VOR troubleshooting

[Exocetid]

I have a KX-170B driving a KI-209. In January, while I was doing familiarization training on my plane, my instructor tuned it to the VOT at KSAT and found it off by around 10 degrees. My KX-155 drives a cheaper KI-201C and it is right on the money. I recently retested the two and now find that the KI-209 is off by about 30 degrees while the 201 is still dead on.

Is there any sort of troubleshooting I can do before I turn it over to an avionics shop?

 

[weirdjim]

Is there any sort of troubleshooting I can do before I turn it over to an avionics shop?

Just about the only thing you can do without good test equipment is to see if the VOR antenna is actually using a real splitter (a little box about half the size of a cigarette pack that will say "splitter" on it). Some folks think that a plain old Tee without any matching circuit will work, but I'm here to tell you it will NOT.

Jim

 

[Exocetid]

Just about the only thing you can do without good test equipment is to see if the VOR antenna is actually using a real splitter (a little box about half the size of a cigarette pack that will say "splitter" on it). Some folks think that a plain old Tee without any matching circuit will work, but I'm here to tell you it will NOT.

Jim

It was working and I am pretty sure it has a splitter; but that brings up another point, what is the layout of the system?

Each radio has two coaxes, comm and nav, at the back of the tray. So, do the two comms go to the two comm antennas on the roof and the two navs go to the splitter and thence to the cat's whisker on the vertical stabilizer?

What I really want to know is could this be a connector problem (old plane, hard landings) or is it pretty much a need for bench work on the radio? Is there an easy way to isolate it to the radio, or could it be the indicator?

 

[weirdjim]

Each radio has two coaxes, comm and nav, at the back of the tray. So, do the two comms go to the two comm antennas on the roof and the two navs go to the splitter and thence to the cat's whisker on the vertical stabilizer?

What I really want to know is could this be a connector problem (old plane, hard landings) or is it pretty much a need for bench work on the radio? Is there an easy way to isolate it to the radio, or could it be the indicator?

Yes, the two comms go to the vertically polarized antennas on the roof and the navs all go to the single catswhisker on the vertical fin.

If both radios are not affected, it is most probably not a connector problem. THere is no easy way that I know of to isolate it to the radio or to the indicator EXCEPT perhaps to listen to the audio of the defective nav versus the good nav on a weak signal. If there is a lot of static/noise on the defective radio vs the good radio, the odds are a problem with the radio. If both signals sound about the same signal to noise ratio, the odds are that it is in the head. Remember, it has to be a WEAK signal.

Jim

 

[gismo]

It was working and I am pretty sure it has a splitter; but that brings up another point, what is the layout of the system?

Each radio has two coaxes, comm and nav, at the back of the tray. So, do the two comms go to the two comm antennas on the roof and the two navs go to the splitter and thence to the cat's whisker on the vertical stabilizer?

What I really want to know is could this be a connector problem (old plane, hard landings) or is it pretty much a need for bench work on the radio? Is there an easy way to isolate it to the radio, or could it be the indicator?

The problem is almost certainly in the indicator. The KI-209 has a built in nav converter (takes the "video" from the radio and demodulates it to radial info) and the problem has to either be there (there are adjustments for the demodulator) or, much more likely, a mechanical problem where the OBS card isn't turning with the sensor that feeds the OBS angle to the nav converter.

 

[Exocetid]

If both radios are not affected, it is most probably not a connector problem. THere is no easy way that I know of to isolate it to the radio or to the indicator EXCEPT perhaps to listen to the audio of the defective nav versus the good nav on a weak signal. If there is a lot of static/noise on the defective radio vs the good radio, the odds are a problem with the radio. If both signals sound about the same signal to noise ratio, the odds are that it is in the head. Remember, it has to be a WEAK signal.

Jim

I haven't yet pulled the unit out to reseat, that may cure it--or might it? Also, on the KX-155, which I did pull and reseat, I noticed that it uses a edge connector and those are notorious for bad connections over time.

 

[Exocetid]

The problem is almost certainly in the indicator. The KI-209 has a built in nav converter (takes the "video" from the radio and demodulates it to radial info) and the problem has to either be there (there are adjustments for the demodulator) or, much more likely, a mechanical problem where the OBS card isn't turning with the sensor that feeds the OBS angle to the nav converter.

Is it possible that the GS works when the course needle doesn't? Would that tell me anything other than the obvious?

How do you test the GS without getting on an ILS course?

 

[weirdjim]

The problem is almost certainly in the indicator. The KI-209 has a built in nav converter (takes the "video" from the radio and demodulates it to radial info) and the problem has to either be there (there are adjustments for the demodulator) or, much more likely, a mechanical problem where the OBS card isn't turning with the sensor that feeds the OBS angle to the nav converter.

Gee, I'd probably give it even odds to be radio or head.

Jim

 

[John Collins]

Gee, I'd probably give it even odds to be radio or head.

Jim

I would guess the KI209 has a problem, odds 98.763 +/-.001.

 

[gismo]

Gee, I'd probably give it even odds to be radio or head.

Jim

While I can conceive of a modification to the radio that would cause the indicator to read 30 degrees off, a failure of the existing design that would cause that is hard to imagine. The radio simply provides the unfiltered output of the AM demodulator to the indicator. The circuitry that decodes that signal and determines what radial being received along with the part that compares that radial to the one selected on the OBS is inside the indicator not the radio. The radio can mess things up by producing a weak and/or noisy signal to the indicator but that would cause a wandering or stuck in the center needle, not a 30 degree error.

I suspect that the fact that in some setups (unlike the KX155/KI209 being discussed here) the nav converter is located inside the radio not the indicator and in that case the failure could be there (in the radio) or in the indicator in the form of an error of the OBS output to the radio. There's also a third configuration where the nav converter is a completely separate box connected to the radio and the indicator (e.g. the KX155/KN77/kni520 setup I have as my #2 navcom).

I'll go John C one better (even though he knows much more about avionics than I do) and put the odds at 99.9% indicator with the .1% residual resulting from the potential for a misstatement of the symptoms.

 

[gismo]

Is it possible that the GS works when the course needle doesn't? Would that tell me anything other than the obvious?

Yes. The GS is almost completely independent of the VOR/LOC function. About the only things it has in common with VOR/LOC reception/display is the radio's power supply and frequency selection electronics.

FWIW, it's also quite possible for the LOC function to work perfectly while the VOR indication is in error or absent. Those two share the same receiver paths but the decoding is completely different and there's no OBS input to the process. Strangely, the FAA doesn't require any periodic testing of the LOC or GS functions (and yes they can malfunction is a way that gives an erroneous display without any warnings).

How do you test the GS without getting on an ILS course?

Any avionics shop has the ability to generate a test signal which can be used to determine proper GS (as well as VOR and LOC) function. You can also get a tester for around $1000 that does the same but it's not certified for use in the required 30 day VOR check AFaIK.