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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 Ham Member 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
  11. M0PIW

    M0PIW Ham Member QRZ Page

    OK, by way of an update, TX is fine into a dummy load. I've visited a friend today using their tribander and exactly them same problem on RX though SWR is fine, flick to the same antenna on there FT1000 and there's not a problem RX or TX.

    Unfortunatly I don't have access to a sig gen, I have tried connecting the antenna and then checking the boards for flex or loose connections with no joy. [​IMG]

    All those jumpers are fine and doing a great job?

    Really no further forward..
  12. WG7X

    WG7X Ham Member QRZ Page

    OK, this is getting fairly strange.

    Just to re-cap: On dummy load transmitting is OK?

    Then when you go to a friends house with a "known good" antenna the receive goes dead when connected to an antenna that the normal station rig uses OK?

    If that's the case then we know only one fact: The problem, whatever it is, is in the 706.

    That is only one step towards fixing it.

    My706 started exhibiting strange behavior with fluctuating SWR and power out on VHF which was eventually traced (not by me) to a cold solder joint on the VHF final.

    Not that it has anything to do with your problem...

    I'm out of ideas, mate.

    Maybe someone else?

    73 Gary
  13. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Open up the 706 and locate the MAIN UNIT (MAIN 1) and disconnect P1 from J101.
    Connect the antenna shield to the chassis of the 706 and connect the center conductor of the coax to CP101.
    See if the reception returns. If so then you have a problem in the FILTER UNIT. If not then it's farther down the RF path but shouldn't be too much farther then this point.
    Make sure you have a dummy load connected to the usual antenna jack or disable the transmit so you don't accidentally transmit into an open.
    Service manual here at; and the information is on page 10-2. The FILTER UNIT is later in the manual.
    Wait a sec....could be the wrong model. Look at this service manual and see if it's the correct one;
    If so then you need to disconnect P1 from J1 and connect the antenna to the jack J1 (page 11-2). The earlier models of the 706 are a bit more difficult to work on as far as test points are concerned.
    Anyway, see what that does.....
    Get back to us with the results.
    Hope this helps
    Last edited: Sep 8, 2013
  14. M0PIW

    M0PIW Ham Member QRZ Page

    Thanks Gary

    Tried exactly what you suggested and the fault is still there when the antenna is connected to J1...
  15. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Not sure if you are having the problem with or without the pre-amp enabled. If the pre-amp is on then apply the antenna center conductor through a .01ufd blocking capacitor to pin 6 of IC1 (uPC1658G).
    Also check the voltage at the top end of R7 and make sure the pre-amp selection voltage is within specification. See if that helps.
    Also, what is the s-meter reading when you're using the antenna? Could be something overloading the entire frontend and causing the receiver to simply blank out.
    Have fun
  16. M0PIW

    M0PIW Ham Member QRZ Page

    Thanks again...

    Pre-Amp and ATT for that matter are functioning perfectly. I don't think there are issues with overloading, the S-Meter reads as it should in terms of RX.

    Strong Broadcast Station
    +60 with Pre-Amp to the point of overloading
    S7 without no overload

    Connect the sheild
    S0 with Pre-Amp just about readable
    Nothing without preamp

    Connecting the chassis to the PL259 shield without it being connected to the SO

    With preamp S7???

    I hope that makes sense? To summarise, the shield of the PL connected to the shield of the SO drops from +60 to 0, the shield of the PL not connected to the SO but bridged to the chassis drops +60 to S7. The anttenuation is not as severve when the PL is bridged to the chassis rather than the SO

    R7 - Whereabouts?
    Last edited: Sep 9, 2013
  17. KO6WB

    KO6WB Premium Subscriber QRZ Page

    My eyes are really getting old.....Sorry, R7 is for the selection of the appropriate band filter, so just ignore the goof up.
    The voltage is at the cathodes at the junction of D12,D13 and looks like D28.
    It could be there is a filter being selected when it shouldn't be but darned if i can figure out how that'll cause this problem.
    Here's the area I'm looking at and remember....Old eyes......
    00000 ic706.png
    Hope this helps some but it's a head scratcher for sure.
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