VGA cable fault - solved
We have more than 100 classrooms with ceiling mounted data/video projectors. One room recently had a problem with the projector not "seeing" the computer. In autosearch, it would flash an image for a fraction of a second, then continue searching. Hmmm. Bad sync. Bypassing the VGA cable made it work, so we were down to either repairing the existing cable or pulling another through the conduit... not fun and very time consuming. We usually try for a repair to get a room operational ASAP, but how do you find a break in a cable? Most of it is in the wall and not likely to be damaged or faulty from the factory which should have shown up right away. All I had with me for troubleshooting was my toolkit and o'scope. Cables usually break at the connectors, so I had two choices... the user end or the projector end. Wiggling it at either end didn't make it work with signal on it. Hmmmm. What to do? A TDR would be nice. Perhaps cable capacitance could provide a clue. I scrounged a 47K resistor out of my toolbox and hung one end on the scope calibrator (half a volt 1kHz square wave) and attached the probe to the other end: a quickie low-value cap checker. The scope ground was attached to the cable shell and pins probed with the scope/resistor combination via a jumper lead at each end of the cable. Each used pin showed some capacitance indicated by a rounding off the edges of the square wave... all but the one that broke inside the connector. Pin 13, a sync line, was open at the projector end! Back to the shop for a replacement connector and the room is again operational. R&Ring VGA connectors is no piece of cake, but pulling a new cable is a lot worse. The original was likely damaged by too hard a pull.
Ray Carlsen University of Washington, Seattle