Simon was one of the first popular electronic games. Invented by Ralph Baer and put on the market by Milton Bradley in 1978, Simon sold well for year after year. Its simple gameplay of remember the randomized sequence of colors not only challenged the short-term memory of its players but their ability to recall sequences quickly. These devices sold so many units that obtaining a vintage unit with its three switches and three buttons is not hard to find and will not break the bank. Recently I had acquired a "vintage" Simon and could see that it was in need of repair, so I decided to document the steps I took to repair it.
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| Simon - Looks in Good Shape |
Simon is a simple device, simple enough that it can usually be repaired. At the heart of Simon is a micrcontroller or an ASIC chip which implements the relevant functionality of the micrcontroller. Older units will use a 28-pin TMS-1000 microcontroller but the later and more common units will use a smaller 16-pin DIP ASIC with the label MB4850. If this chip is dead, then good luck finding a replacement because the chips were pre-programmed at the fab. The MB4850 has been decapped and reversed engineered but drop-in replacements are not readily available.
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| First Sign of Trouble |
The other chip is a SN75494 (older boards use a SN7494), which is an off the shelf part and used to drive the four lamp lights and the speaker outputs. Simon runs off both a 9v battery (microcontroller) and two D-cell batteries (SN75494, lamps & speaker). Pocket Simon uses an MB4850 and SN75494 but takes two AA-batteries in addition to the 9v.
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| That Doesn't Look to Good |
When I saw Simon at a thrift store in its box with its instruction manual included for $20, I thought it would be worth the risk. The box and the device looked in good I opened up the battery compartments and was concerned. I could see that the 9v battery snap connector was missing a terminal, but someone helpfully put a pair of replacement battery snap connectors inside. Then when I looked at the D-cell battery compartment, I could see one of the battery contacts had been seriously corroded and would not conduct electricity as is. Still I figured for $20 I would take a chance and see if I could repair it.
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| Keep Track of Your Game's Components |
Repairing the 9v battery connector was simple enough, I cut off the broken snap connector but I could not solder a new connector to it, the wires would not "take" solder due to some black coating. So I removed the wires from the board, found a pair of wires long enough and spliced one of the functional battery snap connector onto it. After fitting some heat-shrink tubing over the splices I had a 9v power source for the MB4850.
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| Mainboard Top Side, Doesn't Look Too Bad from this Perspective |
The battery corrosion was worse than I originally thought after opening the device. In order to open the device you must unscrew the four screws on the sides and remove the switch covers. Milton Bradley told people to do this if they needed to replace a lamp and even included a spare bulb in a cavity near the 9v compartment. Battery corrosion is among the most common non-simple repair issue it seems with these devices. The corner of the PCB under negative D-cell terminal was covered in blue battery acid corrosion. I dropped the bad contact into a cup of white vinegar and let it sit for an hour. After drying it I found that the metal, although not coated in shiny copper anymore, could still conduct electricity. I did not need to break out the sandpaper, dremel or a metal polish but it is a good idea for a more aesthetically pleasing look.
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| Battery Acid's Wake |
The corrosion had destroyed four traces on the corner of the board, the acid ate away at both the solder mask and the copper traces underneath. Two of the traces were outputs, one to the blue lamp, one to the red lamp. The lamps all share the 3v line from the positive terminal of the D-size batteries and the 3v line to the red lamp had been destroyed by the corrosion. The other contact on each lamp goes to an output pin on the SN75494. The ground plane near the ground pin of the SN75494 (pin 8) had been eaten away so I soldered a wire from that pin to the closest ground. These schematics for Pocket Simon and the TMS-1000 Simon was very helpful even though there slight differences between them and MB4850 Simon.
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| Fixed Mainboard |
I applied white vinegar several times and scraped away loose corrosion, although on further reflection I should have deposited a slurry of baking soda and vinegar on the worst affected areas. Having enough conductive metal surface to make a connection with the negative terminal was a concern, I scraped until I found some metal and then applied a lot of solder to try to give more surface area make a better connection. I did not solder the terminals to the board, they are held in by a screw to make contact with the circular points on the PCB.
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| Closeup of Fixed Area, Top Yellow Wire is +5v, Black Wire is Ground, Others are Signal Outputs |
When I had finally soldered all my fix wires, four in all, I turned the machine on to hear the correct beeps but one lamp was always on and one never turned on. Two lamps on the opposite side did turn on and off as they should. I figured I must have wired my fix wires incorrectly because swapping the lamps showed that all four were still functional. After a second try all the lights turned on and off but when I pressed the buttons on one side correctly, the game would end with the "razz" sound. When I pushed the button above or below the button where the lamp lit up, the game would register that as correct. At that point I knew I had reversed the outputs and a quick wire swap later and I had a fully functioning game.
Helpful Simon repair videos include the following:
https://www.youtube.com/watch?v=QJkzujFKz08
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