[RodB]

Motorboating sources are many, so you can start with an IF alignment, make sure the AGC is properly connected, find out if the motorboating occurs at all the tone switch settings. Usually occurs somewhere before the first AF circuit. It's a low frequency feedback. Might be because the tube shields are off or a dirty tube pin connection or grid cap. The grid cap wire for the detector tube should be inside the tube shield against the tube.