Is Not Your Fault……..
In some very rare repair cases there are electronics problem that can't be solved. You have spent countless of hours trying to find the solution and even have replaced all the boards but the equipment seem like still have problem. The problem can be intermittent or even solid problem. You could blame yourself for not being able to solve the problem. Let me tell you, it is not your fault, it is the manufacturers fault. In other words, it is the circuit board design fault! The circuit board could have less grounding and that's why certain manufacturers will issue a bulletin to all their repair techs to add in the extra grounding in the mainboard or etc to solve the problem. It can be adding another extra component in the mainboard or any circuit board in the equipment.
This is why, even when you have replaced all the boards in the equipment and the equipment still would not work. The board that you have bought could be from older version. I still could remember a case back in the early 90's when I was still a computer motherboard repair tech for a famous company from Singapore. The problem with the motherboard was intermittent no display after pressing the reset button. There is no way one could solve the problem because it was the design fault. I've got a message (repair bulletin from Singapore) saying that one needs to add in a small resistor between two pins in one of the ICs. That's it and the job done and that motherboard worked well.
In the above case, if you are trying to find the solution (assuming if you did not get the answer) i guess even till old age we still could not repair the fault. When you repair any equipment be sure to check if there is any update be it hardware or firmware because this can truly save your time in electronics troubleshooting.