706mk2g weird swr power problem

Discussion in 'General Technical Questions and Answers' started by KG7HVR, Oct 28, 2018.

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  1. KG7HVR

    KG7HVR Ham Member QRZ Page

    I bought this radio earlier today at ham fest. Worked fine then while using it with my sb220 I noticed a infinite SWR reading on the radios display but I was still putting out 1200 watts out of the amplifier. My tuner showed a flat match.
    I shut it off and checked everything.
    and hook it up to a dummy load and it was still doing the same thing infinite High SWR no power out now but I can see it was drawing current on my power supply for a second. I did full reset. Same thing . Uhf vhf works fine.
    I hooked it up to my mfj 929 auto tuner and it was putting out power in tune mode but radio still showed bad set.
    I think it was already messed up or the amp did it. I've had odd things happen with this amp. The finals appear fine .
    It does this 160-6m
  2. WZ7U

    WZ7U Ham Member QRZ Page

    The only thing I can think of is that the SB220 perhaps lacks a tuned input. For example, I have a 706llg that feeds an old Dentron GLA1000. This amp doesn't have tuned input, so the 706 sees near infinite swr when keyed up into it.

    The options include:
    A. Build a tuned input pad for it
    B. Buy a used tuned input matching device for it
    C. Put a small tuner inline between the amp and the radio

    I chose C, because while it's not the most technically correct way to do it, it does get me on the air somewhat QRO. Be advised, doing it this way could have another set of problems associated with it; namely, spectral impurity. So far, no one has complained or mentioned my signal being particularly dirty. I would hope someone would mention that if they noticed it on a pan adapter/sdr.
    Here is an idea of what I'm referring to when I mention Tuned Input Device - [​IMG] Dentron made a CM-U and a CM-1 or CM-2, all are very similar in that the switch changes the input impedance relative to band. There are a few articles out there regarding tuned inputs for amplifiers, and most newer amplifier designs incorporate this feature into them (or so I've been led to believe).

    The amplifier sub would be a good place to bring the subject up, since those guys are foremost authorities on such things. https://forums.qrz.com/index.php?forums/amateur-radio-amplifiers.89/

    Good luck figuring out the best alternative for your SB220. But know this, you don't want to put a 706llg final into a highly reflective load; those finals are near impossible to get anymore.
  3. KG7HVR

    KG7HVR Ham Member QRZ Page

    The radio doesn't work now. The amp isn't even hooked up
  4. KG7HVR

    KG7HVR Ham Member QRZ Page

    Maybe I wasn't clear enough. This happen while driving the sb220 and now the radio no longer works no matter what
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The SB-220 operates 160m through 6m? Then, it's heavily modified since the original amp was only 80-10m.

    Do you know what modifications were made? "Sounds like" something from the amp spit power back into the Icom and fried the HF TX section of the Icom. Does the Icom still work okay on 2m/70cm?
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I think he was talking about the radio.

    I seen someone post that it was ok to use the amp in bypass on other bands. 160m and 6m. I would never recommend that.

    I would test using a dummy load for starters.

    Good Luck.
  7. WZ7U

    WZ7U Ham Member QRZ Page

    Awww crap. Might be too late then. That sucks! :(
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    That does suck.

    The nice thing about that radio is that it has a internal fuse for protection.
  9. CX3CP

    CX3CP Ham Member QRZ Page

    if the radio draws any significant amount of current when key, but no output, that sounds like you have something wrong between the Pa Output and the SO239 antenna connector. Check the LPF unit on all HF bands, and also check the jumper cable that you use for connect the amp and/or the dummy load.
    it's not going to be the first time that there are problems with jumper cables.
    WZ7U likes this.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Good Idea. Often the only band that works is where the problem is.

    Chill to some good music when frustrated.

    CX3CP likes this.

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