Sony Plasma Wega KE-37XS920 No VideoSignal

If you still have your receipt for repair...take it back for repair again.
This time, you should have some kind of warranty on the last repair, so you should have to pay minimal (if anything) to get it fixed again.

If it is the logic board, insist that you are given the faulty one....that way you can see that it has been done and you can then dispose of it at your leisure, "knowing" you have not beem conned into paying for services not given.

Hope this helps