Early Philco radios up to around 1932 used a novel method of winding the RF coils. The secondary coil of enamelled copper wire was wound directly on the form in solenoid fashion. A plastic strip was wrapped over the cold end of the winding and the primary coil of cotton-covered bare copper wire was wound on the plastic strip. The drawing below shows the method.
The strip was made of Celluloid (cellulose nitrate), the only thin plastic available at the time. This clever construction anchored the primary and provided a level surface to wind it on, but had a fatal flaw. No one knew that Celluloid would slowly decompose over the years, especially under warm, humid conditions.
When Celluloid decomposes, it releases nitric acid which rots the cotton cover and eats away the bare wire of the primary. Luckily, the enamel coating protected the secondary wire, so it is usually in good shape. You should routinely check the continuity of early Philco coils and not be surprised to find open primary windings. Even if the coils seem OK, they may be in precarious condition. I have had coils open up after an hour or so of playing.
Bad primaries are easy to rewind. Disconnect the wiring and remove the coil from the set. Carefully count the number of turns on the primary if your model is not one of those in the table below. Don’t try to count turns by unwinding; the wire will break and crumble. Count by dragging a pin or needle over the coil surface so you can feel each turn as the pin passes over it.
It’s also a good idea to make a sketch of the coil before taking it apart. Remove the primary wire and remains of the plastic strip. Salvage as much of the strip as you can to use as a pattern. Immerse the entire coil in dilute ammonia and rub it gently with a soft toothbrush to remove any acid residue. Rinse it thoroughly with water, and let the coil dry.
The number of turns for three models I have restored are given below:
Cut a new plastic strip of the same dimensions as the original with notches in the same places using the remains of the original as a pattern. The plastic used in blister packages is about the right thickness. Unexposed photographic film which has been boiled to remove the emulsion can be used. 32 gauge enameled magnet wire has the same diameter as the original cotton-covered wire used in Model s 20 and 70. Model 90 used slightly thinner wire. Any motor repair shop should have this wire.[Ed. Note: Enameled magnet wire can also be obtained from your favorite antique radio restoration and repair sites on the web.]
Put the new plastic strip around the secondary coil in the same location as the original and use a bit of tape at the top to hold it temporarily in place. Solder one end of the wire to the starting lug, bring it straight up, anchor it in the bottom notch, and wind on enough turns to hold the strip in place. Remove the tape and finish winding the required number of turns, anchoring the wire in the upper notch. Bring the wire down to the other coil lug and solder it.
Wind the turns close together with no gaps using enough tension to make a tight winding. Give the new primary a coat of lacquer to make sure the winding doesn’t slip.
by D. K. Owens