Black screen on 15" late 2013 to mid 2014 logic board swap

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Hi!

I just replaced my late 2013 15" MBP 2.3GHz logic board to a mid 2014 2.8GHz (661-00680). The replacement board looks mint, like new.

The charging light on the cable works, the fans are spinning up, but I'm getting nothing on the screen and no boot up sound.

I tried a SMC reset, PRAM and NVRAM reset. Tried an external monitor, no output. What else can I try? How can I confirm if it's a bad board or if I messed something up? Are additional steps required for this swap?

Thanks!

 

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Anyone with some board repair experience care to chime in? Any basic things I can check with a multimeter? I have since put my 2.3GHz board back in the laptop and that works just fine.