If you replaced Tristar and are confident that it was a successful replacement (and a good part!), then the other possibility is the PMIC and associated circuit that has to drive the current. Unfortunately, there is no schematic available so it's hard for me to reference anything for you.
You will either have to probe until you find it (check the coils to see if they are solidly anchored) or outsource it to someone who already has a lot of experience on those devices.