How to unroot my TF101

The backlight on my Asus Transformer broke and i need to send it in for repair. I bought the tablet used already rooted. I have no idea which root it is, and I can't see anything on the screen. I tried unrooting using the method provided here , but it didn't detect my device. Also it won't go into APX mode and I can't see anything that's happening on the screen. I was thinking I could connect the tablet to a TV, but I don't want to purchase an HDMI cable before I have a sure method that will unroot my tablet. The serial starts with B6O, and since it's the backlight that's broken I can still see very limited parts of the screen.