This is one of the earlier batches that has a 3R3 (3.3Ohm) current limiting resistor. It’s on the right of the orange terminal block. It was originally put in place to limit the current on the 5VAC line but since then the more recent RPis have been drawing a lot of current so we had to remove that resistor. You can solder a wire across that resistor to bypass it, which should solve the power issue.
But before you do that, there is an easier route which involves using a USB power adapter.
– On the orange terminal block, take out the wire that’s closer to the common (COM) terminal, and also take out the common wire, then directly connect together.
– The other wire (farther away from the common terminal) should stay in the orange terminal block.
– Then plug in a microUSB cable connected to a USB power adapter to your RPi, so that it draw power directly from USB; and also plug in the 24VAC adapter, so that it can power your solenoid valves.
This way, the power to RPi is separated from the power to solenoids, and it should solve the booting issue.