I don't think it's been mentioned so far, but those Chinese kits use AD584s that have been rescued from scrapped equipment. That's how they get the price down; they're not fakes, as far as the community can tell. The date codes on the ICs is the clue
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Lots about these on the EEVBlog forum. For example:
http://www.eevblog.com/forum/beginners/t17704/
Failures aren't unheard of, but they seem to be very rare. To have 2, of different designs from different suppliers, seems to be particularly unlucky. I went down the DIY route - which was probably for the best based on your experience
