. Two current-carrying plates are separated by a non-conducting material. Electrolytic capacitors are marked by a positive ( + ) sign toward one end to indicate their placement in a circuit (Example 5).

Examples. Capacitors and Electrolytic Capacitors.

Example 6. LED

Choosing Replacements

There may come a time when you have to replace a resistor or capacitor in your computer. To do this you must obtain a part of equal or better quality than the one you are replacing. For resistors, ask for carbon film, 1/4 watt, five percent tolerance, which can be found as part of the Radio Shack 271-1300 series. If the Technical Reference Handbook specifies wattage higher than 1/4 watt, make sure you get the higher wattage or else you'll watch the resistor darken, smoke, and snap in half. You may, on the other hand, substitute a higher-wattage resistor for a lower-wattage one if it physically fits in place. Make sure that the resistance you choose is within five percent of that specified, unless you are familiar enough with the circuit that you know you can substitute another value safely.

When you replace capacitors, 50-volt ceramic disks are fine for the small ones scattered throughout the board. Radio Shack sells these in its 272-120 series. The Technical Reference Handbook parts list specifies 12-volt types for about 30 capacitors, but erring in the direction of a higher voltage is wise. Where 16-volt electrolytic capacitors are specified, 25 or 35-volt replacements will do fine. Also, since electrolytic capacitors have a percentage of error of +100, - 50 percent at best, don't hesitate to replace a 220 mf capacitor with a 270 mf capacitor.

Displays

One of the prettiest parts of electronics is its human interface—light-emitting diode (LED) and neon displays. Photo 5 is a collection of these displays, including (at the top left) 10 red, orange, and green single LEDs, from jumbo to micro sizes; (second row) nine seven-segment numeric LEDs; (third row) three seven-segment, multi-character, "bubble" type displays, popular in calculators a few years back; (fourth row) a jumbo seven-segement LED and a multi-character neon display; (vertical row at right) three larger neon displays from desk calculators; (top center) a bar-graph LED used for hi-fi level controls in place of meters.

The schematic symbol for a light-emit-ting diode is the same as that for a diode

(see Example 6), with a few arrows representing light emission.

Diodes and Transistors

Diodes and transistors are the building blocks of logic gates and other integrated circuits, but I've left them till last because you will find so few of them in your computer. Photo 6 shows (at top) three bridge rectifiers, which contain four diodes each; (left group) zener diodes, used for controlling voltages; (middle group) small-signal switching diodes, found in certain interfacing and logic circuits and for switching unwanted signals out of a circuit; (right group) rectifier diodes, for transforming alternating current top pulsing direct current. The larger diodes at the right can handle higher power.

The schematic symbol for an ordinary diode consists of an arrow pointing at a bar, showing that electricity may only flow in one direction through these device. You may see either of the two symbols in Example 7; the surrounding circle is more common in older schematics. There are

Example 7. Diodes variants on these diode symbols, the most common of which is that for the zener diode. A zener diode conducts electricity in one direction until the voltage rises over a critical point. Then it undergoes a temporary breakdown, letting the voltage conduct backwards through the device. It is an electronic safety value. The broken lines at the edges indicate a zener diode (Example 8).

functions. In Photo 7 you can see three general types: (top row) audio transistors; (second row) four small-signal switching transistors; a tiny radar transistor; and five high-power transistors.

Transistors usually have three leads, because they are switching or amplifying devices. Two of the leads represent entry and exit points of some type of circuit; the third lead controls the flow of current between the entry and exit points. Switching transistors flip on and off almost full; audio transistors follow the waveform present on the third lead and mimic its level. I won't go further with the theory because of the many types of transistors, and the numerous considerations needed to design circuits using them. Among the schematic diagrams, see Example 9.

Example 8. Zener Diode

Transistors were the breakthrough over 30 years ago that made your TRS-80 possible. Although you will find few individual transistors inside present-day computers, there are still some used for non-logic

Example 9. Transistors

More to Come

In a future column (I'm not promising next month anymore!) I'll present a rundown on what to do when your computer breaks down, some precautions to prevent it, and, when things get desperate, what replacements you can make that will get you up and running. Included will be how even a rank amateur with 10 thumbs (except my friend Paul) can remove and replace a bad IC.

Updates.

I'd like to thank the hundred or so readers who wrote after reading my report of the minimal response to February's reader survey, and to those who reacted with dismay at my suggestion (in July) that I might be moving away from writing about the TRS-80 Model I in favor of the Color Computer. Yes, I probably will, and here's why: I feel the Color Computer is faster and more powerful than Model I, II or III. More on that sometimes later.

To those who wondered: Yes, the interview with Dr. Lirpa was an April Fool joke. No, it wasn't a five-volt salami, it was a daisy ham borrowed from the Roxbury Country Store (thanks, Neal). Yes the program does increase keybounoe rather than feliminate it. No, the photo wasn't a secret shot to me, it was my wife Claire (put on your glasses, folks!).■

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