IC-7300 plus linear amp warning

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

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Looking through the schematics to see if yours is missing a cap in the keying circuit... there could be a single point where ICOM has a wrong value loaded or yours has a wrong value part loaded. Seeing this trouble out of another 7300 would be interesting. Anyone here have a scope and can take a picture of their 7300's release characteristics?

    4ms is an odd delay. Maybe they are firing the release from the wrong variable in software (i.e. calculating the amp's key release delay from the PTT release instead of the "decay envelope finished" or some such jazz). If it's a hardware fault, shorted diode in a D/R/C setup near the driver transistor's lair...

    Seeing exactly the same timing out of another 7300 with different software would answer a few questions. It seems like we have to answer them ourselves anyway as ICOM doesn't seem too interested in fixing this.

    As for my radio's weird Key On character around 15ms, I didn't even look until reading this post a few days ago because I thought that in 2017, this kind of problem shouldn't happen, especially with a computer running the whole operation. Heck, you can delay the modulation channel a few ms and have the "perfect" compressor in the audio chain like we've been doing in the studio for a couple of decades now.

    Looking at the attack trace on my scope now, I'm pretty sure what's happening is that the power is set with a constant "in the middle somewhere" voltage and then switched to the actual detected voltage around 15-18ms. During that switching time, the value is 'up for grabs' and the output power is indeterminate until switching is complete and the ALC detector and timing have settled in. They did it this way to speed up the Key On timing.

    Since I'm not operating QSK, I'll use a 20ms turn on delay... some of the spikes are truly amazing.
     
    Last edited: May 27, 2019
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    From what I'm seeing in the very dense 7300 schematics is that all of the timing for bias and relays is performed in software. It looks like ICOM has to update this or you have to make a sequencer.
     
  3. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, I'm certain it is an error in the firmware and that it affects every single 7300 in the world.
    After-all, I wasn't the first to notice.
    It was apparently reported to Icom back in 2016 but I suspect was only seen as affecting full QSK CW and as the 7300 is terrible at this, I was never going to use it.

    I'm not much of a programmer but can easily imagine either picking the wrong variable or subtracting a value instead of adding could cause this fault.

    But I am not holding my breath and will progress my own solution by modifying the amp keying in an external box.
    Not a sequencer as such, just to delay the amp relay off time.

    Now I am looking at ALC overshoot but need to keep reminding myself "This is an Amateur transceiver, Stop looking so closely"!

    Frank
     
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Stop looking so closely or you'll find more problems! :p

    Another relay arc-life tester from ICOM, ~~~~ "I Can Only Monitor!" ~~~

    Yeah, it takes less time to toss a few components together and just make it go then to send another email into their "auto-circular-file" system.

    I'm glad I looked at mine. I wouldn't have done so until this thread came along. Now I know the 7610 should have the TX delay on to avoid overshoot. :) Zzzzzt

    The 7300's a nice radio anyway and a couple bucks worth of components will calm it down a bit.
     
    Last edited: May 27, 2019
  5. G0GSR

    G0GSR Ham Member QRZ Page

    Just found this link in another forum:

    http://www.w0qe.com/amplifier_timing.html

    Someone else doing timing measurements.

    It looks like Icom has a history of getting the amp switching very wrong but its a lottery as to what older models are afflicted or not.
    Nothing has changed now with the latest batches.

    Frank
     
    KD2ACO likes this.
  6. G0GSR

    G0GSR Ham Member QRZ Page

    I have completed my (hopefully temporary) fix for the IC-7300 amp switching problem but would really like Icom to fix this instead.
    But in the meantime....
    I have a box powered from the "Acc" socket containing a bit of simple logic and a beefy FET to switch almost any external amplifier.
    As an added bonus, it removes the contact bounce from the 7300 keying relay and adds a 10mS gap after RF ceases before un-keying the amp.
    The total delay is around 14mS as Icom un-keys about 4mS before RF ceases.

    With-delay-RTTY-annotated.png
    This is RTTY mode showing the un-keying period.

    With-delay-CW-30WPM-QSK-annotated.png
    This is full QSK CW at 30WPM. A single "dit".

    So I am happy now using this until Icom get this fixed.

    Frank
     
    AB6RF and KD2ACO like this.

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