Radio issues: carrier only, no voice

[GauzeGuy]

Any ideas on what would cause a complete loss of the ability to transmit? I've tried swapping out headsets, using different radios, using different headset jacks and the ship's mic. The mic results in unreadable, very faint communication, using the headset plugged into either front headset jack results in carrier only, no voice.

 

[ScottM]

Something wrong in the radio's modulator circuit is a possibility if you have tried all pathways of audio in and swapped out microphones.

 

[flyingron]

Either that or the signal path between the mic jacks and the radio is discontiguous. Is there an audio panel? Does it affect all radios connected to the audio panel? I would suspect either a failure of the panel (especially if it's the piece of crap Garmin one) or of the tray wiring before looking inside the radios.

Of course, any radio shop worth a clue could isolate it down pretty quickly.

 

[Archammer]

Of course, any radio shop worth a clue could isolate it down pretty quickly.

+1. You have completed all the troubleshooting to isolate the issue to the intercom system. Because you are getting some type of feedback from the mic / communication out, means that it's some type of failure - as mentioned by ScottM & FlyingRon - rather than a simple inadvertent button push or isolation setting.

To the radio shop you go.

 

[weirdjim]

Any ideas on what would cause a complete loss of the ability to transmit? I've tried swapping out headsets, using different radios, using different headset jacks and the ship's mic. The mic results in unreadable, very faint communication, using the headset plugged into either front headset jack results in carrier only, no voice.

OK, before we go jumping around like a double-jointed hooker on dollar night ...

Describe the audio system of the aircraft. Is there a true audio panel, not just a bank of passive switches? Is there an intercom between the mic jack(s) and the radio? You say there is "carrier but no modulation". How do you know there is carrier? How far away can the carrier be heard by an aircraft in the air (i.e. not just a handheld in your airplane or the unicom/tower on the airport)? Confirm that the hand mic modulates, however distorted and weak, but that a headset has no modulation whatsoever.

Until we know the answers to these questions, we are just doing a rectorandom troubleshooting job.

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[PilotAlan]

Any ideas on what would cause a complete loss of the ability to transmit? I've tried swapping out headsets, using different radios, using different headset jacks and the ship's mic. The mic results in unreadable, very faint communication, using the headset plugged into either front headset jack results in carrier only, no voice.

Also, have you made sure the intercom is off, forcing its failover mode? If there's a difference when intercom is on vs off, then you have definitively isolated the issue to the intercom.

 

[weirdjim]

Also, have you made sure the intercom is off, forcing its failover mode? If there's a difference when intercom is on vs off, then you have definitively isolated the issue to the intercom.

He never said he HAD an intercom.

.

 

[SoloEqs]

Pull out the radio, put it on your workbench, then connect antenna, power and jumpers for the mic audio and PTT. If the problem persists, off to the radio shop. If the problem is GONE, it's somewhere in the in the plane -- failed wiring, connectors, switch or settings, etc.

 

[weirdjim]

Pull out the radio, put it on your workbench, then connect antenna, power and jumpers for the mic audio and PTT. If the problem persists, off to the radio shop. If the problem is GONE, it's somewhere in the in the plane -- failed wiring, connectors, switch or settings, etc.

That is generally easier said than done. That implies that you have a bench setup with the power/audio connector, RF connector(s), a power supply capable of handling the transmitter, and the manuals that show you what the interconnect pins are. Most folks don't have the luxury of that sort of arrangement, so in vitro troubleshooting in the aircraft may be the only option.

Speakin' o'which, I would like to find a KX-170 spare rack or at least the connector if anybody has one they'd like to swap for some old radios.
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[flyingron]

Yep, and easier way is to just swap another similar model radio into the existing rack if it comes to that point.

 

[douglas393]

Yep, and easier way is to just swap another similar model radio into the existing rack if it comes to that point.

Had the same problem with my radio in the G1000 two weeks ago. Com1 received but did not transmit anything but a carrier. Com2 was perfectly fine. Spoke to an avionics shop, recommended I pull the breaker, and reset to see if it fixed the problem. No joy. Took it to my avionics shop, switch radios, now Com2 not working, and Com1 was fine. The broken radio went to the old radio resting area at Garmin, and a "new" refurbished radio took its place and now all is fine again in Mudville... well except now Otto has taken a vacation, evidently a problem with the autopilot not being introduced properly to the new radio, hopefully atleast. That darn G1000.

 

[GauzeGuy]

Thanks for the replies.

To add to the plot, I visited the plane today and when cool, the radios worked perfectly. Not really my problem, but I was trying to not cause the owner a major repair bill. I figured if there was anything basic that would be the cause, I'd just take care of it. Since I know the radios will work, I'll shut everything down that is not absolutely needed to reduce heat. That should be more than adequate to make it back.

 

[SoloEqs]

That is generally easier said than done. That implies that you have a bench setup with the power/audio connector, RF connector(s), a power supply capable of handling the transmitter, and the manuals that show you what the interconnect pins are.

I was thinking more along the lines of a $3 pack of alligator leads from Harbor freight and some paperclips, a 12V battery, a 26" chunk of wire (or rubber duck) for an antenna and a copy of the pinout from online.

All he needs to connect is the two DC leads, the antenna and the MIC HOT and PTT, and only for about 3 seconds -- not long enough to worry about SWR on a spike antenna that close to 1/4 wave. It will be immediately obvious if he's got modulation or not.

 

[SoloEqs]

Thanks for the replies.

To add to the plot, I visited the plane today and when cool, the radios worked perfectly. Not really my problem, but I was trying to not cause the owner a major repair bill. I figured if there was anything basic that would be the cause, I'd just take care of it. Since I know the radios will work, I'll shut everything down that is not absolutely needed to reduce heat. That should be more than adequate to make it back.

Okay, if it works when cool, check all of your connectors, then use a hairdryer to warm things up without changing any settings, switches, etc., and don't forget to heat your mic and the jack.

If you're not the only guy with a key to the plane, my first suspicion is that switches got reset, but you could have a thermal intermittent.

 

[denverpilot]

Just check the connectors going into the rack.

99% of the time the problem will be found at a connector.

 

[GauzeGuy]

Well, made it back to KBJC from KRAP. I shut down all non-essentials in the radio stack. I kept one radio going when needed. That worked pretty well, no trouble at all. The shop is thinking the avionics fan failed and caused the overheating, but will dig further into things now that it is home.

 

[bluerooster]

I was thinking more along the lines of a $3 pack of alligator leads from Harbor freight and some paperclips, a 12V battery, a 26" chunk of wire (or rubber duck) for an antenna and a copy of the pinout from online.

All he needs to connect is the two DC leads, the antenna and the MIC HOT and PTT, and only for about 3 seconds -- not long enough to worry about SWR on a spike antenna that close to 1/4 wave. It will be immediately obvious if he's got modulation or not.

What ya gonna clip th' Gator clips to on a card edge connector, without clipping to two contacts? Then if it has D-sub connectors, how ya gonna clip to that? And then ya gotsta have at least th' pin-outs, for that radio.
and a proper power supply, and a good dummy load. along with some form of measuring equipement, to find out if the carrier is being broadcasted.

IIRC the aviation radios are SSB with suppressed carrier, so if the carrier is being broadcast, you have internal troubles.

 

[weirdjim]

What ya gonna clip th' Gator clips to on a card edge connector, without clipping to two contacts? Then if it has D-sub connectors, how ya gonna clip to that? And then ya gotsta have at least th' pin-outs, for that radio.
and a proper power supply, and a good dummy load. along with some form of measuring equipement, to find out if the carrier is being broadcasted.

IIRC the aviation radios are SSB with suppressed carrier, so if the carrier is being broadcast, you have internal troubles.

No, I don't do lashups either, not with a few thousand dollars of incinerated silicon on the table.

Aviation radios are full carrier, full sideband AM (Ancient Modulation), the last bastion of regular AM in the spectrum.

Jim

 

[denverpilot]

No, I don't do lashups either, not with a few thousand dollars of incinerated silicon on the table.

Aviation radios are full carrier, full sideband AM (Ancient Modulation), the last bastion of regular AM in the spectrum.

I'm glad someone pointed that out. LOL.

As far as alligator clips on card edges, masking tape easily takes care of that problem. Some people have no imagination.

I know Jim won't do it, but he also has an avionics business, and can write off test gear, including trays with wiring harnesses.

Frankly my money is still on a loose connector.

 

[Tom-D]

Take it to a good shop, and get it fixed.

before some do gooder here shows you how to ruin it

 

[denverpilot]

Take it to a good shop, and get it fixed.

before some do gooder here shows you how to ruin it

Agreed. My comments aren't what I'd do with my own aviation radios. Just the more expensive ham gear.

Of course, those same radios have cost $3K at so-called "good shops" and still aren't fixed. LOL.

 

[GauzeGuy]

Take it to a good shop, and get it fixed.

before some do gooder here shows you how to ruin it

Thanks, but I got it back home and that's the end of my involvement with it.

The only reason I asked was to try to prevent unnecessary expense on the part of the owner -- plus preventing delays in my return or leaving the plane there because of radio issues. I'm flying a 182RG for under $150 hr/wet; I'd like to help keep it that way. :wink2:

Since the flight school has in house maintenance, they'll be able to figure it out. Again, speculation at this point is an avionics fan caused overheating and thus the problem. Since I was able to maintain comms after shutting down all unneeded avionics, that would make sense. We shall see.

 

[weirdjim]

As far as alligator clips on card edges, masking tape easily takes care of that problem. Some people have no imagination. .

I have a great imagination. I've toasted my fair share of expensive silicon with alligator clips biting through masking tape. The smell of expensive burning silicon is only second to "napalm in the morning" (Hello, VietNam, Robin Williams).

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[denverpilot]

I have a great imagination. I've toasted my fair share of expensive silicon with alligator clips biting through masking tape. The smell of expensive burning silicon is only second to "napalm in the morning" (Hello, VietNam, Robin Williams).

Heh. Your clips have sharp teeth.

 

[SoloEqs]

What ya gonna clip th' Gator clips to on a card edge connector, without clipping to two contacts?

Well, Blue, there's this newfangled stuff called "tape," and it insulates stuff.

Then if it has D-sub connectors, how ya gonna clip to that?

If it's the male side, you just do it. If it's the female side, you clip to the paperclips that you've put inside the holes.

Remember, we're only talking about 5 or 6 leads, and they're likely to be spread out a bit.

And then ya gotsta have at least th' pin-outs, for that radio.
and a proper power supply, and a good dummy load. along with some form of measuring equipement, to find out if the carrier is being broadcasted.

Pinouts can be found online or in the installation manual -- or, as a last resort, maybe you could look and figure out the wires. A proper power supply is called a 12-volt battery. You don't need a dummy load when you have an ANTENNA, and a 1/4 wave antenna for aircraft band is about 24 inches long. For measuring equipment, you listen on another radio for SOUND when you talk into the mike for a couple of seconds.

IIRC the aviation radios are SSB with suppressed carrier, so if the carrier is being broadcast, you have internal troubles.

Your familiarity with the emission type fits perfectly with your expertise with homebrew test setups. Aviation radios are AM.

 

[Henning]

Any ideas on what would cause a complete loss of the ability to transmit? I've tried swapping out headsets, using different radios, using different headset jacks and the ship's mic. The mic results in unreadable, very faint communication, using the headset plugged into either front headset jack results in carrier only, no voice.

The magic smoke escaped so the pixies can't fly your message out.

 

[SoloEqs]

I have a great imagination. I've toasted my fair share of expensive silicon with alligator clips biting through masking tape. The smell of expensive burning silicon is only second to "napalm in the morning" (Hello, VietNam, Robin Williams).

The clips I use for tight spaces have been doctored with wire cutters, needlenose pliers, etc, so the only teeth are the ones I want making contact. The Harbor Freight clip leads are cheap enough to make this cost effective.

BTW, the "napalm in the morning" quote was from "Apocalypse Now."

 

[weirdjim]

You don't need a dummy load when you have an ANTENNA, and a 1/4 wave antenna for aircraft band is about 24 inches long.

22 inches, and if it isn't a decent distance away from the radio, you will get a whole bunch of unpleasantness as that RF feeds into every circuit in the radio by direct radiation.

For measuring equipment, you listen on another radio for SOUND when you talk into the mike for a couple of seconds.

Only if the radio is a significant distance away from the DUT or you will be listening to what is generated inside the radio, not what is being radiated.

Your familiarity with the emission type fits perfectly with your expertise with homebrew test setups. Aviation radios are AM.

To be fair, if this guy's experience is with long range military radio, they do use SSB-SC, but civilian and UHF military are still good old double sideband full carrier AM.

.....

 

[SoloEqs]

.....

Once upon a time, when I was young and dinosaurs ruled the Earth, Motorola had the "PT" radio model series. These essentially were a lunchbox with a radio transceiver inside, a hand mike and a 1/4-wave spike antenna on the top. They still make those antennas, 30 years after they stopped making PTs! This is pretty much the setup that I've suggested.

The power output was in the same range as an aircraft radio. As long as the case is on and closed, there should be no RF problem with a spike antenna.

The problem is lack of modulation. If anything other than dead carrier comes across (even if you're just hearing the IF), the radio is working OK and the problem is external. It only takes a couple of seconds to know.

BTW, now I use PT cases as extenders for HTs -- I strip out the transceiver and put in an audio amp and a power supply / charger circuit, then fill that big battery box with NiMH batteries and put an HT holder on the front. One lead from the PT case is voltage out to run the HT, one lead hooks it up to the hand mic and audio, and one goes to the antenna (sometimes I put in an RF amp, too). This turns a 4-hour HT into a 12-hour HT (or a continuous-use mobile or portable base) for disaster use. There's also room in there for a digital-mode interface. And I used a big "paramedic" radio case for a Yaesu FT-100D HF / VHF / UHF rig, doing the same thing (batteries, charger, amp, digital, etc).

 

[weirdjim]

Once upon a time, when I was young and dinosaurs ruled the Earth, Motorola had the "PT" radio model series.

If anything other than dead carrier comes across (even if you're just hearing the IF), the radio is working OK and the problem is external.

Gonset had the Gooney Bird series which was an AM 118-127 MHz rig, the 2-meter FM version, and the VHF commercial band version.

I respectfully disagree that the final can be dead with modulation of the earlier driver depending on the design providing milliwatts of output, which can be heard for a quarter mile or so.

 

[BillTIZ]

Coax to the antenna, broken ground?

 

[SoloEqs]

Gonset had the Gooney Bird series which was an AM 118-127 MHz rig, the 2-meter FM version, and the VHF commercial band version.

I respectfully disagree that the final can be dead with modulation of the earlier driver depending on the design providing milliwatts of output, which can be heard for a quarter mile or so.

My point is that modulation doesn't require the final PA -- even just the IF should give enough output to pick up on another nearby radio. If there is ANY modulation (intelligible or not) it will prove that the radio is operating properly and that the problem is either the mic, connectors or wiring.

 

[weirdjim]

My point is that modulation doesn't require the final PA -- even just the IF should give enough output to pick up on another nearby radio. If there is ANY modulation (intelligible or not) it will prove that the radio is operating properly and that the problem is either the mic, connectors or wiring.

I'm sorry, but that's just not so. I'll leave it with the note that I've been on the design end, the installation end, and the repair end of aircraft transceivers since I was 16 ... back when the rocks were being delivered to Earth.
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[denverpilot]

I think what he's trying to say is if the final PA is blown you can still hear the exciter if you're close enough. Mixing in terms like "IF" (Intermediate Frequency) totally blows his credibility as a radio technician, though.

The IF is just an oscillator circuit buzzing away at whatever the radio's design engineer decided to utilize.

The problem is, Jim's right. AM (double side band) doesn't exactly work that way. You're thinking FM or Phase Modulated.

Here's three block diagrams from three different quickly Googled internet sources for a basic high level AM transmitter. You may want to reference any of these and point to this "IF" on the diagram that you believe "can be heard" to test AM modulation...

A brief look at them after grabbing them would indicate the easiest one to work from for this discussion is the last one. Where would you place an RF probe and an AM receiver to "hear the IF"? (Hint... nowhere. You could use a scope to look at the audio between the modulator block and the RF Power Amplifier, but you won't hear it on a receiver nearby.

In AM you're modulating a steady double sideband carrier. It's there all the time. Not PMing or FMing at an IF stage by "bending" the frequency and then multiplying the resulting FM/PM frequency up to your desired center frequency.

You're mixing up modulation types.

 

[denverpilot]

P.S. That middle one was so small on my screen I had to go blow it up to see what the blocks were. That's a digital radio and there's digital to analog going on there. Not really useful for my point, but kinda a neat diagram to ponder, nevertheless. Never trust Google's results. Ha.

 

[weirdjim]

P.S. That middle one was so small on my screen I had to go blow it up to see what the blocks were. That's a digital radio and there's digital to analog going on there. Not really useful for my point, but kinda a neat diagram to ponder, nevertheless. Never trust Google's results. Ha.

There is a subtle effect that has bitten me on the behind more than once...in a bipolar RF transistor final amplifier (which almost ALL of them are) the UP modulation (RF peaks) are much less energetic than the DOWN modulation (RF minimum). We compensate for that by modulating not only the FINALS, but the DRIVER as well, but only for the peaks. You can hear that driver a quarter of a mile away with really good modulation while the final is blown all to hell.

There is NO substitute for a set of calibrated pads and a diode detector at the output of the pads at the milliwatt level where the diode is still linear. You can put an antenna on the thing (which, by the way, if it isn't a GOOD antenna and not a coathanger cut to length, will activate the VSWR protection circuits and shut the final down) so you will get minimum, if any, power output, but enough to hear on a receiver across the taxiway or across the field.

The IF argument, of course, is really not germane. You can't hear the IF until there is a RECEIVED, not a TRANSMITTED signal.

Jim

 

[SoloEqs]

I think what he's trying to say is if the final PA is blown you can still hear the exciter if you're close enough. Mixing in terms like "IF" (Intermediate Frequency) totally blows his credibility as a radio technician, though.

The IF is just an oscillator circuit buzzing away at whatever the radio's design engineer decided to utilize.

Uh . . .no.

The IF is the intermediate frequency. You're talking about the IF oscillator, which PRODUCES the IF.

Modulation is present on the IF, and an IF can be heard outside the radio if you're close enough (or have a good antenna). Cops spot illegal radar detectors in commercial vehicles by transmitting a modulated signal in the appropriate bands, then "sniffing" for that modulation on the IFs found in common radar detectors. The next time you pass a DOT truck scale, look for an itty-bitty Yagi antenna aimed down to hit the cab of the truck.

However, that said, you're right that I was thinking about the signal before the PA.

 

[SoloEqs]

The IF argument, of course, is really not germane. You can't hear the IF until there is a RECEIVED, not a TRANSMITTED signal.

Yep, you're right, I was thinking of the signal from the exciter.

Damn, I came off as ignorant as an appliance-operator Bash-book Extra, not a General who's been building radios for 40 years or so.

BTW, I just noticed your 182's N number. Cute!

 

[denverpilot]

However, that said, you're right that I was thinking about the signal before the PA.

Good catch. I've been spending so many years explaining IF oscillator mixes at multiple FM (really PM) transmitter sites to folks, I live in the IF oscillator world more than past where the mixing magic happens.

Did it to myself once too. Had a GE MASTR II doing standard low-side IF injection on 447.575 receive in the basement and couldn't figure out what local interference was wiping out hearing 145.460 on a handheld nearby.

(Never got around to ordering high-side crystal for it either. Wonder if the new club President has figured out why he can't monitor 145.46 on his scanner yet without a continuous buzz in the background? He lives a lot closer to the 145.46 transmitter site, so I doubt he's noticed now that he hosts the link radio. )

 

[bluerooster]

I stand corrected on the SSBSC thing, I don' know what I was thinkin'.

Any ideas on what would cause a complete loss of the ability to transmit? I've tried swapping out headsets, using different radios, using different headset jacks and the ship's mic. The mic results in unreadable, very faint communication, using the headset plugged into either front headset jack results in carrier only, no voice.

Reckon why there was a difference in modulation between the headset mic, and the handheld?

Bad modulator. Do you get "side tone"?(hear yourself in the headset)
If not, then mabe in the preamp, or amp stage.
Or mabe something as simple as a "wild hair" directing input to ground.(which would be external)