Did you resolder the relay's solder joints on the solenoid driver board?

Did you replace the diode/resistor pair behind the transistor?

If yes and still not working, then you would want to test the CA3081 and the 74LS154 to make sure they pulse high/low as expected.

You can use your DMM. From Pinwiki:

"Normally, all sixteen of the decoder output lines are held high (+5vdc). When strobed, the decoder lowers one of it's sixteen output lines, depending on the pattern of the four input signals."

"With no input supplied (strobe is high), the output lines of the decoder are high (+5 vdc). This puts a voltage at the base of Q1 (this transistor is one of seven in the CA3081 transistor array chip). This turns Q1 "on" and the voltage supplied to it's collector via resistor R1 passes through the transistor to ground. At this point, little or no voltage is present at the base of Q2, and Q2 is "off". With Q2 off, the 40vdc at the coil has no place to go, and the coil remains deenergized."

Using the schematic you could determine what line the relay is on. Then with one probe on the correct leg of the chip (don't short two together) and the other on ground you could see if the state changes when running the solenoid test. If it does - then you know the CA3081 is probably bad. If it doesn't then you can suspect the 74LS154 or the PIA

Oh yeah that is right. Thanks Andrew!

Not useless - now you know how the board works. If the signal never makes it to the CA3081 then yes something wrong with signal path. Could also be one bad output on the PIA. More likely is the input is bad on the CA3081.

If you had a second working solenoid driver board you could swap. If the fully working board also works in the Future Spa then you know its on the other Solenoid Driver Board.