Radio testing ideas

Discussion in 'Instruments / Avionics / Electrical System' started by BBerson, Aug 28, 2009.

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  1. Aug 28, 2009 #1

    BBerson

    BBerson

    BBerson

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    I have an old Narco 810 Com radio. Other pilots will sometimes tell me the transmission is weak, other times they say "loud and clear". So I decided to do some simple testing with an ordinary multi-meter.

    I first disconnected the antenna cable and checked the antenna and cable for internal shorts. None found, the ohm meter did not move with one lead on the center wire and the other test lead on the external connector.

    Then I removed the radio and tested the radios antenna output wire with the ohm meter and found the meter went over to zero ohms, as if it had a dead short inside. I tested a couple of other radios and they did not test this way.( no indication at all on other radios)

    Is this a good way to test a radio?
    It still works, even with a dead short apparently. How can it work at all, I am wondering?
    thanks
    BB
     
  2. Aug 28, 2009 #2

    djschwartz

    djschwartz

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    Unfortunately, no it is not. You really need proper radio test equipment to tell if a radio is working properly or not. Some radios have transformer coupled output. That will read as a dead short on a simple ohm meter. Others have a capacitor coupled output. Those will read open.
     
  3. Aug 28, 2009 #3

    Topaz

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    Not to mention that the problem may be between the radio and the antenna, or that the antenna installation itself may be to blame. Many factors beyond just the radio.
     
  4. Aug 28, 2009 #4

    BBerson

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    Well, that answers my question about the dead short, thanks.

    Any easy way to test the antenna? I did check the antenna for shorts, none found. The antenna is inside the fiberglass wing, hard to inspect.

    Other than that, would you take the airplane to a radio shop or just send the radio to the Narco factory for service?


    BB
     
  5. Aug 29, 2009 #5

    djschwartz

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    You need a device known as a "Network Analyzer" to really test the antenna. Sometimes these are also called "Cable Testers" or "Antenna Testers" but those terms are also often used for devices that are of no use for aircraft band antennas so you really need to know what you're looking for. If you knew that the radio transmitter was good you could used a device called a "VSWR Bridge" to test the antenna; however, finding one of those that is properly set up for the aircraft band is difficult. Inexpensive bridges for the Ham and Marine VHF bands typically will not work for aircraft band.

    The COM810 is no longer in production. I'd get quotes or at least put limits on how much you're willing to spend before sending it for repair at either choice. Back when I worked as an avionics tech, which was a long time ago, Narco was about the poorest quality and near the worst service in the industry. Their only claim to fame was they were cheaper than anything else. You may also want to consider how long you're willing to wait for it to get fixed. Obsolete products can end up sitting a long time waiting for parts.
     
  6. Sep 3, 2009 #6

    roverjohn

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    Ham guys are all dying off so finding a tester at a garage sale should be a snap. Look for sales where they have a 100' tower in the back yard. I think mine was $1
     
  7. Sep 4, 2009 #7

    BBerson

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    Today I noticed other pilots could hear me just fine locally when my engine was off (motorglider). So I think maybe the engine ignition or alternator may be creating noise.

    I should test the engine only with the alternator off. Not sure if I have an alternator circuit breaker. If I do, then that would be a good test, I think.
     
  8. Sep 4, 2009 #8

    Topaz

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    I would think that engine-generated (electromagnetic) noise would interfere with reception, not transmission. Still, check around your spark plugs, too. If the steel braiding is frayed, it could be a major source of noise.
     
    Last edited: Sep 4, 2009
  9. Sep 4, 2009 #9

    bmcj

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    Could we be dealing with cockpit noise and a marginal microphone?
     
  10. Sep 4, 2009 #10

    BBerson

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    Huh, never thought of that. Can anyone confirm that electromagnet noise has no effect on transmission?

    I installed 5K resistor plug caps when I converted to auto type unshielded spark plugs. The 5K resistor should eliminate spark noise.

    Maybe I should put some foam on the microphone. I have two headsets, one David Clark and the other is a cheap Flightcom headset. Doesn't seem to matter which headset is used, however.

    I measured the cockpit noise, at full throttle it is 90db.
    But isn't 90db about average
    BB
     
  11. Sep 4, 2009 #11

    djschwartz

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    Yes, that's correct.

    I would recommend the following experiment:

    Find some one on the ground who is in a fairly quiet place to talk to while you're overhead near by. A friend in an aircraft on the ground with its engine off or a Unicom at a friendly uncontrolled field will work. Have them note down what they hear during the experiment.

    First, key your mic but don't speak. Do they hear nothing or perhaps a small amount of background noise; or, something more noticeable? If it's more, does it sound like random noise, like the background hiss turned up loud or is it a more or less steady whine or buzz?

    If they hear a lot of just random noise the two possibilities are inadequate noise cancellation in the microphone or a defective radio.

    If they hear a whine or buzz, then it could be either a ground loop allowing alternator noise onto the mic circuit; or once again, a bad radio. For a good description of ground loops and how to avoid them (at least I think it's good because I wrote it!) go to the EAA Chapter 902 website, click on "Builder's Corner", and then "Wiring Tips".

    EAA CHAPTER 902

    The most important part of this is that the mic and phone jacks SHOULD NOT be grounded to the airframe. They are grounded only at the radio or intercom. If the jacks are in an aluminum panel you can get insulating shoulder washers from Aircraft Spruce or others.

    Next, speak a clear test phrase. Have the listener note how the voice sounds. Is is clear but weak? Strong but garbled? Both weak and garbled?

    If the voice is anything but strong and clear the two possibilities are again the mic and the radio. In some older radios there is a "modulation" or "mic gain" adjustment that, if set improperly, will cause the voice to be either clear and weak if set too low or very strong and garbled if set too high. In most newer radios there is no adjustment. If you have an intercom it may have adjustments that have the same effect. If the voice is both weak and garbled then it is most likely the radio.

    Note that a defective intercom can cause the same symptoms as a radio. If you have an intercom in your aircraft you should have a set of jacks that bypass the intercom and go directly to the radio. Perform this set of experiments using both the direct connection and the intercom to isolate the problem.
     
    Last edited: Sep 4, 2009
  12. Sep 5, 2009 #12

    BBerson

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    Thanks Dave,
    Now I have at least three more things to check: intercom, mic phone jacks for improper grounding and mic gain adjustment. This could take some time.

    The airport does have an automated Unicom with a radio test feature. The unicom will record and transmit back any test. I can try your suggestion to key the mike without speaking.

    The radio works well enough within 5 miles. I only had trouble talking to ATC about 15 miles away. But I usually don't need to talk to ATC that far.

    It's just nice to have a crystal clear radio, if possible.

    Can the alternator affect the transmission? I receive just fine, in fact some other airplanes sound as good as an FM car radio.
    BB
     
  13. Sep 5, 2009 #13

    djschwartz

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    It can either by being the source of noise if a ground loop exists or by simply being broken and not keeping the system voltage up where it should be. While most of the newest radios, and some of the better quality older ones, have good internal voltage regulation some cheaper and older radios were very sensitive to power supply voltage. If the voltage got low their transmit power would go down and the signal would become distorted. The really old Narcos weren't very good in this regard. I've never worked on a COM810 and I no longer have access to any factory data on such radios so I can't comment on it. But either way if the alternator is keeping the voltage up that shouldn't be a problem.

    FWIW, I use a King KY97 com in my Stephens. I've had it for nearly 20 years. It works great. The radio is serving as a ground station right now while the plane is undergoing a complete rebuild.
     
  14. Sep 5, 2009 #14

    BBerson

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    The alternator holds the voltage about 14 volts, I have a panel volt meter. But this is a motorglider, with the engine off the voltage is 12 volts. Voltage doesn't seem to be a problem even at 12 volts. That might be something to consider if I buy another radio.
    Perhaps the radios sold for gliders are designed for battery power.

    The Narco 810 has a 14 volt sticker on the back, I wondered if 14 volt was better than 12 volt. If it has a regulator it should not matter.
     
  15. Sep 5, 2009 #15

    djschwartz

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    All nominal "12V" and "14V" systems are actually the same thing. The same is true with 24 and 28 volt ones. The so-called 12V battery has a voltage of 12.6 volts when fully charged. They require about 13.5 - 14 volts to charge. And the battery voltage will drop to between 11 and 12 volts when it is being discharged depending on load. Automotive applications typically referred to the charged battery voltage, which is close to 12, while aircraft convention was to refer to the charging voltage output by the alternator, which is closer to 14.

    With their better internal regulation many of the newer radios will have a wide operating voltage rating of 11 to 32 volts, which means they work for both 12 (or 14) and 24 (or 28) volt systems. This also means they typically behave better at low voltage when running off the battery alone.

    You might want to check the battery voltage while the radio is transmitting and all other electrical devices you use are also turned on. All radios draw a lot more current when transmitting than when they're receiving. Depending on the size and condition of the battery the system voltage could get quite low under these conditions. If the voltage drops much below 12 while transmitting the radio's performance will like suffer a bit. If it drops below 11 then there will be a serious degradation in transmitter performance even with the newest radios.

    Your idea of checking with the glider community to see what radios perform best under battery only conditions is a good one. That's not a factor normally given much consideration in the power plane world.
     
  16. Sep 5, 2009 #16

    BBerson

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    If I can go to 11 volts on the battery, there should be no problem. I will check the volt meter while transmitting. I had a two hour engine off soaring flight on thursday and the battery and radio worked well. Engine started fine after landing, so the battery held.

    I forgot to mention that a couple years ago I talked to a Narco factory technician about ideas for maintenance. He told me to check the radio display contacts. He said the display contacts get corroded and this limits output power. I cleaned the contacts with a pencil eraser as he had suggested and that seemed to help.
    I did that again last week and it seemed to be corroded again. Maybe this needs to be done annually.

    Anyway, I was impressed by the free assistance I got from Narco. I thought it was a good radio. The radio has gotten wet a few times from leaks in the canopy seal. I take it out each flight now to protect it from moist salty air.

    The Narco tech said the radio is not designed for water.... but that would be nice in this climate.

    This radio testing is kind of interesting. Any books available about aircraft radio service? What does it take to get into avionics service?
     
  17. Sep 5, 2009 #17

    Topaz

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    OT, but nice! Ridge day, or just good thermals?
     
  18. Sep 5, 2009 #18

    BBerson

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    Just thermals. Or perhaps thermals with a bit of "convergence zone". I say a bit of CZ because the lift was over a large area near the end. I circled the airport several times in lift most of time, but had to come down to pee. Convergence zone is an area where wind comes around the mountains from both north and south and collides and the air is forced up.
     
  19. Sep 5, 2009 #19

    Topaz

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    I hate it when that happens. :gig:

    Fun, isn't it? As you know, we have a rather nice one at Elsinore. :)
     
  20. Sep 5, 2009 #20

    Dan Thomas

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    It sure will affect transmission. I have unshielded mags on the Jodel, with shielded plugs and leads, and the radio picks up ignition noise from the unshielded distributor caps and messes the transmission up. I get complaints about it.

    Dan
     

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