This was an odd issue, but I solved it.
So here's what happened...
The units were unplugged, and one of them had the fuse inspected. The fuse was fine. There were no obvious faults or burn/scorch marks. To me, it was odd is that TWO units had the exact issue, which is why I though it was MAYBE just fuses, not the boards. The odds of two VCR mainboards dying simultaneously has odds on par with winning the lottery.
I plugged one back in another room, directly into the wall outlet, just to see what would happen ... and it worked.
Perfectly, like nothing had happened.
So we put the fuse back into the power supply on the other deck, the power supply assembly back into the VCR, and close it up. Plugged it in. It worked!?
As it turned out, the 9-month-old APC 1500 $150 backup developed an uncorrectable fault. The power output was only about 90W, and apparently these VCRs won't power up with low voltage. This was the second APC unit that died in the past year, and we're going to stop using them. Several more APC were tossed for capacitor issues, and battery leakage issues (UPS ruined). Even though APC is supposedly the best UPS, I don't see it. We'll be using Cyberpower moving forward.
That was scary. Losing both newly repaired AG-1980P decks would have been hard to deal with again.