I had similar odd readings on my 817N...twice...
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High SWR, odd power and swr readings...turned out it was the finals on their way out.
Thankfully they are relatively cheap and quite easy to replace...but be warned, i replaced mine, all was working perfectly, until i plugged the battery pack back in, then it started all over again...
Yes the finals had blown again...!
I ordered a new power board, but before fitting it i checked the battery pack and one cell had died, seems it dumped the current straight into the mosfets..what a crazy "Undocumented Feature", but since replacing them for a second time, and buying a new battery pack all has been good for the past 2 years or so..
So check your battery pack and don't leave it in there constantly is my advice.
And use some high quality, non conductive thermal paste, mine was very dry which doesn't help moving that heat into the sink.
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Hope that helps
Gavin