Repairing a Stern MPU-100 Pinball Board With Battery Corrosion Damage

Step 3: Removing Bad Components
After scraping and sanding the majority of the corrosion off the board, you'll have gotten up-and-personal enough with the board that you should have a pretty good idea just how far the battery acid made its mark.  If you found any additional components that were visibly damaged, add the appropriate IC, resistor, capacitor, etc labeling [on the PCB] for each component to your list.

Next, you'll have to use a variety of techniques to remove the components.  Soldering and desoldering through corrosion isn't an easy task to say the least.  If I had to do it over again, I would pre-add solder to each of the solder pads of the components I was desoldering.  Yes, I am saying to add new solder in order to help you remove old solder.. but just a little bit -- you should see the old solder start to mix and melt with the new solder.  This will make it significantly easier to heat up the solder again when you desolder.

You can clip the leads on the components, but I strongly recommend clipping them as high as possible so you can grab the lead with needle nose plier as you apply the solder gun to the other side of the board.  You can try using a desolder gun or wick/bulb.. however I had mixed luck due to the old solder & corrosion.  This is where you just have to figure out the technique that works for the situation at hand.  For instance I used sewing needle to help clean some of the component holes or push tiny pieces of the leads out of the hole.

Removing components is a long and tedious process.  If I had to break my time into percentages I would say the initial sanding and removal of the worst of the corrosion was 25% of my time, removing components was 50% and soldering in the new components was 25%.

Step 4: Cleaning Up Further & Neutralizing The Acid
The next step is to clean up the areas you couldn't get to with the initial sanding.  Any areas that still have battery corrosion crystals need to be cleaned up.  You'll want to see copper, but in some areas you may find that the battery corrosion ate all the way through the copper.. or maybe you sanded too much in that area.  If it's a trace, you may need to add a jumper wire.  If it's on the ground of the board, as long as you didn't break the ground path entirely you should be fine.

Once content with the corrosion cleanup, you'll want to neutralize any of the remaining corrosion as best possible.  With nicad batteries (early Bally/Stern used nicad) this involves using white vinegar as the base to neutralize the acid.  Please note that for other types of batteries you might be using a different base & vinegar might actually make it more acidic.  In this case, vinegar is the answer to neautralize.

Finally, go over the same areas with isopropyl rubbing alcohol.  Realistically, the higher the concentration isopropyl alcohol, the better.  But most people have rubbing alcohol as bought in a drug store and that should do in a pinch.  This will help clean the board further and should evaporate quickly since it's alcohol.

Step 5: Ordering New Components
You can order a battery corrosion repair kit from GPE (Great Plains Electronics) at http://www.greatplainselectronics.com/products_kits.asp.  The kit will include most of the components that are commonly affected with battery acid damage.  For most of the kits, a PDF is available for download so you can check if there are any additional components you might need to add to the order.  In my case, I had to order a few additional capacitors.

Step 6: Soldering New Components
Although some people would recommend it, you don't have to replace *every* component that comes in the battery corrosion repair kit.  If the component had any corrosion at all, you should have already removed it from the board.  If there's no visible signs of damage, the solder points and/or the component's leads are somewhat shiny, then its probably okay.

Solder any components from the original list you made (you still have that list, right??) back in by referencing your original list & part numbers / board diagram that were included with the kit.  If your board looks at all similar to mine where the ground that runs along the bottom of the board had its solder mask removed, then it's going to be tough to solder the ground lead for some of the components.  The solder mask that we removed actually helps keep the heat focused to the solder pad.  With a large portion of the solder mask removed, the large ground area almost acts like a giant heat sink and doesn't take solder very well.  So it might not look pretty, but you may need to use a decent amount of solder and sort of blob it onto the end of the lead.

Step 7: Testing
At minimum, with the board unplugged on your bench, desk, etc you should use the ohm or continuity meter on your multimeter to verify traces and component connections on your board for anything you replaced.  If you had to remove any header pins (where the wire harnesses plug in), definitely verify that there are no bridged pins and they match with their corresponding traces.  You can do that by following the trace to the solder pad of another component and touching the header pin with one multimeter probe and the solder pad with the other probe.  If you accidentally short some of the pins that carry high voltages on them, you could destroy all of the ICs on the board.

The best way to test boards before plugging them into your machine is with a bench power supply.  You can build your own out of an AT or ATX computer power supply since most pinball machine boards will just require 5v power to show you something is working and possibly let you diagnose some things on the bench.  In my case with this board and since it was one of the first boards I repaired, I chose the "cross your fingers and hope nothing blows up" method of just putting the board in the machine.  Luckily for me, the board worked & game came back to life!

Conclusion
Fixing a corroded MPU board isn't an easy task.  It's really better if the board just has some minor corrosion or in the case of this Stern MPU board if the corrosion is minimized to things like resistors, capacitors and not IC sockets.  But, any board that uses through-hole components *should* be repairable, it's just a question of whether the time and effort are worth it.  If you've never done a repair like this before or you don't have any soldering/desoldering experience at all, do not attempt such a big repair for your first project.  I spent probably a total of 30 hours sanding the board, removing components, cleaning up the board, soldering in new components.. so it's much easier from a money standpoint to just buy a new board --- but after you do a few repairs like this you will get faster and learn techniques that will help you with future repairs.  Taking a dead MPU board and making it work again is a great feeling, especially when you get to enjoy some games of pinball after the repair!!