The OP sent me a PM a couple of days ago and I responded. I sent a detailed reply on faultfinding this board but I am unsure if he has seen it? I will reproduce it here in case it helps others with problems in this area. This same technique applies for testing of the original board as well as the Homepin replacement although it isn't as easy to fault find the original without the green "tell tale" LED:
I suppose the best way to start the fault finding is to separate the different functions of the board.
Let me say first though, reversing a diode (or LED) usually will not cause any harm. More damage will be done soldering and unsoldering it usually. Generally it just won't work if installed incorrectly. Diodes are designed to operate being reverse biased.
You can easily test the 24" opto board on the bench. Make up a couple of plugs - one with a TX and one with an RX LED soldered directly to the plugs, don't use long wires, just the leads on the diodes. (see picture) Plug them into the board and apply 12V to the correct pins on the connector (+12V to pin 5 and ground to pin 4). With the two IR LEDs facing each other, and only about an inch apart, the green beam light will almost certainly be ON. If it is NOT double check the orientation of your TX and RX LEDs and if still not on, get back to me!!!!!!!
Use a steel ruler, knife or similar to break the beam to confirm the green light goes on and off. Paper won't work here, it must be a metal sheet or similar.
This is the first step to prove that the board is OK and working as it should be.
A further step you could do to confirm that the board is working 100% is to wire a spare LED in series with a resistor (value of resistor is not critical, anywhere between 100R and 2K2). Fit this to the 5 pin plug between pins 3 & 4 (4 is ground) with the Cathode end of the LED facing pin 4 (ground)-(refer picture) and run a jumper wire between pins 1 & 5 of this same plug.
This LED will then turn on and off as you break the beam and PROVE that the OUTPUT from the board (that connects to the switch matrix) is operating as it should.
If you do all of the above you can rule out any issue with the board itself.
Next step is to plug the board into the machine and ensure it has 12V applied. Connect the TX and RX LEDs mounted in the machine. IGNORE anything else as you just want the green LED on the board to come on and off when you break the beam. Forget about the test mode in the machine. Just get the green LED on the 24" opto board working NOTHING ELSE!
If this doesn't happen you have faulty wires or plugs to the TX/RX LEDs or faulty RX/TX LEDs or incorrectly oriented TX/RX LEDs.
Once you have the green LED going on/off by placing your hand in the beam across the playfield, turn your attention to where the OUTPUT of the board goes in the games switch matrix.
Do the above procedures in the order suggested and once you have the green light working by breaking the beam DON'T go back and start changing TX/RX LEDs as you have confirmed they are working.
Move forward logically with fault finding - don't jump all over the place - get each section working, or confirm it is working and move on to the next.
Once working, this section of BSD is every bit as reliable as any other machine - it's just that the board is a bit of a mystery sometimes and it's easy to start jumping all over the place when trying to pin down an elusive fault.
Please let me know how you go.
EDIT: can't attach a picture to this message so please go here: