In the previous installment, we spent much time describing how and why capacitors can give a restoration hobbyist grief. Unfortunately, they are not alone as their closely allied cousins, resistors, will likewise keep you busy. You see, resistors play many roles in electronic equipment. In the most basic sense, they provide a load for active devices such as used in tube or transistor amplifiers.
Resistors are used to adjust DC voltage levels to that appropriate for active stage operation. For example, the cathode resistor in a triode or pentode audio amplifier stage has a voltage developed across its terminals that is a result of Ohms Law (i.e., cathode resistor value in Ohms multiplied by the tube’s cathode current in milliamperes). This developed voltage drop, in turn, establishes a bias voltage on the tube’s grid element, thereby establishing the tube’s operating point.
Resistors are used in combinations to establish various voltages in step with the network/device’s current draw. Resistors are also used to control system gain/loss, to broaden the frequency response of tuned circuits, and to stabilize amplifiers. They are also a key element of feedback networks, where they are often employed to lower distortion in amplified audio or radio-frequency applications.
Due to their wide application within the field of electronics, resistors are manufactured for various purposes. Depending upon the type of equipment being restored, it is likely to include variable resistors (potentiometers), fixed value/general purpose resistors and, in the power supply section, some large wire-wound types. All come in a myriad of values, temperature characteristics, power levels, and accuracy tolerance.
Fixed value resistors may range in value from a fraction of an ohm to many megohms and in form factors that can safely dissipate between 1/8 watt to hundreds of watts of heat. Within the scope of vintage radios and test equipment, the most commonly encountered resistor is the fixed-value carbon resistor with typical power ratings of ¼ watt to 2 watts.
Carbon type resistors, either carbon composition or carbon film, comprise the bulk of resistors used in consumer products and amateur/commercial radio rigs. These resistors are low in cost and relatively easy to manufacture in large quantities. The carbon composition type is made from carbon granules that are compressed into a cylindrical shape, covered with a phenolic plastic, and with the value indicated by a set of color bands. These are available in 1/8 watt to 2 watt power levels and with tolerance values between 5 and 20%. The higher the value’s precision (tolerance), the higher the cost. In the typical amateur or broadcast radio receiver, one will usually find ½ watt/20% tolerance units.
Carbon resistors are not long-term precision parts, meaning their value will gradually drift with the passage of years. Humidity absorption and over-heating due to a circuit anomaly will cause resistors to depart from their indicated value more rapidly. Other resistor technologies have evolved to overcome these shortcoming, however, carbon composition resistors continue to be produced due to their important radio frequency characteristics and ability to survive short term overload conditions.
The passage of an electric current through a carbon composition resistor creates minute electrical noise. In most circuit applications, noise of this level is of little concern however it may become an important factor in sensitive radio receivers, test equipment, and high-fidelity audio amplifiers. Film types were developed to improve the noise performance of resistors in such applications.
A film resistor is made by sputtering either a carbon or metal material onto a small ceramic tube. The part’s resistance value is controlled by the type and thickness of this deposited material. Final resistance “tuning” is accomplished by etching a spiral groove along the tube’s length. The film resistor is coated with a compliant paint and/or epoxy. The part’s value is either screened onto the outer tube or via painted stripes as per a typical carbon composition resistor.
Carbon and metal film resistors are true precision types and are available in tolerances of 1% or less. Available power levels span the range of 1/8 watt to 3 watts. In commercial applications 5% tolerance parts are typically used. Film resistors are far quieter than carbon composition types and have overtaken use in consumer electronic products.
Where resistors of high power levels are needed, those are constructed using nichrome wire wound onto a ceramic tube. The completed part is next coated with either vitreous enamel or a ceramic/cement material to protect the wire from physical damage. In vintage radio equipment, the power rating of such resistors spans 5 watts to 25 watts. In transmitter power supplies, where such resistors are used as bleeders across filter capacitors, the power ratings might span 25 watts to 200 watts.
The ohmic values of wire-wound power resistors are in the order of 0.1 ohm to 100,000 ohms. Due to physical size, the value of such is printed directly onto the completed part.
While most wire-wound resistors are fixed-value parts, some are constructed with either taps spaced along the resistor’s length or a strip of the outer coating is intentionally left open, thereby exposing the nichrome resistance wire. A sliding contact is then provided, which allows movement of a tap to the desired part of the resistor’s value.
Fixed resistors as described earlier are two-terminal devices, with a wire lead or terminal attached to either end of the resistance element. A three-terminal rotary device, termed a potentiometer, allows for both the connection of the whole of its resistive component, but with the added feature of a movable tap. Thus, the potentiometers serve as continuously adjustable voltage dividers and are used often as audio volume and voltage/signal level controls.
Like other resistor components, potentiometers are power rated. Those whose resistive element is a carbon-deposited film are available in 1 watt to 5 watt configurations. Higher power units, using resistive wire, are available at higher power levels.
While potentiometers are assembled principally as single units controlled by a single rotary shaft, it is possible to configure these parts as dual potentiometers each operated by the same rotary shaft. An accessory snap-switch can also be stacked onto many potentiometers. An example of this configuration’s utility is where the snap switch is used to energize a radio receiver and the potentiometer next serves as its volume control.
How Resistors Fail
A fixed resistor is a simple two-terminal part, sealed in phenolic or enamel. It has no moving parts. What could go wrong? Plenty, unfortunately, and mostly due to something other than itself!
While doing your inspection no doubt you’ll spot some obviously wounded resistors having discolored or charred bodies. In some cases, the only thing left of a former resistor is its two wire leads extended into space -- since the resistor’s body had blown itself to bits. Sadly, those resistors were forced to dissipate far more heat than designed…but why?
The usual culprit: a shorted bypass capacitor. A shorted capacitor can force cause the upstream resistor to sink excessive current (higher heat load) and the eventual destruction of the resistor. So, when spotting a damaged resistor, be on the lookout for a nearby shorted capacitor. In fact, shorted bypass capacitors often cause an allied tube to draw excessive current – which can damage both the tube and its associated plate resistor.
Just because all look good via visual inspection, resistors can be sneaky. Keep in mind, the equipment’s designer selected the various resistive values in your specimen project for good reason. They’re used, for example, to establish correct voltage potentials on tube plate, control grid, and screen grid elements. So, what happens if resistors in critical stages shift their ohmic value? Nothing good, coupled with a lot of head scratching.
Make it your practice to check by measurement the actual value of carbon resistors (the most unstable of the lot) to confirm they are within each part’s indicated tolerance. Recently, I restored a unique and somewhat rare radio transceiver: National NCX-1000. It is more like a prototype since so few were made…and of those made, most are known to be inoperable. Thus, it was a red flag challenge.
This particular rig is comprised of many printed circuit boards and most of the board-populated resistors (100+) were ¼ watt 20% carbon composition types. In this one set, I found over half had drifted up in value by as much as 200%!! No wonder folks had trouble restoring these sets! I replaced them all with carbon film 5% parts (along with a handful of electrolytic capacitors) and the rig actually works great.
Take a breath….I doubt seriously you’ll find anywhere near as many out of tolerance resistors in a single project. It took me 40 years of rig restorations to stumble onto this one. The odds are with you….in my case, it was meant to be.