COMM1 interferes with NAV1 CDI

itsjames2011

itsjames2011

Pre-takeoff checklist
Joined
Jul 21, 2014
Messages
165
Location
Chicopee MA
Display Name
Display name:
James
Hey everyone,

I have a KX175B NavComm with a KI-209 CDI/Glideslope indicator. If I tune up a VOR and I am in range to receive it, the indicator works normally until there is activity on COMM1. If I transmit or receive a signal on COMM1, the CDI and Glideslope needles go all jumpy yet if I turn the COMM1 side of the radio off, everything seems fine.

Any thoughts on wether this is an radio or indicator issue? Also is there anywhere that does good work on these old setups that I could send it off to?

James
 
Could be either. Could also be a cabling issue. My guess is RF from the comm transmitter is getting into the wiring between the indicator and the nav radio.
 
itsjames2011 said:
Any thoughts on wether this is an radio or indicator issue? Also is there anywhere that does good work on these old setups that I could send it off to?
Click to expand...

ROn is correct. It isn't the radio or the indicator but the wiring in between. There are LOTS of things you can do to keep the coupling down, but sending the radio or indicator off to be fixed isn't on the list.

If you sent them off, you will get three things back: the radio, a tag that says nothing was wrong, and a bill.

Find an old radio guy (gray around the ears) that understands radio transmit cable wiring, looking carefully at the antenna VSWR, how to get rid of RF in the bundles, and bypass capacitors.

Most of the kids working in radio shops today know little more than how to swap out boxes and not how to fix the problem.

Jim
 
flyingron said:
Could be either. Could also be a cabling issue. My guess is RF from the comm transmitter is getting into the wiring between the indicator and the nav radio.
Click to expand...

Or there's a poor ground (or unshielded cable) on one of the cables. I'd also check the antenna & cable for the COM side as lousy standing wave ratio can cause the cable to radiate inside the cabin.
 
All of those things would cause RF to get in the indicator-to-radio circuitry which is exactly what I (and Jim) was saying.
 
I agree with the others that there is a wide array possibilities, so unless you want to pack up the entire plane and ship it as a unit, there is not much chance you can just "send it off" somewhere and get it fixed. You're going to have to fly that plane to a competent avionics shop for proper troubleshooting.

So, good people -- who knows a good avionics shop reasonably close to Chicopee MA?
 
Being an Extra class ham radio operator I know all about rf coupling, wiring, and VSWR however the symptoms are exhibited when no transmitting is happening at all. Even receiving a signal on COM1 causes the CDI and GS needles to go crazy.
 
itsjames2011 said:
Being an Extra class ham radio operator I know all about rf coupling, wiring, and VSWR however the symptoms are exhibited when no transmitting is happening at all. Even receiving a signal on COM1 causes the CDI and GS needles to go crazy.
Click to expand...


Makes it easier to find, actually. What circuits (and wiring) are active only when COM1 is receiving? Where are they run behind the panel?

If it's that consistent, a buddy standing ten feet away with a handheld can reproduce the problem while you move things in the wiring bundle or hunt down the loose ground wire.

(Keep him far enough away you aren't chasing the effects of his handheld RF though.)

Or just find said grey-haired RF engineer and show them with your handheld.
 
itsjames2011 said:
Being an Extra class ham radio operator I know all about rf coupling, wiring, and VSWR however the symptoms are exhibited when no transmitting is happening at all. Even receiving a signal on COM1 causes the CDI and GS needles to go crazy.
Click to expand...

Well, you've answered your own question.
Turn the AF gain (volume) all the way down and see if that has any effect. Could be as simple as a "wild hair" on the audio output. Or, leave comm1 turned on, but isolated via the audio panel, and see if the trouble still presents.
May not be RF related at all.
 
wsuffa said:
Or there's a poor ground ...
Click to expand...
That would be my first bet. Somewhere there is probably a bunch of ground wire ring terminals held against the firewall by a screw. Find it, take it apart, clean everything carefully especially the panel area where the ground lugs want to make contact, reassemble and see if that helped. 600 grit wet-or-dry is your friend if things are really ugly. For less aggressive cleaning, the eraser on a wooden pencil has enough grit to shine things up nicely.
 
itsjames2011 said:
Being an Extra class ham radio operator I know all about rf coupling, wiring, and VSWR however the symptoms are exhibited when no transmitting is happening at all. Even receiving a signal on COM1 causes the CDI and GS needles to go crazy.
Click to expand...

Oh, well, pardon Ron and I. We've both got a handful of degrees in EE and have both spent our lives in RF, one of us in aircraft RF. So your Extra trumps that of course.

So, Extra, just one question to see how much you DO know. With a 2:1 VSWR, what is the return loss (in dB ) and what is the % of power reflected back down the feedline?

Jim, WX6RST(Extra) for 60 years, avionics engineer for 50 years, specializing in concealed antennas. And the attendant interference problems.
 
Last edited:
weirdjim said:
Oh, well, pardon Ron and I. We've both got a handful of degrees in EE and have both spent our lives in RF, one of us in aircraft RF. So your Extra trumps that of course.

So, Extra, just one question to see how much you DO know. With a 2:1 VSWR, what is the return loss (in dB ) and what is the % of power reflected back down the feedline?

Jim, WX6RST(Extra) for 60 years, avionics engineer for 50 years, specializing in concealed antennas. And the attendant interference problems.
Click to expand...


LOL. Saw that rant coming. :) You really should have told me where the popcorn was in the pantry before posting that Jim. I wasn't ready.
 
Jim
Well, I'm not an Extra, just a General for 55 years. First class with radar for 45 years. 30 years broadcast engineer, Retired Manager of Transmitter Operations for SC Educational TV Network, PBS network for South Carolina. 11 TV stations, 10 UHF, 1 VHF, 8 FM stations and all the weather stations and the state wide microwave. RF from 1 watt Microwave to 5 MW ERP TV..

With a 2:1 VSWR, what is the return loss (in dB ) and what is the % of power reflected back down the feedline?

I think what you're asking for is
RL 9.542 db
reflective 11.1%
transmitted 88.9%

I've made these type measurement many times over the years...But all that is in the past now. High power RF was so much fun until it arced over..

Gerald K4NHN
Cayce, SC
 
I dunno, I find qrp 5w with a good antenna to be pretty fun.
And workin' hf with an antenna at 5500' would be the cat daddy. :D
 
weirdjim said:
Oh, well, pardon Ron and I. We've both got a handful of degrees in EE and have both spent our lives in RF, one of us in aircraft RF. So your Extra trumps that of course.

So, Extra, just one question to see how much you DO know. With a 2:1 VSWR, what is the return loss (in dB ) and what is the % of power reflected back down the feedline?

Jim, WX6RST(Extra) for 60 years, avionics engineer for 50 years, specializing in concealed antennas. And the attendant interference problems.
Click to expand...

First off I'd say I'm very surprised by the attitude coming from you Jim. We've spoke in the past and I never took you as the type to spout off and turn into an ******* for no apparent reason.

I never tried to turn this into a credential battle but please don't talk to me like I'm an idiot.

I stated that this issue happened in a RECEIVE only condition so the idea that I have a bad SWR, an odd multiple length feedline, or some other incidental radiator/RF coupling problem can't possibly be the issue.

On a side note...If you're going to vet someone on a technical subject, you probably shouldn't pick a question that has the answer in a graphical chart within the first chapter of any Gordon West or ARRL License Class Manual.
 
Yeah, I sort of figured any damfool could look to a cookbook response, even though some damfool before you took it out of your hands. I wanted to see if you could do it and give the theory and/or the equations for it, but that doesn't seem to be possible in this NG.

Sorry, but when you come onto the group with an Extra as your total chops that fires my rocket. And, when you negate my BS/MS/Extra/fifty years in the biz, that just makes the whole thing go away.

NOW, let's get back to the problem. It seems that AUDIO is upsetting the applecart. Let's think about that.

Jim
 
weirdjim said:
Yeah, I sort of figured any damfool could look to a cookbook response, even though some damfool before you took it out of your hands. I wanted to see if you could do it and give the theory and/or the equations for it, but that doesn't seem to be possible in this NG.

Sorry, but when you come onto the group with an Extra as your total chops that fires my rocket. And, when you negate my BS/MS/Extra/fifty years in the biz, that just makes the whole thing go away.

NOW, let's get back to the problem. It seems that AUDIO is upsetting the applecart. Let's think about that.

Jim
Click to expand...


I was not trying to negate your experience at all, it's just that the things I was reading really didn't make sense as far as my problem goes. As far as theory ask away, I've built all of the antennas here at the shack along with several transceivers from scratch :D

I pulled the radio out of the airplane today, took a look under it's panels, and did my best to try and look for some kind of grounding bus but was unable to locate anything through the very limited access that's afforded without pulling the entire panel out of the airplane.

Tomorrow I am meeting with an avionics guy named Eckhard Straeter and he's going to have a look so I'll let you know what he has to say.

I'm thinking that the idea of a bad ground could be correct because the radio is having to send a signal to the speaker and perhaps a bad ground letting some of that current seep back in?
 
itsjames2011 said:
I'm thinking that the idea of a bad ground could be correct because the radio is having to send a signal to the speaker and perhaps a bad ground letting some of that current seep back in?
Click to expand...

That is one possibility. However, trying to get that speaker signal into the nav audio is something I can't see.

The other possibility is that an electrolytic in the power supply feeding both the NAV and COM audio amplifiers may have opened up allowing COM audio to get into the NAV signal.

One sure way of seeing if it is speaker ground loop current getting into the mix is to open up both speaker and phones lines while COM audio is present and see if the needle quiets down. If not, the coupling is internal.

Just a couple of random thoughts.

Jim
 
Met with the avionics guy yesterday and he brought a portable vor and glideslope testing transmitter with him. He says the indicator(KI-214) is bad and that the radio needs a lower frequency crystal on the comm side and new volume pots on both sides. He also said the KI-214 is junk and not to replace it with another one. I guess at this point I'm going to be looking for a new(more likely new to me) Nav/Comm and Indicator. Any suggestions? I really want to start working on my instrument rating and I'd like to be able to do it in my own airplane....


weirdjim said:
That is one possibility. However, trying to get that speaker signal into the nav audio is something I can't see.

The other possibility is that an electrolytic in the power supply feeding both the NAV and COM audio amplifiers may have opened up allowing COM audio to get into the NAV signal.

One sure way of seeing if it is speaker ground loop current getting into the mix is to open up both speaker and phones lines while COM audio is present and see if the needle quiets down. If not, the coupling is internal.

Just a couple of random thoughts.

Jim
Click to expand...
 
itsjames2011 said:
Met with the avionics guy yesterday and he brought a portable vor and glideslope testing transmitter with him. He says the indicator(KI-214) is bad and that the radio needs a lower frequency crystal on the comm side and new volume pots on both sides. He also said the KI-214 is junk and not to replace it with another one. I guess at this point I'm going to be looking for a new(more likely new to me) Nav/Comm and Indicator. Any suggestions? I really want to start working on my instrument rating and I'd like to be able to do it in my own airplane....
Click to expand...


Did he explain how any of those things were causing your actual problem? None of that directly ties to a specific electrical problem that would do what you described.

As far as the indicator goes - no advice other than to make sure you get one that'll handle future upgrades like adding a GPS. Some older ones can't be driven by the modern GPS units.
 
itsjames2011 said:
Met with the avionics guy yesterday and he brought a portable vor and glideslope testing transmitter with him. He says the indicator(KI-214) is bad and that the radio needs a lower frequency crystal on the comm side and new volume pots on both sides. He also said the KI-214 is junk and not to replace it with another one. I guess at this point I'm going to be looking for a new(more likely new to me) Nav/Comm and Indicator. Any suggestions? I really want to start working on my instrument rating and I'd like to be able to do it in my own airplane....
Click to expand...

Y'know, having been in this biz since I was 16 (yeah, I swapped Piper Colt hours for Superhomer repair hours) I've learned to distinguish real answers from 6u11$#it answers. None of what I'm seeing here is going to disrupt NAV signals from COM activity. I've got a good KI-201 that you can use to plug in where the 214 is now ... but I don't remember if the connector is the same. I'll send you the 201 to try just for the postage if you promise to return it. I think I can probably check out your 214 on the bench but not for at least another month ... we are torn up moving the lab from one end of the property to the other.

Jim
 
Jim,

I can't seem to find a definitive answer on whether the 201 and 214 have the same connector or not but I would love to test another indicator in there. As soon as we get some nice weather I will go up and take a video of the indicators in action so you can see exactly what is happening.

My intro to airplane ownership has not been a good one so far but I'm hoping this will be the last of the issues for a little bit. The airplane I thought I got a good deal on has already required $11,000 in maintenance in the first 3 months because apparently Helen Keller did my pre buy :mad2:


weirdjim said:
Y'know, having been in this biz since I was 16 (yeah, I swapped Piper Colt hours for Superhomer repair hours) I've learned to distinguish real answers from 6u11$#it answers. None of what I'm seeing here is going to disrupt NAV signals from COM activity. I've got a good KI-201 that you can use to plug in where the 214 is now ... but I don't remember if the connector is the same. I'll send you the 201 to try just for the postage if you promise to return it. I think I can probably check out your 214 on the bench but not for at least another month ... we are torn up moving the lab from one end of the property to the other.

Jim
Click to expand...
 
UngaWunga said:
The two VORs near you don't work?

http://skyvector.com/?ll=42.19278082010722,-72.5340270929241&chart=301&zoom=2

Wow, there's a ton of VORs out there...
Click to expand...

Well, this is a pretty flat part of the world, so the formula for range is

range in miles = square root of (2 times the height of the antenna in feet).

Since the VOR is in essence at ground level, and the rabbit ears on the top of the rudder are about 10 feet above ground, the maximum range is somewhere around 4 miles, maybe a little less to get a good locked signal. That's not very far.

Jim
 
weirdjim said:
Well, this is a pretty flat part of the world, so the formula for range is

range in miles = square root of (2 times the height of the antenna in feet).

Since the VOR is in essence at ground level, and the rabbit ears on the top of the rudder are about 10 feet above ground, the maximum range is somewhere around 4 miles, maybe a little less to get a good locked signal. That's not very far.

Jim
Click to expand...

It really isn't very far. With the on field VOR being out of service, the next closest is 8 miles to the west and the signal has to go straight through a 1400' mountain range...
 
So after replacing all of the coax, checking all of the grounds and even putting an analyzer on the antennas, I was at a complete loss. It turned out there was some sort of RF coupling issue inside the radio. Replaced with a new KX-175b today and all is well.
 
You must log in or register to reply here.
Back
Top