Can you post some pictures of this board - let's see what you're dealing with. Maybe something peculiar will stand out.

For all coils to lock on at power-up you're looking at multiple failures.

Disconnect connectors J1, J2, and J5 from the solenoid driver board. If your knocker is in the head, disconnect it too. This will isolate all solenoids from the driver board.

Measure the following locations on the solenoid driver board - these are the circuit drive for the flipper enable relay. In attract mode the relay should be off. Let us know what you measure.

SDB_Coil_Drive_Voltages (resized).png

Sure does.
The readings you have on U4 indicate it's bad. If you're familiar reading the schematics, U4 is a CA3081 transistor array chip that simply contains seven transistors.
U4 pin 3 is the base of a transistor. It must never go above 0.9 volts. Your reading was 5 volts indicating that pin has internally gone open circuit. This results in that transistor being unable to switch off the flipper enable relay.

When the transistor arrays fail, it's usually just a single transistor failure in the chip. For every single coil to lock on means there was some catastrophic event to damage all the transistors in the arrays, maybe the solenoid voltage got shorted to the 5V logic supply.

These are all the base pins of the three transistor array chips on the board. Any pins that measure higher than 0.9 volts means that chip is bad.

U1 pin 3
U1 pin 6
U1 pin 8
U1 pin 10
U1 pin 11
U1 pin 13
U1 pin 16

U3 pin 3
U3 pin 6
U3 pin 8
U3 pin 10
U3 pin 11
U3 pin 13
U3 pin 16

U4 pin 3 <--- You already measured this one as bad
U4 pin 6
U4 pin 8
U4 pin 10
U4 pin 11
U4 pin 13
U4 pin 16

Your flipper enable relay voltages all look good now.

Time to check the voltages on all those other pins I mentioned that control all the other coils.