I had the same problem with my ATV520 not showing any image to my TV. I contacted the support desk, and the following solution worked for me.
1.Download below firmware and don't unzip.
www.geniatech.com/down-eng/androidtvbox/update_stvmx_20130805_eng.zip
2.Copy all files into SD/TF card root directory
factory_update_param.aml
factory_update_param.ubt
spi_MX.bin
uImage_recovery
update_stvmx_20130805_eng.zip
3.Insert SD/TF card into the box;
4.Press and hold the upgrade/reset button, plug in power adapter;
5.Hold the button for about 10 seconds;
6.Loose the upgrade button, wait for 1-2 minutes(screen has no display at this time),
and then normally you will see robot screen,it is updating the firmware;
7.After finished, the box will restart.
NOTES:
1.Don't use USB disk,only can use SD/TF card.
2.Don't unplug the power in all process until the upgrade is complete.