CDI shows LOC and GS but not VOR

k9medic

k9medic

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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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k9medic said:
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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Does the #2 radio have a glideslope receiver? Not all do and given that the second indicator is VOR only, I'd suspect that.
 
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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k9medic said:
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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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.
 
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.
 
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.
 
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.
 
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.
 
SCCutler said:
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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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.
 
Apologies for reviving this old thread but experiencing a similar situation.

@k9medic, were you ever able to determine the part why the CDI would display LOC indication but not VOR? In other words, I am interested in the lateral display (not the glide slope). Did it turn out to be a radio problem or a CDI problem?
 
The VOR/LOC Converter is undoubtedly failed. This can be a separate box, integrated into the radio or more likely integrated into the CDI.
 
@John Collins, thank you for your reply. You were absolutely correct. Unfortunately the avionics shop blamed the radio after "diagnosing" it. Turns out, the radio was - and is - perfectly fine, and a $125 replacement KI-208 indicator off of eBay took care of the entire problem.
 
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