Repairing a Tomzn or EARU Voltage Protector requires some electronics knowledge and basic tools.
Since each device could be different so that the causes and solutions as well.
Most common causes could be Faulty capacitors, Blown fuses or MOV or resistors etc
Check the device for signs of physical damage, burn marks, or blown fuses.
Use a multimeter to measure input and output voltages, with power supply connected. Inspect capacitors, resistors, and MOV or fuses
Replace faulty components by soldering iron.
I have purchased this Multi-function Voltage Protection device from Aliexpress that was faulty on arrival,
as you can see it is constantly flickering and not working at all.
While inspecting the Circuit board there is No MOV Protection (Metal Oxide Varistor) that gets faulty every things looks in order, all the resistors and IC's are Ok and Nothing is Heating up,
With the Power Supply Connected voltage drop across two Capacitors was in consistent, At Capacitor 1 should have 12v Supply that is step down to 5v at Capacitor 2 via 7805 Voltage Regulator IC in Between both.
The Problem get Solved If we supply 5v to Capacitor 2 from Auxiliary Source indicating either Capacitor 2 or Regulator IC is Faulty.
But the Problem also get Resolved If we supply 12 to Capacitor 1 from Auxiliary Source indicating it is the Capacitor 1 that is Actually faulty while Capacitor 2 and Regulator IC was OK.
it is a 25v, 470uf Capacitor that is need to be Replaced.
Here i have Tested the Solution with a Large size Capacitor for Demo purpose.