4.87 volt on the 24V AC pin is clearly not right. Just to be sure: the 24V AC pin should be measured using AC voltage level on your multimeter — if you measured it as DC, the reading would not be correct.

In any case, if the 5V pin is giving you correct voltage, I think at least the power supply section is working fine. Are you not getting 24V output on any valve, or just some valves? Do you see any visible damage on the board?