I’ve measured one of the solenoids in isolation, and it reads 35 Ohms resistance. Measured from the terminal blocks they’re between 35 and 38 ohm depending on the cable run lengths. A couple of my normal zones are at about 38 Ohm and trigger reliably, but the master solenoid at 38 ohm doesn’t – presumably because another zone is already active.
After writing that, I pulled it out to have a proper look at it. I noticed the voltage across the main capacitor was just sitting at power supply voltage, which seemed wrong, also I could detect no boost voltage at all. A closer inspection of the board shows that QB2 has blown, and possibly QP2 as well. I’m also measuring dead short across across RP2, but that could be QP2 acting up. My best guess is that RP2 has failed short and blown QB2. QP2 might be ok. I can’t see anything else that’s obviously blown, so I guess I’ll grab a couple of parts and see what that does.
Edit: After desoldering some bits, QP2 and QB2 are both blown.