DME help

[murphey]

I've got a Narco DME - it used to work fine. Now, it's identifying distance only sporadically. Local shop checked the unit (and replaced a dying bulb) and said it was fine. This leaves the wiring as a possible problem. From the outside of the aircraft the antenna are solid, nothing wiggles. That leaves the connections and the wiring. Any suggestions that I can do before I spend the money at the shop (and have to fly the aircraft to the shop and get a ride home because I'll probably have to leave it there)?

It's in a cherokee, btw. Now before anyone starts in about getting rid of the DME and getting a GPS, I really like the DME. (I get that on another board....If someone over there wants to pay for the GPS and the install, I'd be really happy.)

 

[ejensen]

Before digging any deeper I would clean all the connection, both on the radio mount and the coax including the connection at the antenna end. Spray cleaner and an soft eraser if you can get at it. Plus check the coax for breaks and kinks. Any work been done around the panel and wiring recently.

 

[wsuffa]

Check the antenna, too. My DME uses an exposed antenna... the insulator needs to be cleaned of exhaust (think lead) deposits periodically.

 

[murphey]

Check the antenna, too. My DME uses an exposed antenna... the insulator needs to be cleaned of exhaust (think lead) deposits periodically.

AHA! I never think of the antenna itself, since it's on the bottom of the aircraft. This weekend's project, then, to clean the belly and antenna, and check the connections.

I've got contact cleaner around here someplace....

 

[wsuffa]

AHA! I never think of the antenna itself, since it's on the bottom of the aircraft. This weekend's project, then, to clean the belly and antenna, and check the connections.

I've got contact cleaner around here someplace....

You're cleaning the antenna insulator, not contacts... good belly cleaner should work fine, esp. if it's removing exhaust residue.

 

[gismo]

If the antenna coax feedline is old it may be deteriorated and that can introduce serious signal loss at the 1GHz frequency used by DME. The coax connectors on the feedline can also become corroded and cause the same problems. It wouldn't cost much to have the coax and connectors replaced with something good and the antenna ground can be cleaned up as well at the same time.

 

[weirdjim]

I've got contact cleaner around here someplace....

NNOOOOOO!!!! PLEASE. Plain old contact cleaner has an intentional residue meant to coat the contacts with a lubricant to keep the switch contacts clean.

That same lubricant at RF (1 GHz.) does NOT do good things to a microwave connector. Rat Shack USED to sell what was called COLOR TV tuner cleaner. Not the plain old TV tuner cleaner, but the COLOR version. I don't know what the chemical difference was but the color version could be used on DME/transponder connectors.

If you can't find a can of that, I'd probably revert to a bottle of 95% alcohol at the drug store and a cotton swab, blowdrying the connector after cleaning. Actually, a mated pair connector rarely needs cleaning; they are pretty hermetic unless they have been laying underwater for a long time. I'd worry less about the connectors and more about the goop (that's a technical term, you'll get used to it) on the outside of the antenna.

Jim

 

[gismo]

NNOOOOOO!!!! PLEASE. Plain old contact cleaner has an intentional residue meant to coat the contacts with a lubricant to keep the switch contacts clean.

That same lubricant at RF (1 GHz.) does NOT do good things to a microwave connector. Rat Shack USED to sell what was called COLOR TV tuner cleaner. Not the plain old TV tuner cleaner, but the COLOR version. I don't know what the chemical difference was but the color version could be used on DME/transponder connectors.

If you can't find a can of that, I'd probably revert to a bottle of 95% alcohol at the drug store and a cotton swab, blowdrying the connector after cleaning. Actually, a mated pair connector rarely needs cleaning; they are pretty hermetic unless they have been laying underwater for a long time. I'd worry less about the connectors and more about the goop (that's a technical term, you'll get used to it) on the outside of the antenna.

Jim

Something that works really well is Freon based flux remover but you're not supposed to release that into the atmosphere anymore and it's hard to find in any case. I'd think that the newer fluorocarbon free flux remover would work pretty well but I'd try it on a hidden painted surface first. That said, IME if a connector is oxidized enough to affect operation it's far more effective to just replace it. The antenna itself can be removed and cleaned in a variety of effective ways and the ground connection can be cleaned up properly at the same time. And as I mentioned earlier, if the coax is more than 10 years old and/or an inferior type (like RG58 and the like) it would be a good idea to replace it as well. You'll spend less money doing that than it would cost to have someone attempt to troubleshoot this problem unless the feedline runs through some relatively inaccessible places.

 

[OtisAir]

I betcha a Mom's spit would work too. Used to work on me, a little dab on a tissue and my Mama could remove dirt, oil, grease, paint, tobaccy juice, and whatever else was on my face!