Atari 5200 consoles with controller problems sometimes need their POKEY adjusted to center the controller. There are several discussions on AtariAge such as: https://atariage.com/forums/topic/300217-considering-a-5200-system/page/2/?tab=comments#comment-4749780

It can be done with a loopback connector and a test cartridge or roughly with Missile Command.

This will serve as the loopback connector but will also let you test your console to make sure all the buttons are registering, and let you turn the potentiometers to see the effect - but it would make the worst controller ever. It can be useful if you have a Wico Joystick or similar that uses a Y connector to get the keypad function from a 5200 controller. This will serve as the keypad.