If you lost an SCR you need to make sure that you do not have any AC in your loop circuit. When you run the car on insp or try to that is, measure with you meter on the loop circuit looking for any AC voltage. If you do have any appreciable amount check your ripple filter. Check the fields and armature separately for grounds and that they have the proper resistance. Look for loose connections in the entire loop circuit. Also check the brushes in your tacho's. Baring all these you may have pooped a drive. Since you lost an SCR you may have many other issues unfortunately.
