Confusing Radio Issues - Any thoughts?

[HamishTHaggis]

Hey out there....I acquired a 172L recently, and it is a delightful bird to fly, but has a couple of odd quirks that we've recently noticed and I'd love to see if anyone has thoughts on what may be causing the issue so we can estimate both the severity and possible repair costs. So here goes:

Aircraft has two MX300 Nav/Com units, essentially identical, and a KA134 audio panel. The issues we've noticed are:

(1) The primary COM 1 appears to be weaker in reception than the secondary. On a recent trip we were handed off to a relatively distant approach control and were not receiving replies. Holding down the test button (i.e. overriding the squelch) made the replies audible but still faint. Switching to COM 2 revealed loud and clear reception on the same frequency. This has happened a few times, in each case relatively distant from the earth station from which we were receiving, and our transmissions are being received (confirmed by relays on guard, second com reception) so it is truly a receiving issue.

(2) The secondary COM 2 seems not to receive ASOS/ATIS/AWOS at all sometimes even though it seems to be a better receiver for most COM frequencies. During the flight on which we discovered item (1), we began to use COM 2 as our primary from that time forward. When nearing our home field, we attempted to listen to the ATIS and received nothing. Switching to COM1 - loud and clear on the ATIS. This has occurred at one other field thus far, and at our home field a few times.

(3) When the audio panel is set to Tx on COM1 or COM2, and to Rx on "BOTH," pressing the mix transmit button sometimes results in squelch noise and our transmissions being unintelligible. This is not 100% but occurs most of the time - probably 85% or more. It is reliable enough that I have developed the habit of turning off "Both" before transmitting anything.

(4) A minor issue but perhaps instructive as the causes of the others - we do have some interference (i.e. reliable, periodic clicking and background noise) that is audible anytime we are monitoring both radios.

My initial thought is that we may have a grounding issue contributing to some or all of these issues, but I'd love to hear thoughts, particularly from anyone who may have experienced a similar combination.

Thanks in advance!

 

[weirdjim]

Somewhere sometime around the time Lindy landed in Paris somebody found a corroded ground on their spark-gap airborne transmitter and ever since then every conceivable avionics problem has been attributed to "a bad ground".

Please, please don't just tell this person that he has "bad grounds". In the first place, you don't know what you are talking about. In the second place, after troubleshooting avionics in everything from Boeings to Jennys, after 50 years of crawling around every airframe known to man, I have found one (repeat ONE) bad ground that was the source of the problem.

Now it is near midnight Pacific, I've had a long hard softball game, I'm tired, and I'm going to bed. I'll help this person in the morning as best I can if you will just give me a bit of time.

Thanks,

Jim

 

[Anticept]

weirdjim: It's because it's a cheap and simple check. Checking for corroded connections doesn't take very long, whereas radio repairs are expensive and it will result in quite a ****ed off customer if it turns out the radio was repaired unnecessarily. I've personally found more than one bad connection related to radio problems and I've only been an A&P for a year.

Anyways, once that's done, I'd have that comm selector box checked. Got one in my mooney that's acting a little funky.

 

[AdamZ]

Somewhere sometime around the time Lindy landed in Paris somebody found a corroded ground on their spark-gap airborne transmitter and ever since then every conceivable avionics problem has been attributed to "a bad ground".

Please, please don't just tell this person that he has "bad grounds". In the first place, you don't know what you are talking about. In the second place, after troubleshooting avionics in everything from Boeings to Jennys, after 50 years of crawling around every airframe known to man, I have found one (repeat ONE) bad ground that was the source of the problem.

Now it is near midnight Pacific, I've had a long hard softball game, I'm tired, and I'm going to bed. I'll help this person in the morning as best I can if you will just give me a bit of time.

Thanks,

Jim

LOL I swear I read the OP post and was going to post that Jim will be around soon enough to respond!

 

[weirdjim]

Hey out there....I acquired a 172L recently, and it is a delightful bird to fly, but has a couple of odd quirks that we've recently noticed and I'd love to see if anyone has thoughts on what may be causing the issue so we can estimate both the severity and possible repair costs. So here goes:

Have these problems existed from the day you got the airplane or have they more recently developed?

Aircraft has two MX300 Nav/Com units, essentially identical, and a KA134 audio panel. The issues we've noticed are:

(1) The primary COM 1 appears to be weaker in reception than the secondary. On a recent trip we were handed off to a relatively distant approach control and were not receiving replies. Holding down the test button (i.e. overriding the squelch) made the replies audible but still faint. Switching to COM 2 revealed loud and clear reception on the same frequency. This has happened a few times, in each case relatively distant from the earth station from which we were receiving, and our transmissions are being received (confirmed by relays on guard, second com reception) so it is truly a receiving issue.

If the MX300s are truly identical (what does "essentially" mean?) then if you swap them one for the other does the problem follow the radio or stay with the position. That is, if we call them MX300a and MX300b, does swapping a for b and b for a still have a weak com 1 (indicating the problem is OUTside of the radio itself) or does the problem follow the radio (indicating that the problem is INside of the radio)?

(2) The secondary COM 2 seems not to receive ASOS/ATIS/AWOS at all sometimes even though it seems to be a better receiver for most COM frequencies. During the flight on which we discovered item (1), we began to use COM 2 as our primary from that time forward. When nearing our home field, we attempted to listen to the ATIS and received nothing. Switching to COM1 - loud and clear on the ATIS. This has occurred at one other field thus far, and at our home field a few times.

What is the frequency of the home ATIS and the other field ATIS? And what do you mean "at our home field a few times"? Does COM 2 sometimes receive the ATIS OK?

(3) When the audio panel is set to Tx on COM1 or COM2, and to Rx on "BOTH," pressing the mix transmit button sometimes results in squelch noise and our transmissions being unintelligible. This is not 100% but occurs most of the time - probably 85% or more. It is reliable enough that I have developed the habit of turning off "Both" before transmitting anything.

This is what I would expect if the MX300s are not interconnected so that both receivers are put into standby when either transmitter is keyed. Think of it like this -- you have two antennas (probably, I haven't asked this yet) for your COM function probably separated by a meter or two. One of them has a receiver that is sensitive down to about a microvolt or two and now you key up the other one that is putting out several watts and jamming a volt or two into the front end of that sensitive #2 receiver. Think of it as a fire truck siren in your kitchen. You can wear the best noise cancelling mickey ears you want and you will STILL hear the siren. The voltage ratio of a volt to a microvolt is a million to one, and that is just force-jamming the front end of that poor #2 receiver into internal hemorrhage.

(4) A minor issue but perhaps instructive as the causes of the others - we do have some interference (i.e. reliable, periodic clicking and background noise) that is audible anytime we are monitoring both radios.

Darned if I know. Least of the problems right now.

My initial thought is that we may have a grounding issue contributing to some or all of these issues, but I'd love to hear thoughts, particularly from anyone who may have experienced a similar combination.

Thanks in advance!

Dammit, we lost 13-8. I was the tying run in the on-deck circle when the batter in front of me grounded into a double play, ending the game.

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[HamishTHaggis]

Thanks for your Reply Jim - I didn't realize there was actually information there initially - tapatalk hid the quoted region so I just saw the "....."

To your questions:

I have only had the airplane since the last week of June. I first noticed most of the problems about 3 weeks ago but that was also the first IFR trip in the plane and the first time we were as far from the ATC transmitters as we were on that trip. The "BOTH" has been around since day one and I presume (with considerable confidence) that all the issues have, but we didn't have occasion to notice until the more recent trips.

The MX300s are identical as far as I can tell with the exception of the functional issues we see, and the fact that one dims in response to covering it's light sensor and the other does not. I have not had a chance to swap them yet but we will do that soon.

Home ATIS is 124.875. I believe we've had the issues with a couple of others but one I know for sure was 120.125. The issue is not 100%. In fact on Friday I had no issues getting the 124.875 on COM2 - we had just washed the plane at another field and were returning to home base. Possibly some water getting into the antenna was helping things? Or unrelated - I am not sure.

YOur point regarding the connection of the radios is perfectly sensible - I know a fair bit about radios, having operated an FM station and done some HAM work but very little about aviation radios in particular. We'll be starting a more involved troubleshooting effort soon, and hopefully have some more info.

Really appreciate your input thus far and would certainly be happy to hear any further thoughts you may have.

 

[weirdjim]

Thanks for your Reply Jim - I didn't realize there was actually information there initially - tapatalk hid the quoted region so I just saw the "....."

To your questions:

I have only had the airplane since the last week of June. I first noticed most of the problems about 3 weeks ago but that was also the first IFR trip in the plane and the first time we were as far from the ATC transmitters as we were on that trip. The "BOTH" has been around since day one and I presume (with considerable confidence) that all the issues have, but we didn't have occasion to notice until the more recent trips.

Most probably was there when you got the airplane, so I'm going to rule out (for right now) a new issue.

The MX300s are identical as far as I can tell with the exception of the functional issues we see, and the fact that one dims in response to covering it's light sensor and the other does not. I have not had a chance to swap them yet but we will do that soon.

Before swapping them, just reseat them. That is, slide them out a bit and then push them back into their respective trays and connectors. See if that does anything at all.

Home ATIS is 124.875. I believe we've had the issues with a couple of others but one I know for sure was 120.125. The issue is not 100%. In fact on Friday I had no issues getting the 124.875 on COM2 - we had just washed the plane at another field and were returning to home base. Possibly some water getting into the antenna was helping things? Or unrelated - I am not sure.

Water isn't usually a fixit. More probably vibration moved whatever the real trouble is just a little tiny bit.

YOur point regarding the connection of the radios is perfectly sensible - I know a fair bit about radios, having operated an FM station and done some HAM work but very little about aviation radios in particular. We'll be starting a more involved troubleshooting effort soon, and hopefully have some more info.

We be here when you get back.

jw

Really appreciate your input thus far and would certainly be happy to hear any further thoughts you may have.

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