[Heathkit] SB-300 Question

[w3wjr at aol.com]

 I just received an SB-300 with a small problem. Before I dig into it, I figured I would ask for hints.
 
Unit works pretty well as long as you leave the outer jacket of the coax disconnected. When its connected the receiver goes deaf.
 
I spent a couple minutes on studying the schematic, but figured I would ask for suggestions first.  I feel I am probably looking for something shorted out between the antenna lead and ground near the antenna input of the circuit?  I figured I would run this past the wealth of knowledge contained within this group.  Its probably a common problem.
 
Thanks
 
W3WJR