Quoted from Pinballdad:

Quick tip. My Dredd had trough optic issues one day. I visually checked and all looked good.Could not get #3 or #4 to trip though in the test menu as I loaded the balls into the trough one by one. I took the front trough board out I think it is receiver, can't remember, and examined real close. I actually needed my magnifying glass (or an ohm meter if I thought of it) to see a resistor termination broken. I mean it was real hard to see. So quick reheat with an iron and it was good to go. There is a resistor on that board for each ball spot. If you take the board out (which is real easy) make sure to check them all. The trough get hit alot obviously with balls falling into it. The shockload transfers directly into the metal guide and then into the boards that mount to it. Over time it is enough to crack solder connections. I may remount my boards on to some sponge rubber washers/spacers in order to reduce/eliminate the impact.
Hope this helps someone. I would only buy the newer boards as a last resort.