Saturday, September 22, 2012

The Shocking Secrets to the Art of Wiring

As you may recall, I mentioned a couple of problems I was having with the headset jacks and the comm radio. The problem with the headset jacks was mostly notable when trying to use the headset microphone on the pilot's side. As I talked into the mic, all I could hear passing through the intercom was a thin, weak, scratchy sound - nothing at all like a voice. When I keyed the mic to talk on the comm radio, it was even worse. It basically shut down any sidetone. The sidetone is what allows you to hear yourself talking on the radio and is critically important. It is so loud in the airplane while flying that you literally (in a non-Biden sense) can't hear what you're saying. If you've never had that experience, you would be surprised at how much difference it makes to the clarity of your speech. And in flying, it's pretty important to communicate clearly with air traffic control.

There are four primary causes to electrical problems, each having its own rule:

  Rule 1: wires cannot have breaks in them.
  Rule 2: wires must be connected to the correct endpoints.
  Rule 3: wires that should go to ground must go to ground.
  Rule 4: wires that should not go to ground must not go to ground.

I started my troubleshooting by verifying that the mic wire met rules one and two. I pulled the plug that contains the endpoint of the mic wire out of the J-box, plugged a spare pin into the correct hole number as shown in the plans, attached one lead of a multimeter to it, then attached the other lead to the mic jack in question. Fully connectivity.  That left rules three and four.

To determine if the jacks were grounded correctly, I went back in the plans to review the installation instructions.  From my review, it looked like one of the jacks was supposed to be grounded to the airplane via the simple method of being in contact with the floorboard. I inferred this from noting an 'isolator washer' on only one of the two jacks.

Hmmm, two potential problems here. The first was obvious: note that the isolator washer is on the outer surface of the floorboard. That implies that the part of the jack assembly that needed to be grounded on the other jack should have good, clean contact with the floorboard. And here's the problem with that: I had painted the floorboards before installing the jacks. There was a thick coat of paint separating the jack and the metal of the airplane. I would need to remove the jack and clear some of the paint off so the jack could get a good ground.

When I removed the jack, I found the second problem. I had installed isolator washers on both the audio and the mic jacks, probably because Van's gave me four of them even though I only needed two.  I had one too many on both sides of the plane. Out they came!

I powered up the plane and the problem was solved! I was able to get a clear, strong voice signal through the intercom from both the pilot's and copilot's headsets.

The problem with the comm radio persisted, though. I pulled out the installation manual for the comm radio to check up on a lead I had gotten from Jackson: there was rumored to be a configuration setting in the radio to adjust the volume of the sidetone. It didn't take long to find, and it was a pretty easy thing to do. Garmin ships the radio at the middle level volume setting, but it's a simple matter to adjust it. You just press and hold the 'MON' button for a couple of seconds, use the large knob to cycle through to the 'SIDETONE' page, and adjust the number up or down. Garmin had set it to a value of 128 (out of 255) and I lowered it down to 50.  That fixed it!  I called Bolton Ground to ask for a radio check and received a very welcome "Read you loud and clear" response.

With the two big problems fixed, there were just a few odd jobs to do to fill the rest of the day. One of the things that has been plaguing me for months is the monkey-trap nature of the canopy. If you were so careless as to get into the plane, lower the canopy and latch it, you were stuck in the plane until such time as someone could remove the handle from the outside to let you out. There was something maladjusted on the latching mechanism, obviously, but the fix was not nearly so obvious. The handle also had the annoying tendency to move positions when the canopy was closed - it would rotate enough that you couldn't open the canopy without having one hand holding the handle and the other lifting the canopy. And that is hard to do!

The way it's supposed to work is you're supposed to use a big pair of pliers to squeeze the tube that the handle goes down through, the idea being that the slightly squeezed tube will prevent the handle from rotating. The problem was that I either got the tube squeezed too tight or not tight enough. I wanted to try something else.

So, first thing first, Pete and I talked over the problem of the unlatchable latch. A number of possible solutions were bandied about, and we eventually agreed that we would attempt a fix on the lowest cost part of the assembly: the outside handle. The idea was to grind a little of it off in order to give the interior latch a little more room to clear the latching block. That worked quite well, so we moved on to the handle rotation problem. I had seen a suggestion for fixing this that involved simply putting a rubber O-ring in the assembly, so it was off to the hardware store to find an O-ring.

While we were there, I went ahead and picked up some number decals to put the tail number on the plane. I'm sure glad it's just temporary - it's really ugly.

The O-ring fit quite nicely and it keeps the handle from rotating. I was afraid the additional thickness would moot the effects of the handle-grinding fix, but what actually happens in practice is that the O-ring just rolls down onto the outer surface of the tube. When the canopy is opened, I just have to roll the O-ring back up the tube.  Piece of cake! I might investigate a more robust fix in the future, though.

Finally, we removed the plastic sheet from the canopy and riveted on the turtle deck skins.

All in all, it was a very productive day.

No comments:

Post a Comment