A few things to watch on early XL100's
Cold joints around the pin transformer.
Cold grounds from main chassis to mother board.
Module sockets crack & go int.
If any of these it should act up when tapping around.
On the 17" sets vert sockets were a BIG problem. They
would crack & cause the module PCB to burn up.
New socket & a module with no dud value was costly.
If none of that start with 'lytics from the output then
back.
73 Zeno