Yes, I've another 3B3. I use it in my 564B, and of course, it sweeps correctly.
Okay, so we know the problem is within the 3B3. I'd start by putting your two 3B3s side by side and comparing the wiring of the plugin connector. Start with the 2 wires for the horizontal output. If those have been switched, check the others, since you now know that someone put these back somewhat randomly.
Make sure you don't get confused and start changing the good 3B3. Unless it's obvious, mark the good one GOOD. ;-)
