Lincoln SW 175 Not Welding ??

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Hello. I am pretty much just a hobby welder, and my Lincoln Square Wave 175 has been with me for about 6 or 7 years (It's not the "pro" version). It's always been great and worked very well for what I do. I'd say I only use it 3 or 4 times a year.     Recently, a friend bought me his aluminum valve cover to weld a fitting onto. No problem, I though. The material was only about 3/16" thick, but for some reason I couldn't get the aluminum hot enough, and couldn't weld it. Ehh...I thought, I'm not the best aluminum welder anyway, so it must have been me.  But today I was just touching up a mild steel piece, and could not get the material hot enough! WTF! I cranked her up to 175 (which should have blown the piece in half) and nothing...The high frequency starts, the arc generates, but it's like it never get's hot. No matter what amperage I put it on (it does however seem to get *somewhat* weaker when I pull it down to 30 amps) - but still not hot enough to start melting the workpiece.   What gives. I opened the cover, checked spark gap, contacts look clean, nothing appears burnt.  What else should I check?  Thanks in advance...  MJB