International projectionist (Jan-Dec 1948)

one million ohms and is sometimes represented by a small omega, u>. If the rating does not appear on the resistor as a number followed by "OHM" or "MEG," we shall probably find spots and bands of colored paint. According to the RMA (Radio Manufacturers' Association) color code, the value of a resistor in ohms is given by three dots or bands of color. Here is how the code is used. The RMA Color Code The first color represents the first figure of the resistance rating, the second color the second figure. The third color represents the number of zeros following the first two figures. But how are the first, second, and third colors distinguished? In one type of resistor the first color is the color of the body of the resistor, the second color is that of the tip, and the third color is a dot placed near the middle of the resistor. The order may well be memorized: body, tip, and dot. In another type of color-coded resistor the colors are in the form of bands, the first band being the one nearest the end. A fourth band is sometimes added after the other three. This band indicates the degree of accuracy of the resistance rating as given by the first three bands and is usually gold or silver (but sometimes brown, red, orange, or yellow) . Table A shows the complete RMA color code. By using this chart you can quickly "read" the rating in ohms of any color-coded resistor. The current-carrying capacity of resistors, given in watts, is a thing apart from the resistance value in ohms. If the current-carrying capacity of a resistor be exceeded, overheating of the unit will result. The smaller resistors are molded from a mixture of powdered carbon and clay: the greater the proportion of clay, the higher the resistance. Like projector carbons, they may become cracked or broken. Excessively high temperatures ruin them by melting the solder which secures the lead wires to the carbon. Among the injuries which commonly befall wire-wound resistors are cracked porcelain coil supports, short circuits, and broken or severely oxidized resistance wire. Testing Resistors It is usually a simple matter to test a resistor. If the resistance value is very low, a battery of two or more dry cells and a d-c voltmeter may be connected in series with the resistor to be checked. However, most of the resistors used in amplifiers have resistances of many thousands, even millions, of ohms and accordingly will not pass sufficient cur rent from dry cells to register on the voltmeter. It is always possible to use high-voltage currents and earphones, but the procedure is rather clumsy. There is a better "kink" that may be used by projectionists who have "non-syncs" with phonograph pickups of the crystal type. Here it is: Disconnect the "hot" lead (red wire) which connects the crystal pickup to the amplifier input. Insert the resistor into the circuit (in series with the pickup), turn on the amplifier and play a record. If the resistor be a good one, music will come from the speakers but with more or less reduction of volume (because the action of a resistor is to reduce voltage). The volume reduction will not be great unless the resistance is higher than 500,000 ohms. No sound indicates that the resistor is "open." Do not be surprised if the non-sync sounds better, even if fainter, with a 500,000-ohm resistor in the circuit. An increase of "load resistance" has the property of improving the low-frequency response of crystal-type pickups.1 These simple tests do not measure the precise degree of resistance but serve only to indicate whether or not a resistor is "shorted" or open. Accurate checks of resistance ratings require the use of a Wheatstone bridge. Wide Variety of Capacitors Capacitors, or condensers, appear in a wide variety of sizes and shapes. A theatre amplifier contains a large number of them — almost as many as it does resistors. In its essentials a condenser consists of two metallic plates facing one another but separated by some substance which will not conduct electricity. This insulating material, called the dielectric, profoundly influences the size of the 1 See "Non-Synchronous Disc Reproduction" by R. A Mitchell; IP for April, 1948, p. 16. T ABLE A. RMA color code chart. COLOR FIRST FIGURE SECOND FIGURE ZEROS ANNEXED TO FIRST TWO FIGURES TOLERANCE Black — 0 (None) Brown 1 1 0 12 Red 2 2 00 22 Orange 3 3 000 3* Yellow 4 4 0,000 42 Green 5 5 00,000 — Blue 6 6 000,000 — Purple 7 7 0,000,000 — Gray 8 8 00,000,000 — White 9 9 000,000,000 — Gold — — 52 Silver — — — 102 Ho Color — — 202 charge which can be held by the capacitor. In the tuning condensers of radio sets the dielectric is nothing but air! The "plates" of tubular condensers are usually long strips of tin-foil, and the dielectric which separates them is waxed paper. The whole is rolled up tightly and packed in a cardboard tube to make a compact unit. Sometimes a black band is printed on one end of the cardboard tube of a paper capacitor. The lead from this end should be connected to the grounded, or negative, side of the circuit for shielding purposes. The small square capacitors contain brass or aluminum plates separated by thin sheets of mica. Mica condensers, as these capacitors are called, are encased in bakelite to protect them from moisture. You have undoubtedly observed that certain tubular condensers have the word "Electrolytic" stamped on them, and also that the leads are designated as positive (+) and negative ( — ). In some cases the correct polarity is indicated by red and black lead wires: the red being positive, the black negative. These capacitors are made by a process of electrolysis which coats the aluminum-foil plates with a thin film of aluminum oxide which serves as the dielectric. When using electrolytic condensers the indicated polarity of the leads must be observed, else the capacitor will be destroyed. Most condensers have a maximum voltage rating printed on them. A capacitor may be ruined if this voltage be exceeded. Moreover, the rating should be three or four times the steady d-c potential applied to the plates of the capacitor, especially if inductive devices such as chokes or transformers be present in the circuit. The standard unit of capacitance is the farad. This is too big a unit for practical purposes, so most of the capacitors in our collection of parts obtained from an amplifier or radio set will be rated in microfarads {pi) or micromicrofarads (fifii) • A microfarad is one millionth farad. A micromicrofarad is one millionth microfarad (one hundred-billionth of a farad) . Capacitor Tests A color code is often used to designate the capacity of mica condensers. The colors have the same numerical significance as in resistor ratings, but the number should be followed by the word "micromicrofarads" instead of "ohms." The sequence of the three colored dots is frequently indicated by an arrow. We shall not attempt to determine the exact capacity values of condensers, for that would require the use of an expensive a-c bridge. It serves our pur(Continued on page 35) INTERNATIONAL PROJECTIONIST • November 1948 ^