90s 9v regulator lost its steps

Greetings, my 9v regulator recently started acting up. It no longer steps the voltage output up per each dial. It now just goes from 0 to 12v no matter what notch it’s on. Fwd or backwards. As you can see see I have upgraded the diodes and a more capable power supply and added a fan for cooling. (I run 6 motors pulling 19 cars so the extra power helps). I have ran this as is for several years so I don’t think it was a mod. What can I test further to check what broke? Thanks submitted by ...