it sounds like the HF is firing as it should but no arc is established as if you'd forgotten the work return or had a very poor work clamp connection, both of which would effect DCEN too although AC can obviously be a bit fussier with regards to work return connection. My inital thoughts were that it could be someone had messed with the settings in the hidden menu but if it still won't establish an arc with AC after resetting to factory defaults i'd be on the phone to Rapidwelding tomorrow. They're a member here FWIW but i would hope that once they've gone through the usual suspects/checks with you (making sure you're not a numpty) they'd arrange collection or exchange of the unitit doesnt do the familiar ac tig buzzing nose,its as if its trying to start and kind of lights an ark crackles and then switches off??