On a radio with an analogue tuning dial, the tuning knob makes a pointer move relative to the dial to indicate frequency. Meanwhile, inside the radio, something called a variable capacitor (VC) is turning on its shaft. The VC looks like a stack of cooling fins moving relative to a stationary stack of cooling fins, all interleaved. A pulley or drum is attached to the VC shaft, and a spring-tensioned, braided nylon rope connects the pulley with the dial pointer and the shaft directly attached to to the tuning knob. A broken rope keeps you from tuning the radio.
Remove and store all fasteners holding together the radio case in saucers or bins. Slide all removable knobs off their shafts and carefully pry the case apart.
Inspect the tuning mechanism, noting the rope path, around which pulleys and shafts it winds and how many times. Find the tensioning spring; make a diagram and notes on a piece of notebook paper. Use a functioning radio exactly like the one you are working on as an example, or find a service manual or bulletin for the radio. Use the 1947 edition of Sam's Dial Cord Stringing Guide included in the references.
Use a crochet hook and needle-nose pliers to remove the old rope. Again, note how many times the rope is wound around each pulley and shaft. Note what kind of knots are used to attach it to the tensioning spring and where the attachment points are. Make a small ink mark above the exact point where the pointer was when the rope broke. Inspect how the pointer clamp is attached to the rope and draw or describe it in your notes. Note how the rope fits in the grooves and spring notch on the tuning drum attached to the VC shaft. After you remove the rope, do not turn the VC: leave it where it is.
Lay the pieces of the old rope out straight and have a helper hold down the ends. Measure each piece and add enough length for knots at the ends. Cut a new rope and tie bowline knots at both ends. Thread one knot loop around the VC drum stud where the rope starts. Use a crochet hook to thread the rope through its entire path and back to the tensioning spring at the VC drum or pulley. Make sure to follow your notes to wind the rope around the tuning shaft the correct number of times. Leave no slack between pulley and shaft points.
Reattach the pointer clamp to the rope at the point marked behind the dial. Turn the radio on and tune it to a station. Wait for the station to announce its broadcast frequency and while firmly grasping the rope to keep it from moving, slide the pointer along the rope to the correct spot on the dial.
Things you need
- Screwdriver assortment
- Prying tool
- Service manual or a working radio like it
- New drive cord
- Crochet hook
- Needle-nose pliers