I wound up getting a Panasonic UTP-2 transcoder to supplement my homebrew solution. Once I had it up and running, I decided I should at least attempt to apply the scientific method to things to see how my handiwork stacked up to a pro solution. So I hooked my cables back up and dug up a tape with color bars on it. I turned on my trusty BVU-870, and... well... The AUTO OFF indicator lit up and the LED counter started blinking weird characters. It wouldn't load a tape and it wouldn't pass video. And since the universe tends toward maximum irony, it had to happen at that moment.
I power cycled the unit several times, and eventually the LED display stopped lighting up at all. This wasn't at all good. And the only help I got from my old friend Google was a suggestion that the moisture sensor thought there was condensation on the video heads. I read through the service manual and found nothing helpful either. I sent an Email to an old TV engineer I know who used to work with a lot of BVU-800s and he had never run into anything like it either.
I had other things to do so I set the problem aside, occasionally turning the unit on and off to see if anything changed. Nothing. A few weeks later, I took a trip to see my folks in Illinois. While I was there I took the opportunity to pull all the boards from my spare machine which lives in a closet there like a heavy but benign troll. Swapping them all out when I got home made one difference. Now every light on the control panel lit up. But it still wouldn't load a tape.
Finally I pull the thing out of my equipment rack and break out the "THORY OF OPERATION" manual (Engrish or Typo -- YOU DECIDE!) and decide to track down and basically short circuit the moisture sensor to see what would happen. That's when I stare at the fuse panel on the back and decide to take a peek inside. And sure enough, there's a 5V fuse which had blown out.
A trip to Radio Shack and $3.50 later, and my BVU-870 runs like a champ. Which makes me simultaneously grateful and annoyed, because I could have fixed it a month ago if the symptoms would have pointed me more firmly to check the fuses. I mean, why the hell should I expect there's a bad fuse when the machine powers up and the fan starts running?? Or something in the troubleshooting section of the service manual, maybe? Not to mention the time I spent digging out and packing up the spare boards to bring back to New York, which now are in a box here in the apartment taking up precious space. Not to mention I now have a U-Matic deck spread across two states and I won't be able to use the spare in a pinch since its guts are a thousand miles away.
Oh well. It works again and now I can do some tests.