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Trak-ball Basics:

This post is intended to show the basic components of the Atari
trak balls for Centipede and Millipede and what to replace to get
one to roll like new again.  

The Atari Mini-TrakBall (Centipede) and Midi-TrakBall (Millipede)
consist of 2 roller shafts, 1 idler shaft, 6 ball-bearings, 2
radial optical couplers, 2 encoding wheels, and a ball.  The basic
setup is shown below.

Key:  E - Encoder wheel is screwed on to end of shaft.
      B - Ball-bearing
      D - Detector Diode

The trak-ball rests in the middle of the assembly and
theoretically has 3 tangent points of contact in the middle
of each shaft.  As the ball is rolled the shafts turn the
encoder wheels at the appropriate rate which is detected by
the logic card and fed back as motion on the screen.

D   __E__
|     |
`--   |         B
      |           .
      B             .
      |               .
      |                 . 
      |                   .
      |     Trak-Ball       .
      |                       B
      B                           |
                               |     |

When the cover is taken off the trak-ball assembly the 3 shafts
are always grooved in the center of the shaft.

    --                              --

At first it may look like it's designed this way so that the ball
rests in the groove. This is actually wear and tear.  The plastic
ball wears down the metal shafts over time which binds the ball.

All shafts should be perfect cylinders as shown below.  This way
the friction between the shaft and the ball is optimum.

    --                              --    Idler shaft

    --                              ----------   Roller shaft

The roller bearing can also get gummed up or rusted.  There always
seems to be an array of dirt and coke spilled into the trak-ball.
If the bearings seem to roll pretty good without binding I don't
recommend replacing them.  The biggest improvement though comes from
replacing the shafts.

The Centipede Mini-Trak Ball is 2 1/4 inch diameter and the Millipede
Midi-Trak Ball is 3 inches in diameter.  The roller and idler shafts
also have a larger diameter in the Midi-Trak Ball.

If you have a Centipede or Millipede and plan on keeping the game,
I recommend buying a new roller set and bearings.  The play improves


The best prices I have found on bearings and shafts are at ANSCOT
Distributing Co.  1-800-456-7277.  Please post if better prices 
are known.

Roller Sets:  Millipede $9.95, Centipede $6.95.
Bearings:     $2.99 each.

The bearing price just kills me but WICO wants $5.00 each.


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