820-3478 iMac 27" Thunderbolt ports not working

[CRC2018]

Got an 820-3478 board in off an iMac 27" 2013 today but neither of the thunderbolt ports will work. If I connect a monitor to either of them monitor is showing connected no signal but there are no mirroring options coming up.
Is this likely to be burned U3210 and U3310?

 

[2informaticos]

Check if you get proper voltages on both Thunderbol vias, pages 30-32.
Is more likely a problem in a common area, than have both vias damaged.

 

[CRC2018]

Think you are correct as with further visual inspection U2800 looks discoloured like it has a burn mark on it. It's got like a brown spot on it.
Is there a source for replacement U2800 or it's a case of pulling one of a donor board only?

 

[2informaticos]

Not idea reallu if is board dependant.
It has a TBT ROM separately.

You can try any, but always better to get from donor board, to be safe...