Angle Encoder
A Complex Solution
Space Encounters uses the Midway Angle Encoder. The care and expense devoted to Midway’s arcade controls in the Bronze Age amazes me every time I need to fix it.
A Yoke control is used in the game to determine Speed and Position. Position is simply left to right. I think a simple potentiometer would have sufficed. I was wrong.
The Encoder wheel is housed in a reader and is connected to the yoke by gears and an axle. The axle has flat sides and a dimple to ensure proper fit for the wheel.
As the wheel is turned, four channels of data are set on by the metal parts or off by there absence. This provides a fairly precise read on the angle of the yoke. This would translate into 16 positions for the Fighter ship on the screen.
I am not sure why a potentiometer was not used. Perhaps they had more experience with “semi-conducting discs” than potentiometers (pots). It may be that 16 graphics were already created and they needed a simple encoder to select the right one of the 16. That was the case with Gun Fight. I am further confused by the use of a pot on the Speed axis. As the yoke is pushed into the machine a pot is adjusted. That reading sets the distance down the tunnel. The picture above is full speed, about as close to the enemy as possible. Normal speed would have the fighter ship near the bottom of the picture. The use of the pot indicates they had some reliability.
Maintenance
Encoder wheels get dirty and stop conducting when they should conduct. The result is erratic ship movements. Maintenance is simple, an eraser or rubbing alcohol are used to clean the metal bits to ensure proper conducting. The difficult part is getting the disc from the housing.
Remove the PCB
It is necessary to desolder the eight pins from the PCB.
Open the Housing
There are screws holding the Axle and part of the housing to the actual disc reader. Remove them and set aside the axle and rotation housing.
There are some stubborn but delicate plastic tab clips holding the encoder housing closed. Try to pry them open with a tiny flat screwdriver and gently open the housing. This will probably destroy some of the tab clips. Don’t Panic. A little glue will hold just fine. It may make the next cleaning more challenging.
Clean the Disc
No magic here, clean the metal parts with a rubber eraser or rubbing alcohol. Nothing static sensitive is on the disc. Get it nice and clean.
Put It All Back
Replace the leads, then the disc and squeeze the housing halves back together. If your tab clips survived, this is an easy thing. If you need to use some glue be sparing and clamp well. The disc must touch the leads to encode.
Attach the axle housing. Solder the PCB back in place. Install in the yoke housing.
Calibrate
Space Encounter has an Encode mode in the test mode. Perform the calibration procedure to ensure full ship movement.