JD500b starting problems.

[PaulJD500b]

I have a JD500b diesel backhoe. It has starting issues and I am losing my mind. Problem came on all of a sudden. It just clicked at first, wouldn't even start with a jump.I replaced the starter relay on the firewall. It still just clicked, but then none of the dash lights would come on and there was intermittent power to the ignition switch, but the machine would start with a jump. Now it won't start at all. Tested the batteries and they would drop to 6v when the key was turned, so replaced the batteries. Then when I tried to connect the new batteries the starter kicked when I attached the positive cable. Figured solenoid or starter, replaced the solenoid, no change. Thought maybe ignition switch, took it out, no change. What is going on?