Now a word about that wonderful PHILCO Wiring used in sets from 1939 to 1942 when production stopped for WWII.  They tell me it was rubber covered, but I really don’t know what it was.  It was just awful stuff!  They tell me that the repair of PHILCO Wiring is not for the NOVICE, but if you take your time, you can do it.  All of the “rubber” insulation breaks down over the years, dries out and flakes off.  If you turn that 40-150 chassis you just got upside down, you will see a lot of bare wiring and partially bare wires that will brake apart when touched.  They even used it in their power transformers.
Some people just put sleeving over the wiring without taking the transformer off.  They slide the sleeving up the wiring inside the transformer covers.
The problem with doing this is the wiring inside the transformer, under the covers, is touching causing a short in the transformer. The only way to do it right is to color code the wiring with paint dots and remove the transformer.  Remove the transformer covers and then put the sleeving on, running the sleeving right up to the paper insulation.
The rest of the wiring can either be replaced or re-sleeved.  Do one at a time and try not to change the length or the route of the wiring.  Use heat shrink sleeving.  If your going to replace the wiring use 20 Gauge Cloth Covered Stranded Wire, do NOT use solid wiring.  If you put a nick in solid wire and you bend it, it will break.  I have seen it break inside of something and you don’t know it broke.  A real hard thing to find!!