Recently, I was having troubles with my ice3. At first it was just some intermittent cases of not seeing the target voltage or not reading the signature. But then it worsened to not being able to see any target voltage at all. I know it was not by board because I could use my dragon on it just fine. Then I noticed that my board's power becomes 0V everytime I connected the ice3.
So I investigated a bit and found that the protection diode across Vtg and Gnd is busted. This was why my board's power lines were shorted.
So I replaced the diode and voila! it's alive!
I do not know the part number of the original diode but I replaced it with 1SS181.
Below is a picture the ice3 with the replaced diode.