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

While you are at it, replace those original large filter capacitors. Look for Bally power supply cap kits like the deluxe kit at BigDaddy and swap them out.

Just two caps so no real need for a kit for that one.
Get a screw terminal cap for $9 and high voltage axial cap at $5 and call it good.

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.

Trying to keep the explanation simple as to what U2 does but it's not easy.

U2 is used to select one of the 15 momentary solenoids. Due to how it is implemented, U2 can select only one momentary solenoid at a time. These 15 momentary solenoids are controlled by transistors are within U1, 3 and 4: Seven transistors within U1, Seven transistors within U3 and one transistor within U4.

Of the seven transistors within U4 -- one is controlled by U2 as a momentary solenoid control, four of the transistors are used are controlled by the "Continuous Solenoid Data" lines from the CPU board and 2 of the transistors within U4 are not used.

The 74154 decoder (U2) is not able to switch enough current to the final output transistors. So the CA3081s and their respective 120 ohm resistors provide additional current to the final output transistor -- the SE9302s.
Actually, the 120 ohm transistors provide the required current to turn on the output transistors, the CA3081 is responsible for turning the power 'off' to the output transistors.

Also note that Bally has drawn the output transistors (SE9302s) improperly. These are actually "Darlington Pair" transistors which means each of those SE9302s has two tightly coupled transistors internally. So each solenoid output is actually controlled by -three- transistors. One within the CA3081 and a pair of transistors in each of the final output transistors. Had to throw that in there to add to the confusion factor. Shown below is a 'Darlington Pair" transistor.

Darlington (resized).jpg