Hope this video helps others resolve the mysterious power trouble issue for some TASCAM model 12 mixers. after some digging, phone calls and research it was found that others had luck replacing the "stand by switch" at their dealer/repair shops. Mine is out of warranty but thru my local dealer we were able to contact TASCAM and they were kind enough to send out a replacement switch. possible it could be a loose Molex connector as well as you will see in the video. good luck and hope this helps.
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and let me know if you have another fix or if this worked for you
:D