With apologies to Fred Foy, Brace Beemer, and the writers of “The Lone Ranger” radio program, “Return with me now to those thrilling days of yesteryear. The Analog Dial Pointer rides again!”…or not, if you’re unable to repair the damned thing.
If you’ve never peered inside the cabinet of a 1950s-vintage radio, you may have missed seeing one of the most frustrating and diabolical technological innovations of the electronic age—the dial-cord drive. The dial-cord drive’s task is simple: translate a control’s rotation into actuation of an electronic component while providing a visible analog indication of the component’s setting.
In an AM broadcast-band receiver, twisting the tuning knob rotates the tuning capacitor and moves a pointer along a frequency scale. The Depression-era advent of molded plastic cabinets and miniaturized components made table-model radios popular, and dial-drive designs proliferated as cabinet stylists’ imaginations ran wild. Complex drives may include a 2- to 3-ft length of dial cord, several miniature pulleys, one or more tension coil springs, and an indicator dial or pointer. A few test instrument designs also include dial-cord drives.
In practice, dial-cord drives suffer from a number of failure modes: cords slip and break, and tension springs stretch, often as a result of users’ overtorquing controls. Over time, extensive use wears and polishes pulleys’ surfaces, which results in slippage. Mice residing in stored equipment use dial cords as nesting material.
Repairing a broken drive requires a supply of dial cord—typically comprising a nonstretchable core overbraided with nylon, or in some applications a bronze or stainless-steel cable. Dial cord comes in several diameters, and selecting the wrong size can prevent a successful repair. Finding replacement tension springs may prove challenging, but don’t overlook junked ink-jet printers as a source. Helpful tools and supplies include a bottle of rosin dissolved in alcohol (an antislippage compound) and a collection of hemostats and forceps (available in outdoor-supply stores that cater to fishing-fly crafters). You’ll need a restringing diagram (if available), manual dexterity, patience… and the persistence of the Lone Ranger! T&MW