CDI Wanders

[jefinley17]

I have an Archer 181 and recently had used KX-155 radios(1-with G/S) installed to replace the original kx170b/kx170 radios. The Com side works great - But the Nav. using the the vor 109/106 heads, - set to a radial good at first and then will start to wander. The tech. has checked the whisker antenna and checked each head on the ground with a tester. said it had a good reading. The needle can go full deflection back and forth - but at a VOT - (2000') it seemed to hold. ILS and GS seen ok as well. My thought is a grounding problem or sensitivity issue with the newer radio. Do the heads have to be calibrated to the newer radios? Maybe the coax needs to be upgraded. Does the splitter need to be grounded? Thought I would put this out there before the shop looks for it.

 

[poadeleted20]

With how many different VOR stations does this happen? Does it change with distance? radials? Have you check the Navigation NOTAMs to see if the VOR station has bad radials?

 

[jefinley17]

All the stations I've used are good. Both (2) VOR heads drift,when both are set to the same station and same radial. I have tried this with several stations and radials with the same result. And holding heading with a gps(gx155). Sometimes the needles drift together, sometime drift in reverse directions. I can reset them and they hold for a while ( less than a minute) and then drift and then recenter then drift reverse then center?(sometime for more than minute with slight drifting and to/from shows some instability) Tracking to a station seems to reduce the problem as you get closer.

 

[weirdjim]

This is almost universally an antenna problem. THink about it...what is the only thing (besides the battery) that is common to both NAV radios? The antenna.

THe tech "checked" the whisker antenna. How? An antenna balun (chunk of coax right at the antenna) that has a broken wire will "look" good to a VSWR meter (if the tech even went so far as to check it) but will give needle wander to some radio designs.

Now the splitter. What make and/or model? Some folks mistakenly believe that a simple metal box with three connectors wired together somehow "magically" splits the signal. T'aint so. You need either a chunk of ferrite or a couple of lengths of a special (72 ohm) coax with a carbon resistor across the outputs to do the job right.

Jim

 

[Hiperbiper]

I have an Archer 181 and recently had used KX-155 radios(1-with G/S) installed to replace the original kx170b/kx170 radios. The Com side works great - But the Nav. using the the vor 109/106 heads, - set to a radial good at first and then will start to wander. The tech. has checked the whisker antenna and checked each head on the ground with a tester. said it had a good reading. The needle can go full deflection back and forth - but at a VOT - (2000') it seemed to hold. ILS and GS seen ok as well. My thought is a grounding problem or sensitivity issue with the newer radio. Do the heads have to be calibrated to the newer radios? Maybe the coax needs to be upgraded. Does the splitter need to be grounded? Thought I would put this out there before the shop looks for it.

Check the cheap things first.
Check the coax cables going to and coming from the splitter. It wouldn't be the first time someone put the antenna lead to the GS port...
Look for pulled cable ends, cables bundled with electrical wires, ect.

If the antenna/coax/splitter system worked with the 170's there is no reason to suspect they somehow crapped out when you connected new radios to 'em. If everything was working prior to the change look at what you changed, not what was left intact.


JMPO
Chris

 

[jefinley17]

Thanks to all for your input.
I agree with the antenna being the issue. A little more background....kx155 nav/com without the g/s was installed first(shop #1) and the VOR for that unit started the wandering and the KX170B unit (still installed) VOR worked o.k. Then the second KX 155 with G/S was installed weeks later (shop #2)(when the shop removed antenna,bench checked-rebuilt the balun as a solution to the drifting to radio installed 1st) and then both VOR,s started the wagging on the flight home. Both radios were bench checked (with some minor repairs) also the Archer on earlier 181 has a vent in the tail which can take on weather and is where the 31 yr old coax runs from the tail. I'll see if your suggestion help this weekend and see where stuff was bundled..... I'll let you know what I find.

Thanks again, Jim

 

[weirdjim]

If the antenna/coax/splitter system worked with the 170's there is no reason to suspect they somehow crapped out when you connected new radios to 'em. If everything was working prior to the change look at what you changed, not what was left intact.

JMPO
Chris

There have been isolated cases where a difference in the internal design of the radios causes one model to wander and another to be rock solid with a defect in the antenna. However, if the radio shop took the antenna off the aircraft and actually did a rebuild on the coax balun, then I would suspect (as you did) that something in the installation went sour. I'm interested in the comment that with just one of the new radios installed, the new radio wandered while the old 170 was rock solid. That's an interesting clue that I don't have an answer for except the difference (as I noted above) in the internal radio design itself.

I'm leaning more and more right now to that splitter. Here's a good test. Remove the splitter entirely and connect the antenna directly to one or the other of the new radios (connect the coax connectors with a barrel). If that cures the problem, you've got splitter problems. If it doesn't, more investigation is necessary.

Jim

 

[jefinley17]

The solution to any problem is perseverance..... and a little help from your freinds.
It seems inconceivable both installations resulted in the same error. while checking the spliter - finding no change in either nav, I decided to check the continuity of the coax to the spliter. Pulled the radios, checked the nav coax to spliter and nothing. Then I decided to check the com. side coax to spliter and it zeroed the meter. I am saying "why would the com. coax go to a spliter when I have 2 com antennas?" So I crawled into the tail and disconnected one of the com. antennas and how about that... the signal to nav #1 goes dead! Then the other com antenna and Nav #2 no signal.
How does this happen? Well I checked the older Kx170 radios and the nav antenna input is on the back left (when facing the display) and on the new radios Kx155 they put the com input on the back left. I guess no one was checking what antenna went where. So the local shop swaped them out for me right away.... and the result was rock solid VOR tracking... No sway or drift. no charge!
What I don't understand Is why the com's side worked so well on the nav whisker antenna with a split signal?
Maybe the this can help the next guy with a radio change--- the more I learn the less I find I Know!!!
Anyway thanks for the help.

Jim

 

[weirdjim]

It seems inconceivable both installations resulted in the same error. while checking the spliter - finding no change in either nav, I decided to check the continuity of the coax to the spliter. Pulled the radios, checked the nav coax to spliter and nothing. Then I decided to check the com. side coax to spliter and it zeroed the meter. I am saying "why would the com. coax go to a spliter when I have 2 com antennas?" So I crawled into the tail and disconnected one of the com. antennas and how about that... the signal to nav #1 goes dead! Then the other com antenna and Nav #2 no signal.
How does this happen? Well I checked the older Kx170 radios and the nav antenna input is on the back left (when facing the display) and on the new radios Kx155 they put the com input on the back left. I guess no one was checking what antenna went where. So the local shop swaped them out for me right away.... and the result was rock solid VOR tracking... No sway or drift. no charge!
What I don't understand Is why the com's side worked so well on the nav whisker antenna with a split signal?
Maybe the this can help the next guy with a radio change--- the more I learn the less I find I Know!!!
Anyway thanks for the help.

Jim

It is pretty easy to narrow things down when you've only got one variable with one degree of freedom.

Things to remember: You CANNOT use a splitter to connect two coms to a single nav antenna. You are most fortunate that keying up the transmitter on the coms didn't blow the front end out of the other com. MOST fortunate. You might want to pull the splitter apart and see if it is a coax splitter with a carbon resistor between the split outputs (probably french fried if it is) or a full ferrite balun splitter that could probably absorb the transmitter output. Replace the resistor (100 ohms) if fried. If ferrite, just give a little prayer.

When checking out a coax for open/short, take BOTH ends of the coax apart and put a known resistor value (I like something like a few hundred ohms) at one end so that a short or an open at the other end won't show up as a "good" coax.

Glad you found the problem.

Jim