CDI shows LOC and GS but not VOR

Discussion in 'Avionics and Upgrades' started by k9medic, Mar 12, 2019.

  1. k9medic

    k9medic Pre-takeoff checklist

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    Here is yet another “new” finding with my airplane.

    We have dual Narco Mark 12E radios with one VOR/LOC/GS CDI and a second VOR only CDI.

    The #1 CDI will not display a VOR indication. We have swapped radios in the stack and the problem exists on both radios. Here is where is gets interesting.

    When we swapped the number two radio into the number one position, we also lost the glideslope ability.

    Any thoughts?


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  2. jsstevens

    jsstevens En-Route PoA Supporter

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    Does the #2 radio have a glideslope receiver? Not all do and given that the second indicator is VOR only, I'd suspect that.
     
  3. SCCutler

    SCCutler Administrator Management Council Member PoA Supporter

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    Did all 12Es have a GS receiver, or was that optional? If it was optional, it would not have made sense to buy the glideslope-equipped radio to use with a CDI-only indicator.
     
  4. k9medic

    k9medic Pre-takeoff checklist

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    I’m not sure on the second radio. Any way to quickly check?


    Why would the second radio not at least display a VOR signal when put in the #1 slot?


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  5. jsstevens

    jsstevens En-Route PoA Supporter

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    I'm not sure but I read your original email to say you lost the glide slope. That is quite likely no glide slope receiver in radio #2.
     
  6. EdFred

    EdFred Touchdown! Greaser! PoA Supporter

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    Is there a notam on the vor? Was up with a student and couldnt get a vor to come in. It was worse than the notam said. Tried another vor and voila, worked perfectly fine.
     
  7. flyingron

    flyingron Touchdown! Greaser!

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    No, not all 12E's have glideslope. It was a pretty common practice to only have the #1 have the GS (both receiver in the radio and indicator). The #2 should, however, work with the localizer.

    As for the problematic indicator, despite their similarity in presentation, the LOCALIZER and the VOR signals are completely different. It's not inconceivable that one works without the other.
    When you swapped the radios, did the VOR and LOC work? If the LOC worked, then there's probably something wrong with the #1 radio itself. If nothing worked, its probably the indicator. Of course, the wiring harness could also be a problem. If people have been yanking radios and reinserting them willy nilly, I'd take a very close look at the connectors on the back of the radio and their mates in the trays.
     
  8. k9medic

    k9medic Pre-takeoff checklist

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    When I swapped them the #1 CDI did not show VOR movement. It did show LOC movement. No GS either.
     
  9. Ryan Klems

    Ryan Klems Pre-Flight

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    The 12E had a version with and without glide slope (looking at the back of the radio, you'd have 3 antenna connectors vs 2 when GS equipped). Nearly 100% guarantee that your second did not have a glide slope receiver since it was connected to a lateral deviation only CDI. I think you are indicating that both the #1 and #2 12E's work with the #2 indicator to display VOR indications? If so then it's likely internal to the #1 CDI, or the circuit for the OBS not round tripping between the radio and CDI. I'd shine a flashlight in the #1 tray and take a good look at the pins. Any green creeping crud? The molex card edge connectors can also lose tension over time and not make good contact with the connectors on the back of the radio. Someone who knows what they are doing could carefully tweak them with a pick to provide more tension. You don't mention what CDI's you have installed, but I'd guess probably a ID825 on the #1. Since you are getting localizer indications, it can drive the needles, so that circuit is working, I'd focus at looking at the OBS pins 11, 6, 5, 8 on the 103 connector which should be the upper right when looking at the tray from the front.
     
  10. k9medic

    k9medic Pre-takeoff checklist

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    Thanks Ryan!
     
  11. SCCutler

    SCCutler Administrator Management Council Member PoA Supporter

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    One other thing: The 12E is capable of displaying (on the radio's digital display) which radial you are on when you are receiving a VOR signal; you might check to see whether each radio will properly interpret and display the radial. That's a pretty good indication that the radio's OK.
     
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  12. k9medic

    k9medic Pre-takeoff checklist

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    I never realized that! I've put 30 hours or so on the plane since I bought it but it's always been back and forth to the same location. Enter waypoint into GPS and go direct.