Key Switch Problems and Solutions:
On the Space War game, the entire user interface consists of 21 magnetic key switches. The key switch is comprised of a simple plastic housing, a key plunger with a button on it, a small ring magnet, and a reed switch. The reed switch is normally an open circuit. When the player presses the key, the key plunger moves the internal ring magnet downward toward the center of the reed switch. In this position, the magnetic lines of flux are parallel to the reeds inside the glass body of the reed switch. This causes the reeds to become magnetized. The tips of the reeds are then attracted to each other and so they bend towards each other until they touch, thus closing the switch. The advantage of such a switch is that it is physically robust, immune to dirt and water, and has a working life of several millions of cycles.
A common problem known to Space War machines is the failure of one or more of the key switches. Due to the age of the Space War machines, the magnets inside the key switch housings may have become extremely weak with respect to their magnetic field strength since they were manufactured. Worthy of note is the fact that the original magnets were so weak to begin with that each switch required two of them stacked together to produce a strong enough magnetic field to activate the reed switch. If, over time, the magnets have become too weak, they may activate the reed switch only intermittently or not at all. In this case, the key switch can be repaired by replacing the magnet with a stronger one. If a stronger magnet cannot activate the reed switch, there is no other choice but to replace the reed switch. Sometimes, the reed switches themselves do fail.
Outer Diameter : Inner Diameter : Thickness : 0.375" (9.5 mm) 0.250" (6.35 mm) 0.039" (1 mm)
Suitable Replacement : SuperMagnetMan R1005A
Reed Switch Dimensions:
Glass Body Length : Glass Body Diameter : Normal State : Switch Type : 0.6" (15.1 mm) 0.089" (2.26 mm) Open SPST
Suitable Replacement : Hamlin Electronics MDCG-4 12-33 available at Digikey : HE503-ND
Replacing Magnets and Reed Switches :
The key switches. are relatively easy to disassemble and reassemble so it is possible to replace the magnets and the reed switches as required. I highly recommend that if you replace one magnet that you replace them all. If one of them is found to be too weak to function, then they are all going to be very weak and are likely bordering on failure anyway. The same cannot really be said for the reed switches. If one has failed, it doesn't necessarily mean that the rest of them are also likely to fail. However, if your goal is to make the machine as reliable as possible for the long term, replacing all of the reed switches with new ones is good preventative maintenance.
Remove the nuts that secure the key switch body and remove the key switch body from the PCB:
To open any key switch, it must first be removed from the PCB. To remove a single key switch, remove two nuts on the bottom of the PCB and lift the key switch body off of the PCB. There are no wires attached to the key switch body. The reed switch is not attached to the key switch body so it will remain in place on the PCB. Lift the key switch body straight up from the PCB so as not to damage the fragile reed switch. The nuts may have been painted to prevent them from falling off or the screw threads may have become corroded. This may cause significant resistance, enough to cause the screw to start turning inside the plastic housing. If this happens, there are two things that you can try to secure the screw while turning the nut. First, you can add a tiny bit of super glue around the top of the screw head and allow it plenty of time to cure before attempting again to remove the nut. Alternatively, if you are handy with a dremel motor tool, you can carve a slot into the top of the screw head and then use a flat screwdriver to hold the screw while turning the nut.
In the photos below, the numbered key switches. are shown. The numbered key switches are made as two rows of 5 key switches each. The procedures described below apply the same to the single key switches as well.
Open the key switch body :
The bottom of the key switch body should simply pull straight away from the top section. The fit is tight so you might want to use a small flat screwdriver to pry the top and bottom sections apart. Gently insert the tip of the small screwdriver blade in the seam between the top and bottom sections and twist it to pry the sections apart. Do this a little bit at a time in several locations around the entire body so that it comes straight apart. Switch to a larger screwdriver once you have opened up some space.
Here's what the original magnets look like. Sometimes they are complete rings. Many of the magnets in my machine had split. Note that they are painted red on one side. This denotes the north magnetic pole. The polarity of the magnet is not important with respect to operation of a reed switch, however, both magnets have to be stacked together such that they are both in the same direction. That's easy to determine since they'll just stick together on their own.
Installing the new magnets :
The vertical position of the new magnet is critical to proper operation of the reed switch. Be sure to arrange the magnets exactly as follows. Each key switch had two black ceramic magnets stacked together. Remove one of the old black magnets from each key switch and place a new magnet under the remaining black magnet. At this point, install the key switch body top section back onto the bottom section and install it back onto the PCB. Install the retaining nuts.
Correct placement of new magnet :
Incorrect placement of new magnets : Do not install the new magnet on top of the old one as shown below. The new magnet must be placed below the old magnet as shown in the photos above. If the new magnet is installed on top of the old one, the key switch may activate when released.
Preparing new reed switches for use :
When installing a new reed switch, care must be taken to precisely bend the leads to the correct shape. Also, the lead that extends from the top of the new reed switch to the PCB must not be made of magnetic material. The leads of the new reed switch are plated steel and so they are magnetic. The bottom lead is okay because it will not interact with the magnetic field of the ring magnet. However, the upper lead presents a problem. Because it would pass through the ring magnet right next to the reed switch, much of the magnetic field flux would concentrate within the steel lead wire. To prevent this, the steel lead wire must be replaced by a non-magnetic lead wire. The lead wire that is welded to the top of the old original reed switch can be cannibalized for use with the new reed switch Simply cut it off of the old reed switch and solder it onto the new reed switch as shown in the photos below. Once you have prepared the new reed switch, simply solder it into the PCB.
Note also the precise position of the switch contacts. They are in the exact center of the glass body of the reed switches. It is very important to maintain this positional relationship. The contacts in the center of the reed switch must be positioned at a very precise height above the PCB in order for the magnet inside the key switch body to properly activate and deactivate the reed switch The range of motion for which the reed switch must operate correctly is only about 4 mm. If the reed switch is too high or too low or if the magnet inside the key switch body is too high or too low, the reed switch will not operate correctly. It may instead turn on when the key is not pressed or it may activate when the key is half way down and then open again when the key gets all the way down. Of course, proper operate is for the reed switch to be open when the key is not pressed and closed when the key is fully pressed.
Testing the Key switches:
The easiest way to test all of the key switches before installing the entire switch assembly back into the machine is to connect a continuity meter (ohmmeter) to the ribbon cable and listen for the beep when each respective key switch is pressed.
Take a piece of any PCB male header pin strip that has a 0.1" pitch and 0.025" square pins, either double row or single row, and plug it into the control panel ribbon cable connector. It is easiest to use microhook leads on the continuity meter but you can of course use simple probes.
Refer to the connection table "CCPU Input Paths" on page 3 of this document : Vectorbeam_Space_War_Control_Panel_Hookup.pdf
For each key, connect the meter to the ground pin (pin 26) and to the pin designated in the connection table (under column "Ribbon wire #") for the key being tested. The meter should beep (or read very close to 0 ohms) when the key switch is pressed down. The meter should stop beeping (and read open circuit or OL) when the key switch is released. If all of the key switches now work properly, you can install the entire switch assembly back into your machine with 100% confidence. If you still have a problem with a key not working after installing the switch assembly back into your machine, it is very possible that there is a problem on the CCPU board. One of the electronic inputs might be bad. The only cure for this kind of problem is to have the CCPU serviced (ask me nicely by email for help).
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