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IC-7300 plus linear amp warning

Discussion in 'Software Defined Radio (SDR)' started by G0GSR, May 21, 2019.

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  1. G0GSR

    G0GSR Ham Member QRZ Page

    From, someone noticed that there was a problem with amplifier un-keying which can potentially damage your amp changeover relays.

    IC-7300-Amp-keying during RF output+annotation.png

    I confirmed the fault and have attached my own oscilloscope plot showing the amp keying line "releasing" almost 4mS before RF output ceases. In other transceivers, there is a delay of at least 10mS in un-keying the linear amp AFTER the RF ends.

    Its only 4mS but this is a serious problem as any amp that has no protection from "Hot switching" could burn its changeover relays with this fault.

    TX delay found in most modern radios only affects the amp switching ON, not OFF. This delay is working as expected on the 7300.

    I think that Icom have made a serious mistake here.

    KU3X and AI3V like this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    KD2ACO likes this.
  3. AA4PB

    AA4PB Ham Member QRZ Page

    A simple work-around might be to add a relay or other keying interface that has a capacitor to keep the relay closed for 4ms after the radio's PTT opens.
  4. W9WQA

    W9WQA Ham Member QRZ Page

    would that slow down the relay going in to transmit?
    would that be a similiar problem.
    this is news to me. curious as i have a BIG 12 relay in a home brew amp.
    AG5CK likes this.
  5. AA4PB

    AA4PB Ham Member QRZ Page

    It can be designed so that it doesn't slow the relay closure time. One of the issues will be making sure that the capacitor charging current doesn't overload the radio's PTT circuit. You could use a buffer transistor supplied by an external 12vdc power source to drive the relay or you could go all solid state, depending on the amp requirements.
  6. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, and I have tried a simple capacitor circuit and it does indeed work.
    You can achieve this by wiring a 100uF capacitor to a 13 pin DIN plug pins 2 and 3 (+ve to pin 3) then plugging this into the ACC socket. No need to even take the covers off.
    It works but has a side effect when I use my "tune switch" to give me a 10W carrier to tune the ATU.
    When releasing the tune switch, the radio briefly switches to TX again but its no big deal, I just don't like it.

    But I am so concerned about this that I am now building a keying interface to delay the relay OFF time by 25mS using a monostable.

    I really think that Icom should fix this however.

    VE3CGA and AG5DB like this.
  7. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, you should be worried.... but:
    The nature of this fault means that AM, FM and RTTY are the modes most at risk of burning relays.
    SSB less so unless there is audio when you de-key. (like the click from the hand mic PTT switch?).
    Digi JT or FT modes, no problem.
    Semi break-in CW or SSB with VOX, there is no risk.
    Full break-in CW; Risky. This fault was seen very quickly after the 7300 was first released. It has been reported to Icom who have ignored it.

    WB5YUZ and K0UO like this.
  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    Is there an adjustment in the menu, by chance?

  9. AA5CT

    AA5CT Ham Member QRZ Page

    Early model radio or late model radio?
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I've seen this problem and variations with so many radios that I just key everything with this box and be done with it.

    K0OKS, K3RW, WB5YUZ and 3 others like this.

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