The bane of the later prewar Philcos is the extensive use of rubber insulation on the wiring. Philco even used this wonderful stuff in the power transformer leads of many 1939 models. Fortunately, they soon switched back to cloth in 1940 for their power transformers. Unfortunately for us, they kept using the rubber-insulated wiring throughout the rest of the set in every home model up until domestic radio production ceased in 1942 for the duration of the war.

Likewise, many Atwater Kent sets made after 1930/31, most older Majestic (Grigsby-Grunow) sets, and Zenith radios made after 1938 or so make extensive use of wiring with rubber-coated insulation.

Figure 1. Looking under the chassis of a 1939 Philco, model 39-7T. The power transformer and first IF transformer have been removed.

These sets are not for the novice! The rubber insulation breaks down over the decades, causing it to dry out and flake off. Turn the chassis of the typical 1939-1942 Philco upside down and you’ll likely see many bare, or partially bare, wires, with some old rubber insulation residue on the bottom of the cabinet. The insulation that remains is typically very dry and brittle, and will break apart when touched. This is a potentially hazardous condition, because if one of these sets is plugged in and turned on, and certain wires are touching each other, a fire could result!

Figure 2. A closer look at the wiring under the 39-7T. Notice how the rubber insulation is dried out and cracked.

Remember: An old radio should never be plugged in and turned on until, at the bare minimum, its electrolytic capacitors are replaced and the power transformer checked for signs of overheating.
The rotted rubber insulation issue must be addressed in order to have a safe and successful restoration. There are two ways to solve this problem:

1. Replace all of the wires in the set with new wiring.
2. Remove the remnants of the rubber insulation from each wire, one at a time, and replace with heat-shrink tubing.

Figure 3. Would you trust these wires on this 1939 Philco power transformer?

Let us examine option 1 first. 20-gauge solid wire is an excellent substitute for the original wires, and is available in various colors so that the radio will still look original underneath. Stranded wire should be used for wires, which must flex in use, such as the wiring going to pilot lamp sockets. Again, 20 gauge is fine.

Each wire should be replaced, one at a time, with a new piece of the same length as the original, routed in the same manner as original. This is important! If you merely cut a piece of wire and stick it in, regardless of length and/or position, the radio may not work properly (if at all) when you are finished!

Note: It is very important that you only replace one wire at a time! If you get in too much of a hurry, it will be easy to forget which wire went where, and you may end up with a non-operating radio!

Figure 4. The wires on this transformer are going to receive new insulation in the form of color-coded heat shrink tubing. Remove enough insulation going into the transformer so that you can extend the tubing at least 1/8″ inside the wrapping. This can be done with a very small flat-blade screwdriver, using it as a pick. Work very carefully so that you do not damage the paper insulation!

Replacing the wires running to pilot lamps is a problem all its own, as the pilot lamp socket has rubber insulation inside which has deteriorated by now. See the page on Rebuilding Philco Pilot Lamps for more information.

Figure 5. All of the wires of this Philco power transformer have been resleeved. What is left of the original rubber insulation is shown to the left of the transformer.

Figure 6. The reassembled power transformer is ready for reinstallation in the 39-7T chassis. The wires look much better now.

Now let us turn to option 2. This is actually somewhat easier, as you only have to unsolder one end of the old wire, remove the remnants of the old insulation, slip on new heat-shrink tubing, heat the tubing to make it shrink, resolder the wire back in place and move on to the next one.

The tubing should be shrunk to fit, as it will make the new insulation easier to bend. Plus, the form-fit of shrunk tubing makes the wire look original (if the proper color is used). And, yes, heat-shrink tubing is also available in various colors and lengths.

Figure 7. The completely rebuilt Philco 39-7T chassis. All of the wires that previously had rubber insulation now have color-coded heat shrink tubing instead. The radio now works perfectly, and there is no worry of a piece of insulation falling off a wire.

I have used both options (rewiring and resleeving) in the past; after restoring many of these Philco models, I formerly preferred resleeving; but in the past few years, I have decided that I prefer rewiring as much as possible, saving resleeving for very hard to reach areas such as power transformer leads and wiring underneath coil assemblies. Just remember that as long as you are careful and pay attention to what you are doing, either rewiring or resleeving will work for you.