The middle pin of the patch switch is connected to the middle layer through the countersunk hole of the welding pad. You can try it like this: The multimeter is set to the test break, and the tip of the countersunk hole is fixed with the tip of the watch pen, and the other watch pen is pointed at the peripheral solder joints. If the ringing point with small on-resistance is found, the line can fly here. If this method does not work, we have to dismantle the same kind of new machine to test.