Many people in this thread have gotten a defective ATU-10 from Chinese vendors on eBay/AliExpres/....? These have quality components but bad manufacturing (cold solder joints etc.). I too got one of these bum units from eBay.
BUT, here is the encouraging story: The seller hison-818 was absolutely great to work with. We went through and ruled out user errors, he understood my configuration in which I was testing the ATU-10 (Rig to ATU-10 to a known good Dummy load with plenty of power capacity). Within 1 day of triaging the problem, he dispatched a new ATU-10 after locally testing it in the same configuration. I received it yesterday (5 days later), worked perfectly. Last night i WSPR'ed on multiple bands with plenty of hearing k1jbd reports on WSPRnet. He did not ask for any additional charges, postage or even to return the old unit (which would be very expensive from US to china from a postage point of view).
Based on the above I can highly recommend hison-818, a honest vendor, who has excellent customer service and cares about the customer.
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73 de k1jbd
bammi
WS1M - bammi