Now It's more than month since I've happily fixed my AppleTV replacing its broken internal power supply with external power adapter. It works great and the entire solution looks pretty good.
I purchased common purpose power adapter MeanWell P66A-1R1B 5В/8.5А (~$40) (http://www.megachip.ru/pdf/MW/P66A.pdf) and reconnected power cable between internal AppleTV power supply and motherboard directly to this external power adapter. The broken internal AppleTV power supply was just thrown away. Everything works just fine and stably. Any suitable external power adapter AC/DC with output voltage 5V and currency >=7.2A can be used as replacement.
If you need more info, or want to see photos how the result looks like - please response, I can provide more details here, or in person.