Sony KDL-40V5100 No Image
 

A customer brought this set in a few days ago, she had taken it to a previous shop where they replaced the power supply and mainboard, but failed to fix the problem (yet they still charged her $250). She brought the two old boards, and the supposedly-new ones were still installed.
Initially, the relay would click and the power light would turn green, but nothing else would happen. I was able to get the backlight to power by tricking it (3.3V to backlight_on) but nothing else happened. I noticed that once I returned the backlight circuit to normal, the backlight would now power by itself.
Both power supplies produce the same symptom, and at this point I can rule them out as they appear to be functioning correctly. But there is a difference with the mainboards. With the original board installed, the power light and backlight come on and stay on, but nothing else happens - no click from the speakers etc). With the "new" mainboard, the same happens but the set powers off after about 10 seconds, with no error blinks.
I tried to find some evidence of output to the T-con board using a scope, but there appears to be nothing at all - no discernible blips etc when various functions are pressed.
I have used this method in the past to successfully diagnose plasma sets, so at this point I'm inclined to suspect a bad mainboard. I also would have to conclude that the previous shop either received a bad board (which they should have returned under warranty) or installed a known bad board and charged the customer for a new one (in which case they shouldn't even be in business).
Is there anything else I should check before ordering another mainboard? The customer doesn't want to gamble much more money on this set nor do I want to gamble much time on it.
Thanks for any advice.