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Thread: electrical problem

  1. #1

    Default electrical problem

    New to ham so don't know much. Anyway I mounted a Yaesu FT-2500M in my Humvee. The SWR is around 2.0. My problem is that this radio has three output levels 5,25 and 50 watts. I installed a watt meter and it shows only about 2.5,10 and 20 watts. Sounds like Almost half the output. Any ideas as to the problem? Also can you point me to where I could send it for repair.

    Thanks for the help.

    Trooper

  2. #2

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    As an added thought, If the antenna system is not well matched, some radios have a lowered output level to prevent damage to the radio. I do not know enough about this radio to say for sure, but it is something else to check.

    KD9KVS

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  3. #3
    K7KBN's Avatar
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    This is where someone suggests a known-good dummy load of proper power rating. Connected in place of the antenna it'll show you a lot of things.
    73
    Pat K7KBN
    Semper ubi sub ubi.

  4. #4

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    Quote Originally Posted by K7KBN View Post
    This is where someone suggests a known-good dummy load of proper power rating. Connected in place of the antenna it'll show you a lot of things.
    OK I replaced the Power amp and the power levels are still at about 5,15 and 25 watts instead of 10,25 and 50 watts. I have attached an external watt/swr meter for these readings and the swr is less then one. Any ideas?


    Ken

  5. #5
    Super Moderator 5B4AJB's Avatar
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    Sounds like the coax is damaged, you have a braid shorting at the far end, or the D.C. cabling isn't man enough.
    Please tell me you wired it through fuses directly to the battery...

    Either that or it's a setting somewhere?

  6. #6

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    First, I'm shocked. The Mitsubishi M67781L final is rated for 40 watt. 50 watt is the absolute maximum rating. Yet, the yaesu technical supplement clearly states it runs at 50 watts. I cannot believe they would design a radio to run a device at its absolute maximum rating. Must be a rugged part...

    You say you replaced the power amplifier. So you tore the radio apart and swapped the M67781L final module? Being yaesu decided to push that device to its limits, I sure hope you used good heat sync compound!

    Break it down for us in detail. Tell us what gauge wire you used to power it, how long that wiring is, and what you connected it to. A 50w radio isn't going to work right on a cigarette lighter circuit! Does the display dim when you key up? Can you check the voltage while you key up to ensure it isn't dropping? Next, tell us what kind of coax you are using (should be printed on the side and likely hard to see), how long that is, and if the cable itself without the antenna passes a continuity test. Assuming that last step checks out (tell us about it anyhow), we want to know the SWR reading too. You say it had an SWR under 1 in post #4, that's not possible. This makes me think you are not using the meter correctly, the meter is broke or the meter is not rated for use at VHF. Are you using a "radio shack" swr meter? If so, those buggers are useless at VHF! The higher you go in frequency the more inaccurate those HF SWR meters get.

    We need more info.

  7. #7

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    Tell us about the antenna, mount and mounting location as well.

  8. #8

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    Hello everyone, lots of questions to answer, I will do my best.
    The radio is mounted in my military Humvee which is 24 volts and I have a 12 volt converter just for the radio and it reads 12 volts at the radio.
    The radio power wiring is factory and looks like 12 or 14 gage about 6 feet long.
    The antenna is a 2 meter firestix using about 16 feet of RG58 with no markings. My watt/SWR meter is a MFGJ812B. I was wrong on the SWR It was not adjusted properly. Anyway the three settings and there power and SWR are as follows:

    Setting low Mid High
    Should be 10 25 50
    Watts 3 10 23
    SWR 1.3 1.9 2.9
    I can see that the SWR is not good at the higher settings.
    The ant is mounted on the rear of the Humvee on a military mount. The Humvee is an aluminum body with a canvas top. I checked the cable for shorting looks ok.
    When I changed the PA There was plenty of heat sync material left.
    The display does not dim when I key it.
    I'm not sure how I can measure the voltage drop when keyed.

    Hope I have answered all the questions.


    Thanks

    Ken

  9. #9

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    The way to measure the voltage when keyed is to somehow find or otherwise expose a small amount of wire near the radio to connect the meter to. Does the voltage converter have a power rating marked on it?

    As for the heat sync material... Its super bad practice to reuse existing heat sync compound due to the high probability of air bubbles being trapped between the module and heat sync when the new module is put in place. I only know this because I nearly fried a very expensive 8 core CPU doing that. Fresh (and much softer) heat sync goo is usually applied as a small but thick spot in the center so that the part pressing down spreads it and eliminates the possibility of trapped air. Next time, clean everything completely and start fresh. Rant complete

    Increasing SWR with power is a sign of something horribly wrong. Under proper conditions, that does not happen. So many things could be going on, I will list a few. See if anything stands out.
    -Is the antenna mounted along side anything conductive? Is that mount low on the vehicle body? Can you get it above the roof line if its not already?
    -Did water leak into the coax?
    -Loose connection, perhaps worn out PL-259 pin that slides into the SO-239 all sloppy?
    -I assume you have the 5/8 wave fiberglass firestick that should not need matching, but do you have any matching device with a ferrite core that can saturate at those power levels?
    -Harmonics can do this too if the radio has poor output filtering. More off-frequency power is more the antenna sends back.

    That particular coax is super lossy at VHF (but I assume shouldn't be much of an issue at 16'). If you have a high SWR, at low power, its possible not enough of the reflected wave is making it back to the radio to be measured. As you turn up the forward power, more and more reflected power is making it through the lossy coax to be measured. Some people think a low "measured" SWR is all that matters, they don't consider what is lost, not only on its way to the antenna, but also on its way back. This is not only true as a result of coax loss, but also for the loss due to poor connections and damaged coax.

    RG-58, depending on the specific brand/type, had loss anywhere from 3dB to 6dB per 100'. Lets assume 4dB/100', that's .64dB lost on the way to the antenna. At 50watt, only 43 will make it to the antenna on a good day. That's 7 watts guaranteed to be warming up that coax. If I knew the actual SWR, we could see how much of that 43 watts comes back and m of that, how much is turned to heat.

    Heat can change electrical parameters and cause issues too. Feel for any part of the system to get warm during TX (just not the tip of the antenna If anything feels warm (other than the final module), report back on it.

    What you really need to do is connect a TDR to the coax input and watch for impedance changes. That will show if you have a bad connection, where it is and if there is water in the coax. Or more simply, if the coax is old, I would just replace the coax and connectors as that would be about the same price as the final module you had to buy... And if that antenna is mounted to a low spot on the vehicle next to any vertical metal supports, change that ASAP.

  10. #10
    K7KBN's Avatar
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    SWR (where the R stands for "Ratio") is independent of power being transmitted. I agree with Brandon: replace the RG-58 with RG-213. It's larger in diameter, made to military specs, and it should do you a lot better. Just watch the bending radii when installing it.

    RG-8X might work also; it's just slightly larger than the -58 (same size as RG-59, the older 75 ohm cable TV coax). However, the smaller the cable the higher the losses.
    73
    Pat K7KBN
    Semper ubi sub ubi.

  11. #11

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    Brandon
    Just a note, I changed the PA because someone suggested that it could be the problem but things didn't change.

    I'm going to do some more testing as you suggested and I think I will change the coax also.


    Ken

  12. #12

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    OK I installed a new antenna cable (RG 213U) and also got a dummy load. With the dummy load my power is still at 5,10 and 30. instead of 5,25 and 50. I can live with that. The SWR is less then 1.5 on all three power levels. But when I hook up the antenna my SWR Is 2.0,2.5,and 2.5 for the three power levels.

    My guess is the antenna location which is mounted on the rear with about half above the top which is canvas with a metal pipe frame.

    Maybe this is as good as it's going to get.


    Ken

  13. #13

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    If its not a RF heating issue, its likely the meter itself. The detector diodes are non-linear devices and might show inaccurate readings at the lower power levels. Trust the higher power reading and try to adjust the antenna to make it lower than 2.5.

    With base fed vertical antennas like that, the majority of the current is in the lower portion of the antenna. It is this current that is responsible for radiating the signal. If only the top half is above the roof and the lower half is near one of those support pipes, it might be hard for you to tune the antenna (or even make contacts). Mutual coupling to the vehicle will change the feed point impedance and the vehicle will most certainly skew the pattern of what RF you manage to radiate. On 2m, you would probably be better off with a simple 1/4 wave NMO mount antenna (like this one https://www.amazon.com/HYS-TC-HC152V...SIN=B077HRNLHZ) on the roof rather than using a 5/8 wave mounted next to a support pipe.

    Get that antenna above the roof where it can do its job, even if it means abandoning the current antenna for a 1/4 wave.
    Last edited by brandon lind; Sun 2nd Jun 2019 at 12:03.

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