How to troubleshoot the rewinder?

We have two steps to troubleshoot the toilet paper rewinding machine:
First, the photoelectric protection function of a fully-open toilet paper production and processing equipment failed. The machine can still work after emergency treatment. However, because there is no protection detection, the operator is worried about personal safety, resulting in low work efficiency.
Second, remove the circuit board and perform a visual inspection. It is found that the substrate around the power bridge stack has changed color. It is concluded that the bridge stack has been burned out. I replaced the bridge stack and powered on for a test run, but there was still no response. Use a multimeter to measure and find that the newly replaced bridge stack has also been burned out, so you can judge that there is a short circuit. Checking the components one by one from the bridge stack down, we found that a three-terminal voltage regulator block (7805) was broken down. We bought the components to replace them. After powering on, the power indicator light on the circuit board came on. The operator installed the circuit board on the machine and followed normal procedures to check whether the photoelectric protection function worked. He covered the light eye with his hand and the falling knife stopped immediately. He tried it several times and found it was normal.