I checked the unit this morning. It was getting power. And the fuse was still good.
I removed the circuit board and looked for any burned spots or cracks or damage. The unit looks like it's still new.
Then I noticed a connector on a square patch in the upper right corner of the enclosing box was blackened -- actually the sleeve around the connector was burned. This was the yellow wire (to the fuse, I think). I pulled the connector off and cleaned it up. It had originally been connected, but NOT pushed all the way on, so it was just barely connected.
I plugged the wire back in, making sure to push it all the way in, reinstalled the circuit board, connected the power and external chlorinator cell and flow sensor plugs, turned on the power, and it all works again!
You all just saved me $500! I really appreciate the help.