KNS80 not displaying DME information

BigBadLou

BigBadLou

Final Approach
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Lou
Hi guys, a problem in the new (to me) airplane again.
It has the Bendix-King KNS80 VOR RNAV unit. Fancy stuff for its era, obviously.
kns80.jpg


When in simple VOR mode, the tuned-in frequency gives correct indications on the CDI but the DME information is missing, it displays only dashes, such as in the picture below. (and yes, we were close enough to the VOR, we tried multiple VORs, they all were supposed to have DME information)

The user manual is not very helpful. It only mentions that the DME information will be missing if the DME is inop. Heh, great, thanks. I guessed that. :)
The previous owner swears that the DME worked "just recently". (let's not comment on that)

I have two questions for you:
1) Is there maybe a trick to using the KNS that would cause the DME information to be missing?
2) How would one troubleshoot a DME problem? (or is that something for my mechanic and my credit card to resolve?)

Thank you in advance for any useful replies.
 
Have you pushed all of the buttons at random and hoped for the best yet?
 
Make sure the dme antenna is not covered in oil sludge and the coax cable is properly attached.
 
Brad Z said:
Make sure the dme antenna is not covered in oil sludge and the coax cable is properly attached.
Click to expand...

And if that doesn't help, it's possible to have an avionics shop test the DME on the ground. But start with cleaning the antenna.

DME antennas live in crud on the belly of not clean airplanes, and at those frequencies the crud is often a severe SWR or absorption problem.

Especially if the oil sludge on them is full of water too.

Or the antenna is, or the coax is old and rotten, which gets expensive quicker.

All of which can be tested for on the ground with the right test gear...
 
As the others mentioned - first check would be to determine whether there is an antenna or coax problem.
 
mkosmo said:
Have you pushed all of the buttons at random and hoped for the best yet?
Click to expand...
Yup! :D No change.

Brad Z said:
Make sure the dme antenna is not covered in oil sludge and the coax cable is properly attached.
Click to expand...
Belly is nicely clean. But I will try to actually find the antenna, no idea where it is.

denverpilot said:
And if that doesn't help, it's possible to have an avionics shop test the DME on the ground. ... All of which can be tested for on the ground with the right test gear...
Click to expand...
Yup, trying to get some good tips here first and if that doesn't solve it, I will be pulling out my checkbook and bending over.

kgruber said:
Install a loaner KNS80.
Click to expand...
Got one? Those are not easy to find.
 
steingar said:
Buy a spare. They can be had on Ebay for not that much coin.
Click to expand...

Problem with that is, if it's the antenna or feedline, plugging a spare into the bad antenna may just blow up "not that much coin" on the first transmission of the DME. :)

(Which is why I haven't just gone out and slapped a new-to-us DME in the slot where ours is removed from after it croaked... but my failure was the display, so I have a better shot at it being just that, and not RF-related.)
 
Nate is right. There's something unnerving to piping 100W or RF into an antenna cable whose SWR might be through the roof.
And I do NOT want to find out whether the 1980's designers of the KNS thought of a failsafe for an open output. *eep*

I'l try y'all's great suggestions (hopefully tonight) and report back.
 
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