Icom 706 Antenna Socket at fault?

Discussion in 'Radio Circuits, Repair & Performance' started by M0PIW, Sep 2, 2013.

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  1. M0PIW

    M0PIW Ham Member QRZ Page

    OK, bought myself a Mk1 706 with a strange fault.

    With the PL-259 put into the SO-239 socket the recpetion on the radio is perfect, connect the plug all the way so the antenna shield is connected to the radio it goes deaf.

    It doesn't make any difference what is connected to the plug, in fact I connected a short patch lead with nothing attached to the other end, sure enough, level raises as soon as you connect the pin, slightly connecting the shield and it goes again.

    I've tried connecting the sheild really lightly to eliminate the possibility of a loose SO-239 and straight away deaf.

    Hopefull this video will demonstrate the problem. (click below)


  2. KH2G

    KH2G Ham Member QRZ Page

    I suspect the center pin on the 706 is loose enough to slide back and short. I would open it up and chekc it out with it open and watch to see what moves. Just a guess but probably wind up changing th SOo239
    Dick KH2G
  3. M0PIW

    M0PIW Ham Member QRZ Page

    Thanks - I'll take a look and see what happens.
  4. M0PIW

    M0PIW Ham Member QRZ Page

    Socket checked - No issues whatsoever. Nothing is pushing through and continuity is fine...

    Any other ideas?
  5. KH2G

    KH2G Ham Member QRZ Page

    You've spoken about receive, I ask now if it transmits okay. The problem may be the antenna. Consider that you can normally stick a piece of wire into any transceiver and receive with it but to transmit is another story.
    Dick KH2G
  6. M0PIW

    M0PIW Ham Member QRZ Page

    Yes, transmits though SWR is 3:1 no matter what tuning or trimming is added. I'd been cautious of TX, for obvious reasons, until you asked.

    It may mean something to you (or someone) ?

    Thanks for the reply.
  7. M0PIW

    M0PIW Ham Member QRZ Page

    Further update - The same problem appears on the VHF antenna too.
  8. KH2G

    KH2G Ham Member QRZ Page

    If you have a signal generator available, I'd try injecting a signal and see what happens. While injecting the signal, you could try gently moving things about and see if there is a sudden rise in the level. Something seems amiss around the backplane with the 3:1 SWR.
    Interesting problem. Kind of wish I had it on my bench -hi.
    Dick KH2G
  9. KH2G

    KH2G Ham Member QRZ Page

    You might also check the jumpers from the plugs to the chassis and the grounding points on them as it appears on the schematic that they depend on the case to make good contact and for a fact, sometimes those little jumpers can create problems which is why I suggest injecting a signal and moving some stuff around and gentle probing with an orange stick.
    Dick KH2G
  10. WG7X

    WG7X Premium Subscriber QRZ Page

    I would add that the behavior you are seeing is normal when the antenna is questionable.

    what might be happening is that when the plug center pin is inserted but the backshell is not threaded into the plug, the coax it self acts as a random-length antenna. Then when you fully seat the plug and screw it in, the coax is now grounded and the long wire aspect is now defeated.

    I suspect that is what you are seeing, especially if the indicated SWR is fairly high.

    Look to the antenna before you start tearing the radio apart. This is where a good dummy load is very helpful. If your rig transmits OK into the dummy load, that will shorten the list of things to look at. After that is when the signal generator comes in handy.

    73 Gary
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