I N T R O D U C T I O N T O E L E C T R O N I C R E S T O R A T I O N

Transcription

1 I N T R O D U C T I O N T O E L E C T R O N I C R E S T O R A T I O N This is a brief introduction to the various components used in vintage equipments. The basic function of each component is explained, along with simple test procedures and replacement guidelines. G E N E R A L T H E O R Y There is an analogy between electronic current and the flow of water. In fact, that is the origin of the term "current" in the context of electronics. Electronic charge may be likened to a certain quantity of water, and the motion of electronic charges likened to a current of water. This chart shows the correspondences: Item Symbol Unit Symbol Analogue Unit Electronic Charge Q Coulomb C Water Gallon Current I Ampere C/sec. = A Current Gallons/sec. Voltage E Volt V Pressure Lbs/sq. in. As with water, electrons always flow in a full circle, returning to whence they came. Electrons, like water, are neither created nor destroyed. This is why we call them "electronic circuits". All substances may be broadly divided into two classes: conductors insulators substances which readily conduct current substances which do not readily conduct current metals, some solutions air, plastics, paper, wood, waxes, most liquids A conductor may be likened to a closed pipe. A battery may be likened to a pump. Physically, many components are made roughly in the shape of cylinders. They have two or more connecting wires, called "leads". Leads may be arranged either axially or radially.

2 Axial: Radial: All components are used to introduce a controlled amount of some electronic property into a circuit. As such, there are three important attributes associated with each component: Value: Tolerance: Rating: how much of the property is introduced how accurate the value of the component is what stress level the component can withstand

3 The tolerance is universally expressed as a percentage of the nominal value, and is usually symmetric. That is, a 10% component is expected to be within 10% of nominal value, either high or low. So a component with a nominal value of 20 with a tolerance of 10% could have an actual value anywhere between 18 and 22, inclusive. A component may have more than one rating associated with it. As an example, consider a car or motorcycle battery. Its value is its voltage, which in a car is usually 12V, in a motorcycle may be 6V. The rating is its capacity, which is in ampere hours, sometimes stated as cranking amps for so many seconds, which is the same thing. The tolerance in voltage is usually not given. As another example, consider wire. Wire is normally not considered to be a component, though it may comprise a significant part of the circuit. Its value is zero, and it has three ratings: current voltage temperature the maximum current the wire can safely carry the voltage the insulation can withstand the maximum temperature the insulation can withstand During restoration of electronic equipments, it is often necessary to replace components. When replacing vintage components with modern ones, it is important to use components of the same type, general construction, value, and adequate rating. When replacing components, the general rule is nearest nominal value, tolerance equal to or better than original, and rating equal to or exceeding original. Usually, it is also important to position the replacement as nearly as possible as the original was placed. This includes the placement of wires, which normally must be positioned as closely to the chassis as possible. Leads should normally be no longer than necessary for proper placement of components, with perhaps a zigzag in longer leads to allow for heat expansion. Failure to observe proper positioning of leads or "lead dress" can result in weird symptoms and misbehavior due to unintended coupling between circuit components or wiring. A note on values: Over time the standard values available from manufacturers have changed. Generally, if a vintage component has a value which is no longer available, one can substitute the nearest modern standard value without problems, according to the following table. Note that only the significant figures are shown, without regard for decimal placement.

4 Vintage Modern Value Value ===== ===== Example: 0.05 uf capacitor replaced with uf capacitor. Many times the natural unit for a component is much larger or smaller than what is used in a circuit. In order not to have to write many, many zeroes, subor multiple units are used. They have names comprising a prefix (also called a "multiplier") followed by the natural unit name. Examples: Prefix Multiplier Pronunciation ====== ========== ============= µ, u x 1/1,000,000 micro m x 1/1,000 milli K, k x 1,000 kilo M x 1,000,000 mega µµ, p x 1/10^12 micromicro, pico milliamp, kilovolt, microfarad

5 INTRODUCTION TO COMPONENTS: R E S I S T O R S Resistor: a component used to introduce a controlled amount of resistance into an electronic circuit. Resistance: the property of converting electrical energy into heat. Analogue: narrow in a pipe Current High Pressure Reduced or limited current flow Pressure Drop Low Pressure Value: Resistance Unit: Ohm Symbol: Ω, ω Rating: Power Unit: Watt Symbol: W Rating: Voltage Unit: Volt Symbol: V Note: Sometimes in vintage literature, M = x 1,000 instead of K, with Meg = x 1,000,000 Designation: R1, R2, etc. Typical: R2 470K 1/2W Symbols: A note on symbols: Any time a component is adjustable in value, or used as a control, this is indicated by putting an arrowhead on a movable or adjustable portion, or an arrow through the entire symbol. Sometimes, variable components are electronically distinct, but physically connected so that they may be adjusted in unison. Such ganging of components is indicated by a dotted line connecting the adjustable parts. When multiple units are physically a single component, often a dotted box is put around them. See the schematic at the end for examples.

6 Function: reduce voltage and limit current, turning energy into heat. Uses: voltage reduction or division current limiting controls filtering discharging bias heat B+ division, line drop screen resistors volume, tone, brightness B+, AVC B+, grid leaks cathode resistors heaters in tubes Case Styles: Radial Dog bone composition (vintage only) Wire wound (tap optional) Sand Block (vintage only) Axial Composition Flexible (vintage only) [broken] Flat wire wound (vintage only) Special Controls

7 Materials: carbon composition wire wound carbon film metal film vintage or modern vintage or modern modern modern Finding Value, Tolerance and Rating The values of most resistors are indicated using a color code. The reason for this is historical, with manufacturers hiding the actual values behind the codes they used. Atwater Kent was notorious for this. In the early 1930s the Radio Manufacturers Association (RMA) promulgated a standardized color code, variations of which are used to this day. The code uses colors to indicate digits or "multipliers" as well as tolerances according to the following table. Color Digit Multiplier Tolerance - Black 0 x 1 Brown 1 x 10 1% Red 2 x 100 2% Orange 3 x 1,000 Yellow 4 x 10,000 Green 5 x 100,000 Blue 6 x 1,000,000 Violet 7 Gray 8 White 9 Silver 10% Gold 5% The order of the colors is roughly the one they appear in the rainbow. Note that the "multiplier" indicates the number of zeroes to add to the value, as a power of ten. Each resistor has its value given by two digits, a multiplier, and optionally the tolerance is indicated. If the tolerance is not indicated, it is 20%. High tolerance modern resistors use three digits rather than just two, but they are not covered here. To find the actual indicated value and tolerance, one converts the colors into digits and multipliers, and reads off the tolerance, if it is given. For example,

8 suppose the color code on a resistor is yellow, violet, yellow, silver. The correspondences are yellow 4 violet 7 yellow x 10,000 silver 10% The nominal value is then 47 x 10,000 = 470,000 ohms The range of acceptable values is anything within 10% of this nominal value, or 423,000 ohms to 517,000 ohms, inclusive. A resistor so marked which had a value outside this range would be considered defective, and in need of replacement. On dog bone resistors, the colors appear as paint, which is also the only insulation on these resistors. The order to read the colors is Body, End, Dot or BED. A 470,000 ohm 10% resistor would have a body colored yellow, one end violet, and a dot on the body which is also yellow. In this case, actually, there isn't a separate yellow dot on a yellow body. The end opposite to the one colored violet would be silver. The resistor shown is 51,000 Ω 20%. On cylindrical resistors with axial leads, the colors take the form of stripes around the body of the resistor. The stripes are placed closer to one end than the other, and are read in order from that end toward the middle. The resistor shown is 1800 Ω 5%.

9 Vintage wire wound resistors sometimes take the form of a small bakelite box with leads coming out of each end. One end is pointed, or there is an arrow on the body. The colors are read in the direction the arrow points, or from the blunt end toward the pointed end. The resistor shown is 420 Ω 10%. Some vintage wire wound resistors are covered in a flexible cloth cover, having colored stripes woven into it. The easiest way to get the values of these resistors is to read the parts list, since the colors have often changed with time. The one shown is broken. The power rating of a resistor is determined by its size, but this is difficult to evaluate, since resistors have historically been getting smaller for a given power rating, and a vintage 1/4 W resistor may be physically larger than a modern 2 W resistor. The voltage rating of a resistor is usually not indicated, but may be found from the power rating (for modern resistors) as follows: 1/4 W 250 V 1 W 450 V 1/2 W 350 V 2 W 450 V One must be careful that both ratings are adequate for the intended use. The voltage ratings for 2 W and higher power resistors varies with manufacturer, but are at least 450 V. In special cases, consult the manufacturer's literature. Testing Resistors To test a resistor, use the ohmmeter function of a meter and measure its value. Note that sometimes one must disconnect one end of the resistor to prevent interference from other components which may be in parallel with it. This is because ohmmeters pass a current through the component, and measure the resulting voltage, assuming that no other path for the current exists other than the component. The only way to be absolutely certain is to disconnect one end. If you measure a resistor, and find that it is out of tolerance by being low in value, recheck by disconnecting one end. Vintage carbon composition resistors, if bad, are practically always high in

10 value or "open", not low in value. They are frequently bad. Wire wound resistors are very stable with time, and except for the flexible ones with cloth covers, normally do not need replacement. The flexible ones are often broken, as shown above, and must be replaced. Example: A 470K 20% resistor measures 520K. Is it acceptable? 520K - 470K 50K = = 10.6% 470K 470K Since the resistor's value is within 20% of nominal, it does not need replacement. Selecting Replacements Generally, modern carbon composition, carbon film, and metal film resistors may be used to replace any vintage resistor, so long as the values are proper, and the ratings are adequate. Wire wound resistors should not be used to replace any resistors except wire wound resistors. The tolerance of the selected replacement should be equal to or better than the original. Generally, modern 5% resistors may be used anywhere, with few exceptions. Do not trust the power rating of vintage components, even when they can be ascertained. One reason the vintage component may have failed could be insufficient power rating. The author has seen 1 W resistors used in locations in which they dissipated 1.1 W, and frequently 1/3 W resistors are used where 1 W resistors should be used in cathode circuits. The formulas to compute dissipated powers are or where E x E P = R P = I x I x R P = actual dissipated power in Watts E = voltage across the resistor in Volts I = current through the resistor in Amps R = the nominal value of the resistor in Ohms

11 Power ratings of resistors are for operation in open air with no nearby sources of heat. For this reason, always derate the power dissipating capabilities of a resistor by a factor of two or preferably three. Example: The cathode resistor of a 50L6GT tube is shown as being 150 ohms 1/2 W 20% and measures 280 ohms. The voltage on the cath ode is shown as being 7.5 V. What action should be taken in regards to this resistor? = = 80% This resistor is out of tolerance, hence defective, and must be replaced. The power dissipated is P = 7.5 x 7.5 = W 150 Example: Derating by a factor of two, we need at least 0.75 W power handling capability. The nearest standard power rating at least this high is 1 W. The replacement resistor should then be 150 ohms 5% 1 W A 47 ohm 10% resistor used in the heater circuit of a four tube AC/DC set measures 80 ohms. The tubes are 12SA7, 12SQ7, 50L6GT, and 35Z5GT. What action is appropriate for this resistor? = 70% 47 This resistor is defective, and should be replaced. Referring to the tube manual, we find that the heater current is 0.15 A. The power dissipated is P = I x I x R = 0.15 x 0.15 x 47 = 1.06 W Derating by a factor of two, we need a 2 W resistor. A modern 47 ohm 5% 2 W resistor is the indicated replacement.

12 Variable Ganged Variable Multi unit