IC-7300 and Solid State PA

Discussion in 'General Technical Questions and Answers' started by W5ZZ, Feb 23, 2021 at 2:59 AM.

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  1. W5ZZ

    W5ZZ Ham Member QRZ Page

    Hello friends,
    Is anyone using their IC-7300 with a SSPA ?
    Asking because I read somewhere, about an Icom 7300 power overshoot problem.
    Is this a real problem ?

    Thanks!
     
  2. DK4NU

    DK4NU Ham Member QRZ Page

    Hello
    yes, there are peaks in the output of the 7300. Not these ones from ptt switching (there a tx delay can be set). There are peaks from the modulation.
    maybe the alc is to slow. with 100% power adjust there will come peaks up to 140 watts. the lower the outpt power the smaller are these peaks.
    Also the imd3 becomes bad when the alc meter shows more than a little line (max to S3 in the S meter scale).
    i hope i could help you a bit. excuse my bad english, please. 73 de Uwe (DK4NU)
     
  3. W5ZZ

    W5ZZ Ham Member QRZ Page

    Thanks for info Uwe,
    So, if I connect IC-7300 to my homebrew SSPA ( I am using 10db attenuator at input) and set radio to 50 watts , it will probably damage amplifier MOSFETS ?!
    Thanks!
     
    Last edited: Feb 23, 2021 at 2:26 PM
  4. NG1H

    NG1H XML Subscriber QRZ Page

    This seems to be more of a problem with the "turn everything up to 11" operators. Which brings up the question whether any damage was caused by the overshoot versus just poor operating methods.

    If you are having to rely on ALC all of the time to not damage anything the problem isn't with the equipment.
     
    AG5CK and NG0L like this.
  5. W5ZZ

    W5ZZ Ham Member QRZ Page

    So is the radios like ic-7300 rely on ALC to reduce power ?
    NG1H and what is solution in your opinion?
     
    Last edited: Feb 23, 2021 at 5:25 PM
  6. G3YRO

    G3YRO Ham Member QRZ Page

    ALL rigs with Transistor PAs use the ALC circuitry to control the power output ! (even at the rig's rated full power, it's the ALC that sets it at the desired value)

    Back in the days of Valve PAs, ALC was only there to reduce any excessive drive, to stop distortion on SSB. (A well-designed rig didn't have more than a few dBs of excess drive . . . most even disabled the ALC circuit on CW).

    But because Transistor PAs are wideband, the actual gain varies a lot from band to band . . . and some bands you would have WAY too much drive. So you have to have a way of controlling the power output. This is done on Amateur SSB rigs using the ALC circuitry.

    One of the bad side effects of this is that you get very poor Talkpower from rigs with Transistor PAs (which is why I modified the ALC circuitry on my TS-930S).

    Of course how well the circuitry works varies a lot from rig to rig.

    Roger G3YRO
     
    Last edited: Feb 23, 2021 at 6:04 PM
    W5ZZ and K0UO like this.
  7. KD9VV

    KD9VV Ham Member QRZ Page

    You will need to scope your 7300.
    Not all 7300's had this over shoot issue; mine did not.
     
  8. NG1H

    NG1H XML Subscriber QRZ Page

    For CW & digital modes never operated where ALC is activated.
    For voice modes operate where ALC is rarely if ever activated.

    Think if it as ABS in a motor vehicle. If you are operating all of the time so that the only way you don't get in a wreck is if the ABS stops you in time, and then one time it doesn't, who's fault is it?
     
    AG5CK likes this.
  9. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, this is a real problem in most/all radios including the 7300.

    In this thread, I published some ALC overshoot measurements (using a new method) from my 7300, which being a mass produced radio, are all identical.

    https://forums.qrz.com/index.php?threads/ic-7300-alc-overshoot.745475/

    Bottom line is that with the 7300, you need to de-rate any linear amp to about 70% of its full PEP rating to allow for power overshoots when using SSB voice.

    Icom designers also got the external "amp OFF" keying timing badly wrong and have so far, failed to fix it.
    Loads of info on this here:

    https://forums.qrz.com/index.php?threads/ic-7300-plus-linear-amp-warning.659179/


    Frank
     
    Last edited: Feb 24, 2021 at 8:13 AM

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