I wouldn't fret about the extraneous posts...mods have to delete those, and they're not causing any trouble. You've got other things to fret about.
I watched the video.
My short answer is you've got a complex logic control issue going on...beyond me or most lay-persons to tackle. My two bits of advice would be:
1. Verify power supply outputs are appropriate...the power supply needs to be isolated from the rest of the unit (i.e. the outputs should be disconnected from any loads), and all rails should be checked for both DC amplitude and stability, and for AC components on the DC outputs. The DC outputs should be dead stable, and the amplitude of each within +/-0.5VDC of the supply rail's designated output. When checking for AC there should be nearly nothing...a couple millivolts at the most, preferably <1mV. Once outputs are checked out with the supply unloaded, then systems need to be brought back in one at a time and the respective rails checked in the same way again when under load. I know you said the tech you took it to checked the power supply and said "It's all good", but you also stated things that lead me to believe he is not qualified to work on something like your tape machine...unless he tested the power supply as above, the power supply shouldn't be ruled out. If you don't know how to isolate the power supply and use your DMM to do these tests...
2. ...you really need to locate a qualified repair tech and pony up. And for goodness sakes stop doing things like hunting in the dark pulling connectors to see if something happens. I'm not trying to be mean...were you doing that with the unit powered? No....no, no, no. Logic system issues are delicate and complex and you're not going to magically fix it by doing stuff like that...more likely cause more damage. Again, I'm not trying to be insensitive, but I'm a bit passionate about this because there's a hammer mechanic in me that does stuff like that when I'm at a loss, and it's only brought me headaches and heartaches.. You have a nice clean looking machine...do right by it and be methodical and careful in the approach to get it right again. Accept the problem is highly likely not a simple or easy fix. I'm sorry about this...I know how it feels...but accepting it is step #1, and then the work can begin to rectify the problems. I wouldn't power it on again until the power supply is isolated and tested.
What part of the world are you in? Maybe if we know that somebody can advise a good tech or service center to patronize. If it was me, and I cared about the machine and wanted it working again I'd sacrifice a day's round-trip drive to take it to a good shop/tech.
Sorry about your situation. It's a frustrating bitch.