In December, 2000, starting with or shortly after S/N 4150650001, we replaced the 12 mm long quarter-turn studs in the MAC 500 head covers with ones that are 14 mm long to make it easier to fasten the covers.
The 14 mm long quarter-turns can become too loose to lock securely under certain conditions, resulting in the possibility that the head cover can fall off. We have heard of a case in which a plastic cover fell off a MAC 500 that was probably due to loose quarter-turns. There fortunately was no injury or damage.
The quarter-turns are most likely to become loose if newer head covers with the 14 mm fasteners are switched around and used on older units that originally had covers with 12 mm fasteners. The spring retention clips in these units are "used to" the shorter fasteners and become too compressed to hold the longer ones. The same can happen if you replace 12 mm studs with 14 mm studs without also replacing the spring retention clips.
We recommend that you regularly inspect all 4 quarter-turns on MAC 500s to make sure that they lock with the screw slot aligned parallel to the light path. The fasteners must remain locked when you squeeze the head covers together.
Any quarter-turns that do not lock securely shall be immediately corrected. The solution is to replace the spring retention clips, P/N 08240110, "1/4 turn 6mm receptacle". In an emergency, you may be able to pry the clips open enough to hold the quarter-turns.
For applications where the fixtures are installed over people´s head, additional safety is provided by using the optional head cover retaining wire, P/N 50300502. Click here for details.