Head Scratcher - VOR G/S Problem

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Randy Stuart

Filing Flight Plan
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Sep 24, 2022
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Idaho Maule
On a plane I bought a year ago, I have been chasing this problem down for months and still can't find the cause... I have a KX155, Century HSI & converter and switcher (GPS/VOR). The GPS (250XL) indicates just fine but the VOR is not working..
I've swapped the antenna (the code is loud and clear), I've swapped the 155 (tested mine in another plane, works fine), swapped the converter, swapped the diplexer and bussed the switcher... Can't find anything wrong..
I have a test set for VOR, LOC and G/S that transmits the test signals from inside the plane. They make the VOR, LOC and G/S work fine (flags and needles move as expected), but the VOR, LOC and G/S (while flying) does nothing.
Yesterday I flew to Twin Falls, there's a VOR right off the side of the runway.. Good place to try and test it while landing. Nothing!!... While parked on the tarmac I turned on the radios to see if it would do anything, the VOR worked fine..... So I'm thinking maybe the antenna IS bad (even thought I bussed it and tried another antenna... Then we go to leave, I started the plane and the VOR didn't work anymore, taxied and took off right next to the VOR, nothing!! Now I'm thinking it has something to do with the engine running but there's no radio noise or Alt whine and all the other avionics work fine.
Anyone ever see something like this? Anyone have any ideas what to look for. I'm very experienced in avionics and I'm an IA but this is a new one on me. Time to ask the brain trust....
 
What changes with the engine running vs not running? Have you tried engine running w/alternator off?
 
Nothing changes with the engine running other than the engine running, there's no electronics on the engine other than the JPI monitor probes. I have not tried turning off the alternator but that's worth a try.
 
Could a throttle or mixture cable be tugging/pushing/bending on a antenna connection just enough to disrupt the signal?
 
I connected another VOR antenna & balin to the diplexer, it made no difference. There's nothing touching the radios or antenna cables.
 
Randy Stuart said:
I connected another VOR antenna & balin to the diplexer, it made no difference. There's nothing touching the radios or antenna cables.
Click to expand...
Since everything in the cockpit works, you may want to at least check the coax between the antenna and radio with a wattmeter and multimeter. Sounds more like a signal receive loss somewhere. Where's the antenna located?
 
It's a cat whisker VOR antenna on the top of the tail. I checked the continuity of the cable, it's good and not shorted. But I put another VOR antenna with a new cable and balin on the rear deck, it made no difference.
 
Randy Stuart said:
I checked the continuity of the cable, it's good and not shorted.
Click to expand...
Well if you think the cable to the radio is good then given your extensive TS history with no results it looks like you're at the proverbial fish or cut bait point. Perhaps temp run a new cable from the rear deck to the radio and try to make something work. I think that is the only part you didn't swap? Good luck.
 
Have you verified that the antenna leads are going to the proper locations? Verified the tray connectors don't have any bent or pushed back pins?
 
Is the balun hooked to a splitter? If so, is the splitter's "VOR Out" and "GS Out" correctly connected to the radio? I've hooked these up backwards before and it didn't work.

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