Saturday May 18, 2024

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

Repairing a Stern MPU-100 Pinball Board

Author: Wayne Eggert
Date: 04/16/2010


My father bought a 1979 Stern Hot Hand Pinball Machine in non-working condition last October from a pizza shop owner who acquired it from a pawn shop in unknown condition and hoped to fix it up.  The pizza shop owner had a pinball tech look at it and was told it would need a new MPU board (approx $200).  Diagnosis?  Dead except for the General Illumination (in other words, any lights that remain lit constantly throughout the game and are not cpu controlled).  Sometimes when switching the pinball machine on it would make a beep, so it seemed like the sound card might be alive.  But that was it.. nothing lit up on the score displays, it wouldn't start a game. He's dead, Jim.  So, on these pages I will explain how I brought a dead Stern Hot Hand back to life.

Valuable Pinball Repair Resources
Regardless if you're just starting out with pinball machines or if you've got a few repairs under your belt, one of the best set of repair guides for just about any year and manufacturer of pinball machines are over at Clay's site:  His information is like an electronics tutorial rolled into various decades of machines and he goes into some awesome detail on identifying and fixing the most common issues with these machines and even preventative modifications that will help reduce stress on the power supply or boards.  I really can't say enough about these guides & the TOP (This Old Pinball) DVDs are also super cheap for the information you'll learn.  Anyway, for repairing a late 70s or early 80s Bally/Stern game, check out Repairing Bally Electronic Pinball Games from 1977 to 1985.  That's where I got enough information to attempt this repair.

MPU Board Inspection
It doesn't take much detective work to figure out what's causing the MPU board to have issues when there's significant battery corrosion.  After opening the back box my first reaction was.. "wow, I'm probably going to need a new board."  I never attempted repairing a board with battery corrosion before & I thought even if I replace all the components, what if it still doesn't work. 

Here's what the board looked like:

Hot Hand Before

If you notice, there's staining on the left-side of the board and the green protective layer (solder mask) on the board is actually bubbled on the right-hand side where the "STERN" print & stickers are.  The bubbling is actually all of the crystalized battery acid that got *underneath* the PCB solder mask.  So even though the board doesn't look *too* bad in the picture above, there's lots of work to be done.

Here's a closer look at the right-hand side of the board:

Hot Hand Before - Another Shot

Notice the bubbled solder mask & the ground lead of the C37, C38, C39, C40 capacitors with crystalized battery acid.  What happened here is as the battery acid leaked from the cathode side of the battery, it followed the ground of the board.  Aside from possibly dripping on components below, battery acid can also defy gravity.. it's like dipping the bottom of a piece of paper in water -- it wicks upward a bit and follows traces and even connector pins and the wires in the wire harness.

My first thought here was there's no way this board is ever going to work.  I never attempted a repair anything like this before.  But after reading Clay's repair guide on Stern MPU-100 boards and battery corrosion, I thought I'd give it a shot as a personal challenge.  I decided to purchase a cheaper knockoff of a good soldering/desoldering station figuring if I'd be doing more repairs in the future I might as well buy the tools to help to get the job done faster.  Plus I figured if I could fix the board instead of buying a new $200 Ultimate MPU board, then it almost covers the cost of the tools.  Prior to this I'd been using Radio Shack 15 watt irons and a heated desoldering sucker with bulb from Radio Shack.

Step 1: Make A List Of Visually Bad Components
The first step I'd recommend is looking over the board and making a list of any components that you can visually tell have battery damage.  There will be crystalized greenish blue stuff on a lot of the affected components.  If you have a digital camera available, you might also want to take some closeup pictures of the board so you can review it later -- after you've sanded off the PCB solder mask on the affected areas, you'll also lose any component labels so it will be harder to identify the components.  This list will come in use later on when you decide what parts will need to be replaced.  You can often download the owner's manual from IPDB there are often diagrams of the component layout on the board & schematics.  If there's a component diagram, you can print it out and as you identify bad components on your board highlight them on paper -- then refer to those highlighted parts when you're ordering replacement components.

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Bally Fathom repairs
Posted 06/17/11 3:55AM by Anonymous Techdoser
Thanks for your info, much appreciated. - I'm repairing a Bally Fathom 1981 - MPU board looks similar condition to yours.