From these pics it is not easy to tell. The bottom 2 pics don't really tell much, and really don't look like corrosion damage. The top pic is the one you want, but we would need close up of the area where the battery was and surrounding components. That's where you would likely find related corrosion damage.

Intermediate or experienced techs can often repair the boards, even corrosion damage. There is a process to it.

Other than your pics, go through some of the basic checks before attempting to turn on the game. http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm

Then let us know what you have questions on/get stuck on.

Not too bad, looks fixable.

That looks pretty clean to me. When I get a new (to me) Bally, I always start at the rectifier board and get it solid. Then I move solenoid driver board, and then the MPU.

There's a lot of good info here:

http://www.pinwiki.com/wiki/index.php?title=Bally/Stern

The board definitely has corrosion damage. It also looks to me, when zoomed in.. that corrosion made its way up to the 5101 socket up through the ground trace that goes to pin #8 on the 5101 (right-side of C13). The 5101 socket's pin 8 and pin 7.. also look like they have some corrosive green/blue tint to them. When looking straight down on a good socket, all the metal pins in the socket should look shiny. If they're blue/powdery looking, or broken off completely.. the socket is toast and needs to be replaced.

In your case.. I definitely think corrosion made it to the 5101 socket. Doesn't look like it got hit AS BAD as some boards out there, but the 5101 socket will need to be replaced and all the corroded traces on the board cleaned up and neutralized (including outer edge of the board, ground trace going to the 5101, and any pads under the 5101 that got hit).. possibly a few resistors, capacitors, etc that it also worked its way into off of that ground plane on the outer edge of the board.

You can tell where corrosion made it under the PCB conformal coating cause it'll be kind of speckled darker and lighter green. The really nasty stuff is at the outer edge of your board on the ground plane that is somewhat grey/green in color. Again, I think it's not as bad as some boards out there and shouldn't be a bad cleanup job for someone.. but it definitely needs attention.

Those are intentional, yes the schematics show which ones are to be tied together. Using solder wick to remove solder, you would see that the copper traces are joined.

You've got me a bit worried, jimy. I can tell from your posts you are very inexperienced. I've been in electronics for 3 decades. I've repaired these corrosion damaged boards, that involves the neutralizing/cleaning corrosion damage, replacing the reset section components and replacing the socket for the 5101 and to do the job carefully and correctly it takes me around 3 hours, and they have all been successful. Are you sure you are armed with enough information/practice/tools to do this job? Please do take time and ask questions. You did right asking this one.

There is a deep and quite complete reference for the repair of this problem here under the subtitle
"2a. Before Turning the Game On: Removing the MPU Battery and Fixing Corrosion." It would be good if you read it before beginning this job
http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm

In addition to what wayout said, if you dont have practice soldering, a corroded bally MPU is probably not a good place to start.

I would get some electronic kits and junk circuit boards to play with. Build kits, remove components from junk boards to practice etc.

I'd look at it this way... if you have the means to replace the board it is the fast route. There is a lot more wiring, boards and parts to take care of and repair, so if something else goes wrong it can help to speed up the troubleshooting by giving you an out for the MPU. ex. "I just replaced the MPU board with a testing fully working one, so the chances of my problem there are slim"

On the other hand if you want to learn and perhaps save money (I say perhaps because if it becomes a real dog, you might end up replacing it anyway in the long run) you can attempt the repair yourself. The challenge is aquiring the knowledge tools and techniques/practice that are going to take more time.

Deciding which way to go is all you, mostly - "How soon do I want to be able to play the game, and have it play reliably." is a question to ask yourself.