Can AM Carrier Be Safely Turned Up On Newer Icom's ?

Discussion in 'Radio Circuits, Repair & Performance' started by W2BBQ, Nov 16, 2020.

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  1. KM3F

    KM3F Ham Member QRZ Page

    something is going on with the reply function.
     
  2. N8YX

    N8YX Ham Member QRZ Page

    Go look at Section 7.1-2 in the Owner's Manual where the signal flow is discussed. Specifically, the function of "CF1".

    Then tell me which lines into the Signal Board supply an "AM TX" switching voltage that selects the AM Wide (6KHz B/W) filter on AM transmit.
     
  3. N8YX

    N8YX Ham Member QRZ Page

    Can't edit the above, but the Service Manual also details the signal flow on voice modes "through CF1" (the SSB transmit filter). I'll look again at the Signal Board schematics but cannot find where any filter other than that one is switched into the AM TX chain.

    ETA:

    Did a quick test with my '930 and a general-coverage receiver in the shack. Both tuned to 29.100; 5w carrier on AM and Mic Gain at 9 o'clock.

    With the shack receiver at zero-beat to the '930's carrier, both sidebands can be heard. However...there is a 20dB difference between USB and LSB (USB being the stronger of the two). We either have a very asymmetrical "AM" (DSB) filter, or we have an SSB filter in a circuit which allows a bit of leakage. Given Kenwood's penchant for implementing "AM" as USB with carrier, I'm leaning towards the latter but am open to being shown where on the Signal Board another filter is being selected.
     
    Last edited: Dec 25, 2020
  4. G0GSR

    G0GSR Ham Member QRZ Page

    I have a decent external audio processor.
    Frank
     
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So do I :)

    This was with an RE-27nd and outboard audio processor, eq, noise gate, BBE, etc, as well as with just the hand mic. Either way that was the maximum PEP displayed on the a LP-100a.

    Of course a processor should have no effect on peak output anyway. This limitation is in the radio, not the input audio. Screaming into a 100W radio isn’t going to give you any additional peak power out.

    Perhaps it is a firmware revision. I think my 7300 is at 1.10, which us pretty old. Could also just be the different meters.
     
    Last edited: Dec 26, 2020
  6. KM3F

    KM3F Ham Member QRZ Page

    To see if it's ALC doing limiting, observe an 'average reading' outboard power meter. To see more of the peaks, uses a peak reading meter.
    If the average lowers in power at high audio drive peaks, ALC it's working as it was designed to do.
    If the AM carrier is say 25 watts and momentarily dips on voice peaks, that is to prevent over modulation of the system design.
    I too use dbx quality outboard processing but you cannot do anything but raise the average to peak power ratio in a stock unmodified radio design or be allowed over the peak power limit due to the internal ALC system of control.
    The audio bandwidth is a separate parameter usually limited by a filter or DSP action of some sort.
    If all this is kept clean, then fed to an amplifier it will sound very good.
    For example, an AL80B driven with about 5 watts of AM carrier will yield an very useful power increase in the 300 watt range on modulation peaks without being tooooooo hard on the amplifier but be a where is not a long key down time for usage. Especially when processing is used to raise the average to peak drive signal because your looking as near 300 watts out from the amplifier on voice peaks with about 5 watts of high average signal levels of drive. The plate will get bright with time.
    I use a mirror mounted to the outlet cooling area along the side of the amplifier to be able to see the 3-500 tube plate color.
    This is not a broadcast station setup that will take 5 to 10 minute transmissions.
     
    Last edited: Dec 26, 2020
  7. G0GSR

    G0GSR Ham Member QRZ Page

    AM-mod-2.png

    That's a test tone from and at the peak level of my processor.
    The horizontal cursors are set to the unmodulated carrier level (4 div) . 100% mod would be 8 divisions and 100W PEP.

    The IC-7300 very effectively limits AM mod depth (to about 95+%), you cannot overmodulate.

    ALC indication doesn't move with mod. I haven't done a carrier shift measurement as I don't think there is a problem.
    That's not bad considering some radio manufacturers don't take AM seriously.

    Now, if only the audio bandwidth wasn't stupidly restricted, the 7300 would be a good AM rig.

    Incidentally, the 7300 PA "sings" like a small mod transformer would :)

    Frank.
     
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So you were doing your measurements with a test tone? I was doing real world PEP measurements with voice modulation. If I have a chance I will try a test tone.

    However, I don’t think the processor has anything to do with it. As you say, the radio will not over modulate. So all you have to do is put a big sine wave into it until it clips, or no longer increases in amplitude (depending on the limiting).

    Also, as mentioned above, could be firmware version differences.

    Interesting thread. I don’t use the 7300 a whole lot. I prefer to run vintage gear for both AM and sideband. Something fun about getting the vintage stuff sounding great and having people on the other end think you must be running a brand new Flex or Anan, etc. only to tell them that you are actually running a fifty year old piece of kit (albeit with modifications and an audio chain that costs far more than the radio).
     
  9. G0GSR

    G0GSR Ham Member QRZ Page

    No, just this time.. previously using speech and a Wavenode WN-2 which is pretty good with PEP but its average meter reads garbage.

    Processing is all about loudness whilst controlling peaks. Even with audio overshoots less then 0.2dB, the 7300 filtering really messes things up and makes a mockery of any peak control.


    Frank
     
    Last edited: Dec 27, 2020
  10. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Understood.
    Can you elaborate on this? You are using a limiter as the final piece of audio gear?

    Are you seeing the limitation only in AM, or sideband as well? In sideband the ALC seems to do OK, though I get better results with no ALC action, and all limiting and compression in the audio chain.
     

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