You can test all the 6821 on the CPU board using Leon's test rom. You need to burn a copy of the rom, install this on the CPU board, make sure all connectors except for the power supply are removed or set the board up on a bench. Then with a logic probe you can see the outputs from the 6821 toggling. It is fairly common to have one or more ouputs die on one of the 6 x 6821 chips on the board. If you need to remove a faulty 6821 you need to be very careful of the tracks which run between the 6821 pins as there is only about a 1mm clearance.

Hope this helps.