Ok, Sounds like you have the proper book and followed it. You stated you replaced access 4?
Check POT1 power supply for a short to BNEG at ACCESS 4 CA412-5.
Just replacing pot one won't fix a short. both these codes are closely related. I would want to drive it with a meter attached so I could see what's happening at CA412 -5,7, and 8 with respect to battery pos and neg. Code 487 is CA412-5 shorted to bat. neg code 490 is CA412-8 shorted to Bat. Pos. Hope this helps.