The COAX Connector From Hell

Today at work I got to do a few "firsts" for me. I worked on a plane that had a VHF problem which was traced down to a VHF COAX connector attached to the #1 VHF antenna. I have changed the VHF antennas before and I have monkeyed around with the connectors before. My prior experience with these fun little connectors has been realizing that one was bad and MELing the system since we do not carry the right parts or equipment to fix the thing in OAK.

Since this was a #1 VHF problem it was not MELable, and since the previous shift had traced the problem and ordered the parts I had the relative easy task of putting the thing back together. A first for me, although I did do some COAX crimping at my house for TV cabling.

A little description. COAX  (Coaxial Cable) is a shielded electrical wire. The wire has an inner conductor surrounded by a flexible insulator surrounded by the tubular shielding.

The theory is that the electromagnetic field produced by radios etc. is contained within the space between the insulator and the shielding allowing COAX to be run along other wire bundles without fear of much interference. The thing about COAX on aircraft is that the length of the wire is pretty critical and you are not allowed much slack as far as simply cutting it shorter to allow crimping, stripping, etc.

The connector crimps onto the wire but you have to strip the shielded part of the wire back some so that the wire fits into the end of the connector. Looking at the picture on the right the wire feeds into the narrow hole on the left of the connector. First the barrel is placed over the wire, you push the wire into the connector, making sure it contacts the pin inside, push the shielding down over the narrow thing on the left, slide the barrel down over the whole thing and crimp it. The shielding must contact the connector to ground it out.

Not only was it my first aircraft COAX crimp but they also sent a complete Daniels Strip and Crimp set. Most of you know that Daniels makes all kinds of crimpers that we use on airplanes. They also make pin pushers and pin pullers, hex crimpers, strippers, etc. For this fix the company sent me two Daniels sets! It was pretty cool seeing just one complete set but they sent two. I tried to find out how much they cost but could only find one set on Ebay for $3000.

Any way, we used the hex crimper for COAX, we found the right die block to put into the crimper and squeezed away. When we were done we had a perfect crimp and what do you know the #1 VHF was transmitting again!

One of the things that threw me off for a little was that the wire we were working on had a wire tag on it that said #2 VHF. Apparently Boeing changed the locations of the #1 and #2 VHF antennas on the 737-700s somewhere mid production so the wire tags are all jacked up. Watch out for this one as I can see it really messing people up. I'm not sure why they did not change the tag but, whatever. That was the final first, incorrectly marked wires from the factory.

I have to wonder, this is the second COAX that I have been involved in and I know there have been others, why in such a relatively young aircraft series (the -700s) are we having so many COAX issues? Mines is not to reason why, mines is just to push tin!