I bought a used JDX S7800b, I hope it is a "b". from the button layout I would say so.
- charging indicates charging
- USB makes connection sound in Windows
- Rockchip Batch Tool finds a device and shows it in a pink square with "4-3-3" written on it
- Windows can show a "Android"-device, with nothing else
- or Windows can show two "USB-Devices" like drives, but I can't access any of them.
At starting I have only the "JDX"-Logo, nothing else, after a while the device goes black, I think it is a sleep mode or something like that.
I tried the firmware from here http://www.jxdofficial.com/firmware
Display works, USB works. Also the "reset"-button makes the device to re-start, but not to re-boot.
Also "switching off" via software in Windows worked.
I don't know, if there is a chance from a "pink" device to make it green again or change the firmware.