Thank you very much for your help. I tried a replacement Mainboard and Power Board last night (from a local parts supplier nice enough to loan me a few of each to try out) - same problem (which made me think it has to be a firmware issue - but I could be wrong).
Question re LED strips you mentioned:
Does your HiSense/Roku splash screen look like mine (see picture)?
Mine looks "normal" which would indicate LED strips working and display is "Ok".
Note: This splash screen shows for a second then screen goes black for 10 seconds, then repeats over and over forever (bootloop). Thanks again for any help.
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