PTT wiring

[elippse]

It has been mentioned by some that they only use a single insulted wire for their PTT circuit rather that shielded wire and it's never caused a problem. This sort of puts me in mind of a friend who discovered that he had not put safety wire on his prop bolts his previous inspection and found that the bolts were still torqued. Since safety wiring is a knuckle-busting adventure, he decided not to use safety wire again because the bolts were always torqued.

Here's the situation. A single wire on the PTT switch obviously won't work. The switch must also have a ground. Where is this coming from? Do you ground it to the control stick? That really gives a great ground through all the Heim joints! The ideal situation is to ground it to the radio ground which is where the shielded wire works out. The single wire, not being shielded, can act as an antenna and pick up noise from many sources. But if you use a ground from some other part of the plane, which may have noise relative to the radio ground, then both of those noises may couple to the mic lead since they run together through the three-wire mic-ptt-ground cable and can put the noise on the mic lead which is a very low signal level. You won't know about it when you transmit since you get sidetone in your headset, but others can, and it will give a scratchiness to your transmissions. A word to the wise!

[Canadian_JOY]

Paul - you've raised a very good topic of discussion here. I recently have been working on an intercom installation for a friend and the manufacturer specifically recommended using single wire, unshielded for pilot and copilot PTT's. While I don't like this approach for the noise-induction reason's you've cited, I opted to follow manufacturer's instructions. It still causes me to cringe because it is, at best, sub-optimal.

As you've pointed out, a good way to run a PTT line is in a shielded cable. Connect the shield to radio ground and then use your stick-mounted PTT to create a short between center conductor and shield, thus shorting radio ground to the PTT input on your radio.

Another workable method is to use twisted pair, again with one wire going to the PTT input of the radio and the other wire being tied to radio ground. The twisting together of the wires improves common-mode rejection and while not as effective as an overall shield it is a good method of providing noise immunity. If you ever look at telephone cabling (the big black wire strung from pole to pole) you'll find that each pair of copper conductors inside that big black multi-conductor cable is actually a twisted pair.

Common to both these techniques is the need to bring all radio grounds to a single point. Terminate all shields and radio-related grounds at this point. You'll be surprised how clean your audio will be if you stick to this philosophy.

[pierre smith]

Originally Posted by elippse

.......This sort of puts me in mind of a friend who discovered that he had not put safety wire on his prop bolts his previous inspection and found that the bolts were still torqued. Since safety wiring is a knuckle-busting adventure, he decided not to use safety wire again because the bolts were always torqued......

I sure as he@# hope that you talked him into safetying his prop bolts!! This is one of those mistakes we're OBLIGATED to address!.

Best,

[elippse]

Originally Posted by pierre smith

I sure as he@# hope that you talked him into safetying his prop bolts!! This is one of those mistakes we're OBLIGATED to address!.

Best,

Actually, Pierre, I don't safety-wire my prop bolts. I use a stack of four Belleville spring washers two in one direction and two in the other to maintain the over 1000 psi clamping force of the prop to the flange and to provide this clamping force when the prop shrinks or expands. The safety wire keeps the bolts from turning just as applying a force along the bolt does. But if the prop shrinks and starts to slip, the safety wire will hold the bolts for maybe a second or two!

[rocketbob]

One of the stupidest things in my not-so-humble opinion is safety-wiring the oil filter. If the oil filter is loose enough to rotate, its loose enough to spit oil all over the place and no amount of safety wire will help.

[scsmith]

Sorry for the hijack of the original thread, but the safety wire thing is a good topic too. There are "cultural" safety wire things that don't make sense, like the oil filter.

But, on the other side....how many people safety the oil pressure regulator adjuster nut? My oil pressure was about 55 at cruise, so I screwed the regulator in a couple of turns, and it was then 63. A few weeks later, it was down to 55 again, so I screwed it in a few more turns. Same thing a month later. Finally, I screwed it in and safety'ed it through the roll-pin, and my oil pressure setting hasn't changed in 9 months. Conclusion: my regulator adjuster nut was backing off from vibration.

Maybe a moderator could segragate these into a separate thread? Anyway...