RaTTuS wrote:your micro usb lead is rubbish
Tinderbox (UK) wrote:... microusb cable ...
Not sure those are relevant, since the http://www.suptronics.com/xseries/x300.html
has on-board supply feeding directly into the GPIO header.
However, the lightning bolt and lack of PWR LED is indicating that the 5v is below 4.86v.
If it really is low it won't hurt the Pi, and only some things won't work - such as some usb devices.
I'm going to assume that all three supply methods - X300, computer supply via the header, and micro-usb (from a wall-wart?) - can't be wrong**
Also, the first two bypass the polyfuse, so that's innocent.
What else have you connected to the Pi?
If nothing, then about the only culprit could be U4, the low-voltage detector.
** that's not a given. If you're feeding your X300 with spider's silk, and using the same from your computer supply to the header, and
using a rubbish micro-usb lead, there's your problem.