15:19 You can significantly reduce external IF pickup by using a trap filter in the path from antenna to tuned antenna circuit, or if a ferrite rod is used, between the output of the tuned RF and the mixer tube. Almost any european radio after WW2 made use of that. And if you had done the alignments on the german radios you "fixed" by the book, you would have known for setting the trap is the last step in the RF/LO alignment. Go watch Electronics old and new to see how its done.
Hum can also come from way out of tolerance carbon comp resistors in the RF or IF stages. Sometimes a weak tube also can cause hum. Check the whole radio out for these conditions, as it is always the best service technique on a radio that is this old. Duh.
