I had sears come out

I had sears come out three times to fix the problem. Had the power supply line cleaned & board. It worked fine for acouple of weeks and now back to sqare one.Try to turn power on then automaticilly shuts itself off.But it does turn on i have picture sound & color. Sears i have to order a new pc board,but they say that Mitsubishi no longer makes that boaard. i find hard to believe.