I just looked up some voltage mod info for these and this is exactly why the U1 can only be overclocked to 2.0GH/s because it's limited to 0.8v, which the U2 has a 10k resistor already installed from Bitmain in the R2 slot which puts it at 0.9v minimum.

I'm not sure if Bitmain upgraded the R1 resister? What is the standard running voltage of the Antminer U2? Anyone know if Bitmain upgraded the R1 resistor so I would know where to start from?

It's really a shame they didn't make this upgrade easier as it's a SMD resistor, it's going to suck to change, and to change 3 times until I figure out what maximum voltage and overclock frequency to keep a stable temperature with how many fans I have, and then do it 9 more times for the other ones lol.