String ‘em up!

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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

Sources and supplies If you’re determined to repair Grandma’s table radio or a 1960s-vintage sweep generator’s nonworking slide-rule frequency dial, the following sources (among others) of dial-drive belts, cables, cords, and tension springs may prove helpful (hobby-supply and hardware stores also may offer small tension springs): www.adamsradio.com www.dialcover.com Manufacturers’ service manuals generally offer the most detailed dial-cord repair diagrams. Howard W. Sams Co. published two compilations of dial-drive configurations that cover older radio receivers but lack detailed information: books.google.com/books?id=lhpTAAAAMAAJ&dq=editions:UOM39015006086865 books.google.com/books?id=rBpTAAAAMAAJ&dq=editions:UOM39015006086865 When dial-drive repair is impractical, consider substituting a frequency counter. To accommodate a superheterodyne receiver’s intermediate frequency, the counter design must include a frequency offset, as described in this example: www.norcalqrp.org/fcc1.htm An Internet search will yield frequency-counter designs that can be modified. Here are two examples: cappels.org/dproj/30MHzfmeter/30MhzFmtr.html www.qsl.net/zl1bpu/MICRO/COUNTER/index.htm After struggling with a dial-cord replacement, tip your hat in salute of the patron saint of dial-drive designers—engineer and cartoonist Rube Goldberg: www.rubegoldberg.com …and the persistent Lone Ranger: www.lonerangerfanclub.com/radio.html