Quoted from cfh:0. If there's an orange 12 volt filter cap on the bottom board, replace it with a new 10,000 mfd 20 volt cap. Time: 2 minutes.
1. Take the main green ground wire that attaches to the lock in the backbox, and bolt it to the power supply frame. This involves undoing two screws. Note this wire goes down to the bottom board's brass metal "ground central", where all the green wires connect together. Time: 10 seconds.
2. Modify the power supply supply with a 3" piece of wire. Run it from the power supply negative lead of cap C1 to the metal power supply ground frame. Time: 1 minute.
3. Run a 6" piece of wire from the CPU board negative lead of cap C1 to the metal frame of the power supply. Time: 1 minute.
4. Run a 18" piece of wire from the ground plane of the driver board to the metal frame of the power supply. Time: 1 minute. Check out a picture to make sure you do this step right. It's a bit tricky to find the right trace.
5. Optional: Double up the +5 and ground lines on the double sided connector going from the CPU to driver board. Note this is the modification Todd does. Frankly it's helpful but not mandatory if steps 1-4 were done above. But go ahead and do it, as what the heck, it's 6" of wire and four connector pins. Time: 2 minutes
6. Don't listen to Todd anymore in regards to Gottlieb grounds.
edit: got the location off the pinwiki!
I'm doing the ground mods on a Sys 80b right now, using John Robertson's images here: http://www.papinball.com/tips/sys80b_Ground_mods.html?sa=X&ved=0CBwQ9QEwA2oVChMI1NeUzIrwxgIVRqWICh3FkwTK
Most of this makes sense to me when using cfh's list, but can someone clarify where the ground attaches on the 80b power supply? Looks like it comes out the bottom in the pictures, but I can't tell where it's attached. John also uses a different location on the MPU.