This is the War Stories Section of
The Broadcast Archive

Maintained by:
Barry Mishkind - The Eclectic Engineer

Chasing the Interlock

by Michael Barnes

I was a relatively new guy working at a high frequency transmitter site, which was part of a military aeronautical system communicating with aircraft over the Atlantic. One morning, I was told of problem at the receiver site. The equipment rack, which contained the primary receivers, would not power up. They had everybody in the area working on it all day. They called me in as a last resort to see if I had any new ideas. They had high priority missions to communicate with and the system had been off the air since the middle of the night. 

When I got to the site, I saw a bunch of guys poring over the technical manuals and panels pulled loose on the rack. I also saw a curious red substance spattered on some of the racks. I asked what had happened. Seems the rack had a problem in the middle of the night with one receiver, so they called out the on-call guy (who was also new). Part of his troubleshooting was to check the vane interlock on the blower, which exhausted out the top of the rack. He tested it by sticking his fingers in the hole and flipping the vane to see if the rack kicked off. Well, he apparently stuck his fingers in too far, catching them in the squirrel cage.

Okay, that explained the red stuff all over the racks. It nailed his fingers so bad, it bent the squirrel cage. After they took him to the hospital another tech was called in. The only thing he was told was there was a problem and the on-call guy went to the hospital, and the problem must have had something to do with the blower, so he replaced it. After that, nothing worked. 

Fifteen hours later, is when I came on the scene. After getting this history, I checked out the blower. The interlock switch was an SPDT with three wires. Normally, in the non-blowing position, it tripped one relay which showed a fault light, in the blowing position, it tripped another relay providing power to the receivers. I carefully looked inside and pushed the vane down with a pencil. The fault light did not go on! No one had caught this before. I then traced the wiring and found it wrong. I properly connected the wires to the air interlock and all was well. The whole thing took me about 10 minutes.

Because there was an injury (the first guy was out of work for two weeks and lost the end of his middle finger) and the down time was so long and affected the whole system, an inquiry panel was convened. At this panel the tech who replaced the blower was asked if he consulted the manual when replacing the blower? (No), did he label the wires to insure replacement in their original positions? (No), did he make a diagram to be sure he hooked them back up right? (No). So how did he determine which wire went where? He "checked all three wires, they all had +28V on them, so it didn't matter which way they went". He was not-so-cordially invited to work in a non-electronics position after that one.

I eventually ended up teaching the technical school course maintaining that equipment.

Michael Barnes -