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IC 706 MKIIG 17 & 15 meter issue

Discussion in 'Radio Circuits, Repair & Performance' started by WA7ACE, Apr 30, 2012.

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  1. WA7ACE

    WA7ACE Ham Member QRZ Page

    Icom 706 MKIIG: 17 & 15 meter bands, USB or LSB mode, when the mike is keyed the radio puts out no audio and the SWR my meter deflects to infinity and a full ouput carrier is produced as though AM mode selected. All other bands and modes operate normally. I have a second MKIIG for comparison that operates normally on 17 & 15 using the same antenna, leads and all other. So I have concluded it is within the radio. Anyone seen this condition? Any ideas?
  2. KB8YVC

    KB8YVC Ham Member QRZ Page

  3. WA7ACE

    WA7ACE Ham Member QRZ Page

    Thanks for the link to the service bulletin. My radio had the ground spring from the factory but it lost ground contact. I put a bit more bend in it and it now makes a good ground and the radio functions normally again.
  4. KK0DJ

    KK0DJ XML Subscriber QRZ Page

    KB8YVC de KK0DJ... thank you for that post ... I am a 706MKIIG owner as well and haven't seen this issue, but am grateful someone has and has left a trail on how to fix it! Thanks again.
  5. SV9OFO

    SV9OFO Ham Member QRZ Page

    Well, it's not always a remedy. i have thried this along with some more grounding to eliminate 80m oscillations when transmitting with 17-15m filters enabled but problem still occurs. I have to not operate those bands till I fix it.
  6. NE7X

    NE7X Ham Member QRZ Page

    I added the ground wire as described and my IC-706MKIIG still is unstable on 15 meters.
    Any suggestions what else I can try?
    Thomas NE7X...
  7. AC0OB

    AC0OB Ham Member QRZ Page

    Try a purely resistive (dummy) load and see what happens. I have found highly reactive loads will still cause it to oscillate, evem with the fix.

    Phil - AC0OB
  8. NE7X

    NE7X Ham Member QRZ Page

    Yes, when connected to a DC 50 ohm non-inductive dummy load, the IC-706MKIIG transmits on all bands perfect, including 15 meters. However when I connect the IC-706MKIIG to my outside antenna, it transmits perfect on all bands except for 15 meters. On 15 meters when I key the transmitter in any mode, the transmitter goes into full RF power output and all SWR meters read 100% reflected RF power.

    Looking at my Tektronix 496 spectrum analyzer, when keying on 15 meters with the outside antenna connected, the IC-706MKIIG is transmitting everywhere in the spectrum. It appears the transmitter or RF outputs section is going into total oscillation.

    I received an email from Phil AD5X who mentioned this occurs when there is RF feedback due to a poor ground. Phil's comment got me thinking, maybe there is stray RF on the outer braid of the HF coax. So I place a torrid into the HF coax and BINGO!!! this resolved the issue. Now the IC-706MKIIG transmits on all bands perfectly, including 15 meters.

    Placing the external torrid on the HF coax line is a band-aid fix, I would like to identify the real issue. Looks like I still have an open ground or ground loop somewhere inside the radio.

    Thomas NE7X...
  9. NE7X

    NE7X Ham Member QRZ Page

    UPDATE !!!

    I removed the filter board, cleaned all chassis ground mounting studs, cleaned all the PCB foil points where the ground mounting studs come into contact with PCB and added extra ground wires from different PCB ground foil points to the chassis.

    Now when I transmit on 15 meters, the transmitter still goes into oscillation even when I have the torrid choke on the external HF coax lead. If anything, I made negative progress.

    When I transmit into a DC 50 ohm non-inductive dummy load, it still works perfect and does not oscillate.

    Any suggests what next to try would be appreciated.

    Thomas NE7X...
  10. KE3WD

    KE3WD Ham Member QRZ Page

    Thomas, anytime you take a measurement and situation is different from taking the same measurement but into another load, you'v emade positive progress. As I like to say at the Test Bench, "she's trying to tell me something..."

    In this particular case, she may be trying to tell you that there is nothing at all wrong internal to the radio, but your antenna system is likely bringing RF into the shack.

    Antenna, feedline, is there a balun at feedpoint?

    Is the Antenna a balanced antenna (such as a dipole) or what?

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