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Troubleshooting NAVCOM or CDI...

B

bill.hutchison

Well Known Member
Folks - I'm hoping the good ship VAF might have some ideas.

I have an SL30 hooked up to a MidContinent CDI.

Whilst flying about this weekend I tuned in a couple of different ILSs at different airports to see how well it worked. Unfortunately, I had a GS flag the entire time.

LOC would capture just fine. But never could get a GS.

Do you guys have any ideas how I could figure out if it's the SL30 which is borked or if it's the MidContinent CDI?

I know swapping the SL30 with a known-good one would help, but I don't have one laying around and they're hard to source. Wondering if there is any kind of diagnostic I could do to figure this out.
 
GS <> LOC

The Glideslope is a separate radio signal from the Localizer. On the back of a typical Nav/Com you'll have 3 antenna ports: COM, NAV, and GS.

First thing I'd check would be the antenna connections. Is it running to a dedicated GS antenna or through a 'diplexer' that connects both NAV and GS to one antenna? Carefully examine all the cables for issues.

We can go from there.
 
Previous poster: The SL30 has an internal diplexer to separate GS and LOC signals; just one nav antenna input.
Q: Are you sure you didn't accidentally enter BC (back course) mode? e.g., fly left and fly right were not showing reversed? BC mode disables the GS.
Since neither the GS needle nor the flag worked, that would, I think, require 2 separate failures inside the CDI box. But at least look around at the connectors, make sure somehow wires didn't get yanked off.
If you want to fly to CA I'd be happy to swap my SL30, to see if it's the SL30 box or the CDI or wiring. Surely there must be someone closer, though!
You can hunt around for a repair shop that will look inside, or do it yourself (maybe something easy, like a broken antenna cable post-splitter). Unfortunately the combination of Garmin's fixed price repair, plus the age of the SL30, doesn't look favorably on going that route. Even though I believe the SL30 is still the best nav ever made!
 
I'd be tempted to try the following (no guarantees):
First, check BC is off.
1. easy but long shot: Unplug the cables to the CDI and the SL30. Spray all pins with contact cleaner, re-install. Test.
2. Fly to airport with ILS. Park with good view of GS antenna. A good spot would be a runup area at or near the runway threshold, so the GS antenna is 1000' down the runway, so you should have a 'fly up' needle. Unplug the CDI. Use the schematic to locate the 2 pins on the cable connector that drive the needle up/down. With the SL30 powered up, tuned to ILS, put a voltmeter across those 2 pins. You should see something like 0.1 volts or so (set voltmeter to a sensitive scale). If it reads exactly zero then the problem is not the CDI. You could repeat this test with the CDI up/down pins on the backside of the SL30 (to test the harness) although access will be hard.
3. If all that a fails I'd open up the box, just look around for broken wires, broken solder joints, black burned to a crisp parts, etc. Very much a long shot but easy to do.
Good luck.
 
BobTurner said:
I'd be tempted to try the following (no guarantees):
First, check BC is off.
1. easy but long shot: Unplug the cables to the CDI and the SL30. Spray all pins with contact cleaner, re-install. Test.
2. Fly to airport with ILS. Park with good view of GS antenna. A good spot would be a runup area at or near the runway threshold, so the GS antenna is 1000' down the runway, so you should have a 'fly up' needle. Unplug the CDI. Use the schematic to locate the 2 pins on the cable connector that drive the needle up/down. With the SL30 powered up, tuned to ILS, put a voltmeter across those 2 pins. You should see something like 0.1 volts or so (set voltmeter to a sensitive scale). If it reads exactly zero then the problem is not the CDI. You could repeat this test with the CDI up/down pins on the backside of the SL30 (to test the harness) although access will be hard.
3. If all that a fails I'd open up the box, just look around for broken wires, broken solder joints, black burned to a crisp parts, etc. Very much a long shot but easy to do.
Good luck.
Click to expand...

Bob - thanks for the advice. I'll take a look and get back to you. Appreciate the guidance.

Do avionics shops have ways of simulating LOC/GS to test components?
 
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