Got to open things up this weekend. Just wanted to reply back in the event it might help someone else.
First, I was wrong about it backfeeding; I shorted/burnt the triac for zone1 of the expansion board. This seems to have only burnt the fuse on the main unit (which sourced power to the expansion board…no backfeeding).
I applied USB power to the main unit and it tries to start up, so that’s a good sign.
I shorted out the fuse and ohmed out the 24VAC pins…got something in the 6-7 Mohm range, so there are no shorts there. This was done with the expansion board connected but with the valves/outputs disconnected. Reading should be retaken once the fuse is properly replaced and outputs are hooked back up, prior to applying 24VAC.
I placed a digikey order for some spare fuses and triacs. I suspect I could just avoid zone1 on the expansion board but it shouldn’t be too hard to replace the burnt triac.
Triac digikey partno: 1740-1007-1-ND
Fuse digikey partno: F4985-ND
Attached is a photo of the expansion board and burnt triac.
I’ll try to report back in 1-2 weeks after things are (hopefully) back up and running to confirm the above.