Not all miniature lamps are the same.  Inferior imported brands of miniature lamps burn out more easily, are more likely to "carbonize" (turn silver on the inside) and have less consistent appearance and color. Marco only carries high-quality Eiko brand-miniature lamps in all sizes used by your machine.  

Lamps should be replaced whenever they start appearing black or silver on the inside, or when they are burned out, obviously.  We have found it more convenient to change all of the lamps in a machine when changing bulbs, to prevent frustration later!  A typical machine has around 120-140 or so bulbs in it, so plan accordingly. 

Double-check your bulb-type when replacing it. Remember that the bulb in your machine may not be the correct bulb as a hurried machine tech may have put in.  A bulb that burns too hot is at risk of melting your irreplaceable plastics!  #47 lamps burn cooler and last longer than #44 lamps, which are especially recommended for always-on general illumination bulbs found throughout the machine and backbox.