By Earl Schlenk
Back in 1980, we installed a new multi-positioned intercom system in the railyard where I worked. There were about 50 stations on the master intercom in the office of the yard master. The slave stations, (speakers) were connected to the yard master’s office via an underground cable with 50 twisted pairs. Signaling was done by putting a ground, through a push button switch, on the cable pair going to the master console which lit a lamp on the master console, indicating that the station wanted to communicate.
The audio path was controlled via the master intercom. After about a month of flawless operation I received a complaint that this one particular station was constantly signaling the master without anyone pushing the signaling button. I went to the cable terminal at the remote location, opened the cover and checked the cable pair to make sure this station was on, but I could not find any ground whatsoever on either side of the pair, even at very high ohm scales and high break down voltages.
So, I restored the system back to normal and every thing worked fine for another month. I again received a complaint from the yard master about false signals by this station and he was not very nice about it. So again I repeated my testing and no problem could be found. I decided to put this station on another pair just to eliminate the pair as the trouble. For another month, everything worked as normal and then again the same complaint!
So I changed the station’s position on the master console and again the problem returned. I got a cup of coffee, lit up a cigarette and pondered this weird trouble. I went through all the steps, in my mind, from the first complaint, all the actions that I took. My thinking went as follows: I opened the cable box, checked the cable pair and found no problem, closed the cable box, everything worked fine, a period of time went by, and the problem reappeared. What was common to all this testing that was resulting in the problem being cured for some time but appearing again! I realized that the problem disappeared every time I tested this circuit, what could I be doing to correct this when I tested it? Bingo! The troubled disappeared whenever I opened the cable box. Opened the cable box? Hummm. Opened the cable box?… What could opening the cable box do to eliminate this trouble? Could the box be the culprit? This was the only common action I took when I magically fixed the false signaling.
So, I opened the box and took a Sherlock Holms type magnifying glass and checked the inside lid of the box and found a slight discoloration, at about the position that the cable terminal screws were. I slowly closed the box peeking at the side as I closed it and it appeared that the screws may be touching the lid, the lid was painted on the inside, and I deduced that the voltage on the pair was braking down the insulation qualities of the paint and causing the false signaling! The shield of the cable was grounded and was connected to the frame of the box and provided the ground path for the false signaling!
After careful inspection I realized the installer had used the wrong terminal strip for the box he used. The terminals stood too far out and the terminals touched the painted lid! I added some insulation on the inside cover of the box and never had any more trouble with false signaling!