Just wanted to post this in case anyone else is stumped with long opto issues. My long opto stopped working, and the transmitter had failed. I replaced it, still nothing. Replaced the receiver, still the same issue. I looked closely at the board, and R4 had burned up. Instead of 10 ohms, it was 2 Meg. Replaced it, it immediately burned up again. (Amazing at how fast the smoke escapes from these quarter watt resistors!)
I figured a short to ground, checked, could not find anything. Checked the capacitors with a capacitance checker, all checked good. Burned up another 10 ohm resistor. Removed the board again, and then after reading about all the opto board issues, I changed out C7 (100 ufd, 35V), and lo and behold, it worked. Interestingly, I checked the old capacitor out of circuit, it read 99 ufd, and an ESR of 3.4 ohms. So, I am guessing the capacitor was breaking down under voltage, causing the short.
So, bottom line, if your board has a burned R4, look at changing out C7, even if it does read good.