Slow progress

Painfully slow at times over the last couple of days, but at least it’s been steady. But it’s not surprising as the work I’ve been doing is the most critical of the whole project, namely making up the cabling for the new radio and transponder. The fact is that if I mess things up big-time, there’s a considerable amount of money at stake and things are quite a bit more complex than just wiring up an Icom hand-held through a simple interface, which is what was installed previously.

Things are by no means as straightforward as I’d like them to be, except for the transponder that only has two connections (12V positive and earth). But even that has turned out to be more complex than it appears to be, on the face of it. The problem has arisen because the radio and transponder connect into the system via D-plugs that I elected to make up myself.

Although I could have purchased wiring looms already made up, due to the multitude of configurations that are possible, for the radio at least, it seemed daft to pay a considerable amount of money only then to discard half of the installed cabling that would be otherwise unused. Plus I would also then have had lots of external connectors away from the D-plug, which is something I am trying to avoid.

At order time, the supplier offered, and I ordered, two different D-plugs for the radio and the transponder, but when they arrived, they were the same item. On close and careful inspection, I was very pleased to find that the internal cross-connections in the plug required for the radio have already been done for you thus much simplifying the soldering work that’s needed to make up the connections for the power and earth, headsets and PTTs.

However, when I compared the set-up for the transponder, although not all of the pins in the plug are used, the cross connection that is required is different from the ones already in place for the radio and although I was able to un-solder one of the latter and make the new one that’s needed for the transponder, the one that remained could not be cleared. Now it appears that this ‘short’ for want of a better term is across two unused plug pins, but what I don’t know is what’s happening inside the unit on the other side of the plug.

I’m hoping that the supplier knows what he’s doing as the plastic envelopes for the D-plug kits are clearly marked as being for BOTH the radio and the transponder, so I’m hoping that what I’ve done, which is according to the drawing except for the ‘short’ that I can’t take out, will be OK. However, although an experienced avionics engineer would know immediately, I won’t know for sure until I eventually come to connect the unit and its programmed and tested. That’s if it lights up as it should do in the first place!

Having made the decision to modify its D-plug but to leave the ‘short’ that still remained (I had no choice) I made up the connector for the transponder a couple of days ago. While I was reading the transponder installation instructions, I read that it is MOST IMPORTANT not to run its antenna cable in close proximity to any other RT or navigation antenna. I always knew that really and when I ran its cable alongside the Savannah’s existing radio antenna cable I had major misgivings. I therefore decided that I had no option but to revisit the job yesterday and relocate the radio cable from the passenger side (right) to the pilot side (left) of the aircraft.

I felt much happier once I’d done it and although I didn’t have time to do much more afterwards, it was a job worth doing. What I did manage to get done was apply my new panel labels. When I fabricated the new high-level panel in the Savannah, I used labels from a pre-printed sheet that I acquired from a supplier in the UK. I was never very happy with them. Firstly, the sheet was very much aimed at ‘GA’ aircraft so most of the labels were inappropriate for the Savannah and were unused. Secondly, some of the labels I wanted weren’t present on the sheet and I had to make do with ‘approximations’ that weren’t all that satisfactory. And thirdly, the sheet wasn’t cheap once carriage was added into the cost.

So I decided to bite the bullet and buy a little Brother hand-held labelling machine, for not much more than the total cost of the pre-printed sheet actually. I experimented a bit and eventually decided on a style before running off all the labels that I’ll need. Here’s how a few of the labelled-up gauges now look.

I’m very happy with the result and think that the investment was well worth it, certainly given the total cost of the whole project. And I’ll also be able to label up everything now in my home and office area 🙂

So having studied the connection drawing yet again for the umpteenth time and drawn out my own simplified connection plan, today was the big day for cabling up the new radio. I still had an outstanding query, which was quite a major one. Which connection on my Alphatec Pro headsets powers the headphones and which one the mics? I thought I knew the answer and was all ready to proceed on the basis of my assumption when just before leaving for Malbec, an email from Alphatec came in.

At the end of last week I’d sent them a drawing of a monojack with each of its segments numbered asking what is connected to each one and this was their reply. And how fortunate, because it turns out that my assumption was wrong and that the mic and headphone connections are the reverse of what I’d assumed them to be. Shown below is the picture as confirmed this morning by Alphatec.

So what a bit of luck that I hadn’t already blasted ahead and made the new connections up, because as I found today, the soldering involved was intricate and very time-consuming. It took me literally hours to do this afternoon, obviously much longer than an experienced avionics engineer would most likely have needed, but I had to be very sure that what I was doing was exactly according to the connection drawing and also that I made no mistakes, like allowing solder to run onto adjacent pins.

But eventually it was done and as well as the radio’s D-plug connections all being in place, so also were the power supply and earth and the PTT connections. Here’s how it looked – not a lot to show for the hours that it took to do.

So I’m now in a position to connect all the new wiring up, reconnect all of the gauge and switch cabling that’s still in the aircraft and install the new panel. Connection of my new little electronic gizmo will be quite easy but along the way I’ve got to find a way to mount it so it’s secure but accessible should I need to reprogram it in due course. But somehow I think that that will probably be one of the least of tomorrow’s problems.