[MrFixr55]

Hi Rob, 

Put the Unit into Phono and TV and raise the volume control with an open input.  Does the motorboating occur?  If so, issue is in Audio.  See below discussion on shielding.  Also "dress" the speaker cable so it is not near the Audio Ouutput or 1st Audio tubes.

If issue occurs only in radio mode, does the volume control affect the motorboating, or does it only motorboat on weak signals?

Check all shielding, B+ Bypass Caps for RF, Conv and IF.  Check C3.  This is the cap between the AVC Line (Returns of secondaries for IF, Convertor and RF transformers) and Chassis Ground.

Verify that all tube shields are in place.  If no shields were used (No collar around socket), verify that the tubes are metal or are the Zenith "metal glass" tubes.  These tubes have a grey or black coting on the inside walls of the tubes.  This Shield for the "Metal Glass tube or the shell of the Metal Tube must be grounded.  These are almost always Pin 1 and either are connected to chassis ground or jumpered to the Cathode.

Hope this helps.