Here’s some new info I came across while looking through my Philco paperwork. From the “Philco Serviceman”, June 1933 (my comments in [brackets], spellings are as given in the article):
On some models of the 71 and 91 series, difficulty is occasionally experienced with inoperation of the set from 800 to 1500 kilocycles, because the detector oscillator tube fails to oscillate. The remedy for this trouble is usually puzzling to the serviceman, since it is a rather uncommon type of service complaint.
The condition can usually be corrected by changing the type 36 tube in the detector oscillator socket. The tube change alters the overall characteristics of the oscillator circuit in such a way that oscillation is again established, and the set then operates properly.
In some instances the tube change is not sufficient, and it is then necessary to change the [Type 36 detector-oscillator] cathode resistor from 15,000 ohms to 10,000 ohms. This resistor is indicated at (18), Service Bulletin No. 128 [Model 71] and at (21), Service Bulletin No. 129 [Model 91]. The 10,000 ohm resistor is known as Philco part no. 4412 [1/2 watt].
Locations which are subject to prolonged damp weather usually experience this difficulty more than locations having a dry atmosphere. In extreme cases it may be necessary to change the oscillator coil, making absolutely sure that the coil is entirely covered with paraffine to seal out all moisture. [Which is why Philco coils tend to go bad; many were not sealed properly, and the telltale green windings are a dead giveaway. It’s been my experience that when a Philco coil goes bad, it’s almost always the small outer winding wound over, and at the bottom of, another winding. These are easily re-wound, 36 gauge wire is sufficient, and after rewinding, I spray with several coats of lacquer to seal the coil.]
By Ron Ramirez (09/96, R+P Newsgroup message thread.)