ATA 33-13, Panel Lighting

Learned something new today.

We had a crew write-up about several pedestal panels and light plates background illumination inoperative.
  
     
   
The majority of aircraft that I've worked on all use 0-5VAC for background lighting. Typical installations have several power supplies feeding a set number of user devices each.

For the 767 pedestal, there a two power supplies. We verified 115VAC power and grounds. We also verified the dimmer potentiometer resistance inputs. This left is with a possible short on the user side. One by one we removed the associated panels. No help..... Most of the light plates wiring are soldered, so isolation requires disconnecting at least one lead.
  
     


We went for the "Stab Trim Cutout" light plate first, no help..... By chance we had to shutdown aircraft power. Upon power up some of the light plates were lit. We connected the panels which also had lighting. When we hooked up the disconnected light plate..... no lights.

The plate was shorted out. Sensing the short, the power supply shut itself down. Only resetting power via the breaker would cause the supply to output voltage.
  
     
  
To troubleshoot a problem like this, after each user component is disconnected the breaker has to be reset to see if the last removal was the problem.