FT-757GX Tx audio problem

Discussion in 'General Technical Questions and Answers' started by VK3TV, Apr 15, 2016.

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  1. VK3TV

    VK3TV Ham Member QRZ Page

    I have an FT-757 with an audio problem on SSB. From what I have been told by a station receiving my signal is that the audio is clipping resulting in intermittent and distorted audio. During tune up the carrier is constant and clean.
    Watching the the ALC the needle does seem to jump and peak going out of range. Also when talking normally the forward power also seems to be peaking erratically.

    Did some tests into the dummy load with the following results....

    40 & 80m both LSB & USB intermittent and choppy audio. Peaking 15A and Volts dropping 2Volts when trying to maintain a good audio into the mike.

    20 & 10m both LSB & USB good audio out with very little voltage drop and maybe peaking 10-12 Amps

    I have also noticed both on 40 & 80m that when trying to tune through the antenna tuner or dummy load in CW mode, when advancing the drive it goes from zero to flat out at the 3 o'clock mark and nothing in between. On 40 the set is drawing 18A and on 80 25A when it happens.
    All other bands appear to be working correctly with the drive coming in at around the 1 o'clock position and variable up to full power as per the manual.

    Any ideas would be appreciated.

    Mal - VK3TV
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    Is your power supply up to snuff? Measure the voltage when transmitting at the radio. It should have a 13.8VDC port on one of the accesory ports.
    Report back after that with results.

  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Definitely check your PS as Ed suggests above.

    If that checks out I'd think you either have an RFI issue on some bands as in RF getting back into your rig's audio or driver stages or an internal oscillation problem on the problem bands.

    Do you have another receiver you can use to monitor transmissions? How about an oscilloscope or transmit monitor scope? Monitoring either the audio quality or visual quality of your transmitted signal during two tone or mic input testing can help you pin down problems.

    If possible try monitoring your transmit signal while loading into a dummy load and then again into your antenna on the bands that give you trouble. If it's an RF ingress problem some clip on RF chokes on your mic line may help. Also what are you running for an antenna on the problem bands and how far is the antenna from your shack?
    YB3BX likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Dropping 2V at 15A? I suspect that's the problem, right there.

    What kind of power supply is that?
  5. VK3TV

    VK3TV Ham Member QRZ Page

    Oops, I meant to say it was dropping 2volts and drawing 25A+ only on 80m Initial test were on a home brew PSU which I think was a 20A supply.
    I checked today on the Yaesu FP-301 power supply with a standing voltage of 13.53V with the same results.
  6. VK3TV

    VK3TV Ham Member QRZ Page

    Thanks Ed.

    See my reply to WB2WIK. I have now actually tried 3 different supply and all with the same results.

  7. VK3TV

    VK3TV Ham Member QRZ Page

    Thanks for all you suggestions. I can monitor on another radio, every other band is fine except 40 & 80. 80m is definitely the worst, the power supplies just can't handle the current.
    You may be right about RFI, I will have to get hold of an oscilloscope and do more tests.
    I don't think is is an antenna problem as I get exactly the same results as with the dummy load. I have no issues running my FT7 with a 100w amp using the same supply and antennas.


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