NetHeads, I talked to several electrical engineers at work (one of which is also a big ham radio guy) about the wierd tone that I was getting when I keyed my radio mic. All of them frowned when they found out I didn't use shielded cable on all the lines that the schematic calls for (headphones, mic, push to talk, vox squelch, vox volume, speaker out), but the ham radio guy said "that'd do it for sure". It was about four years ago that I wired up my radio, and I thought I'd be smart and use a computer cable with a 15pin D shell connector, so I wouldn't have to bother soldering all those connections. The whole cable was shielded around the outside, so I thought that would take care of the shielding requirement. Apparently it didn't. And the funny thing (in 20/20 hindsight) is that the shielding ended where the cable came to the panel, and the wires split into a T-shape, half going to one side of the panel, and half going to the other. What I had here was a perfect horizontally polarized dipole antenna, perfect for picking up the output from the vertical stab-mounted antenna!
So I dropped by TBE's electrical shop and picked up a few feet of left over "flight" (that means "space qualified") wire, and rewired the thing yesterday, just like the schematic required. Several shielded pairs, and several single shielded wires. It probably took about 3 hours total just to make the harness. Well, it really only took one hour, but I had to do it the usual three times! The first one was for practice, just to perfect the method. I've been soldering stuff since I was a kid, but this stuff required liquid flux to get it to flow it all, and I wanted to see what it took to melt the plastic around the pins and render the thing useless. My soldering pencil was almost incapable of melting the solder, much less the plastic. Apparently Radio Shack rates its soldering pencils like Craftsman rates its power tool motors, optimistically, but then it's 30 years old, so maybe that's the real problem. The second time, I just wasn't convinced that I had good solder joints because of the low temperature (it'd take at least a minute to melt the solder between wire and pin), and I didn't like the way it turned out. I changed to a more familiar solder, and things started to work like they should, so I did another one. Anyway, I've got it all reinstalled, and there's dead silence when you key the mic now, so that was apparently the problem. Having said that, Jeff Scott tells me he's using exactly the same radio without shielding, and he has no problems. Just consider this a warning to just go ahead and shield it, rather than possibly have to do it twice (or four times). I've been thinking that my 20 year old wire strippers were worn out, because I always have to pull really hard when dealing with aircraft wire, and often end up with a couple of strands broken in the process. One of the EEs told me that the secret was to clamp down on the insulation with the strippers, then let up slightly and rotate it 90 degrees and cut it again before stripping. This way the insulation is fully cut, and it comes off a lot easier than the other way. Another helpful thing was the long backshell they gave me, which allows plenty of room for all these shielding connections to ground. You can see the end product at http://home.hiwaay.net/~langford/04071705m.jpg . There's a layer clear shrink tubing on the whole works where it exits the backshell. Oh, and before somebody berates me for soldering these connections, all I can say is "go blow it out your ear!". First of all, they are totally immobilized in this backshell, and just for kicks I tried to break one of these wires by bending it back and forth 90 degrees times, and didn't see any broken wires. That's good enough for me. And if it's good enough for the space station and space shuttle, it ought to be good enough for you too. So now I need to tidy up the installation, and then it's on to installing my new Ellison carb I got yesterday. I'm with Mark Jones, I don't think I'm going to make the Gathering. I had hoped to take a month off from work and finish it, but it looks like we're about to get a big job that I was the main designer on, and it has an extremely short fuze. More than likely I'll be doing overtime, rather than taking time off. Still, I'm very close, and working steadily on it. Now I'll have more time to do it right. I'd rather do my test flying in cooler weather anyway. Sounds like we'll have some new KRs there to fondle though... Mark Langford, Huntsville, Alabama N56ML "at" hiwaay.net see KR2S project at http://home.hiwaay.net/~langford
