Quoted from John_C:From my reading on ferrite cores, you want them to be placed as close as possible to the source of 'noise'. Since we don't know the exact source of that noise, we have to take a WAG.
My suggestion is a small core near each blood target on the left side of the playfield and a larger one near where those targets connect to the main board (the far-left, upper most plug that I identified earlier).
I was going to order some but since you have some and GreenMachine19 has some coming, I'll hold off pending your testing and results.
Not a HW owner, but a recently retired high power RF electrical engineer with a lot of ferrite core (actually more iron cores than ferrite) experience.
Yes - close to the noise source. If in doubt, use two cores, one placed on each end of the wire - then you're covered regardless of the noise source.
I don't want to get too technical, but the cores can get saturated if there's too much current running thru the wire, lowering their effectiveness. This can be eliminated if both the source wire and the return wire go thru the same core. i.e. A solenoid fires and draws 10A, if the 10A comes in on a wire, and 10A leaves on the other wire (opposite direction), but they both go thru the same core...the net current the ferrite core sees is 0A, so it doesn't saturate. So if you think it might be from a solenoid firing, make sure you grab all the wires associated with that solenoid if possible.
If it's just from a switch transient - not really any current, so you'll likely be OK.
Sorry for the ferrite core sidebar - just wanted to add a FIY