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?

  11. NE7X

    NE7X Ham Member QRZ Page

    I understand what you are saying, however I have over 20+ different radios here in my shack and they are all working without any issue. (see my web site http://ne7x.com ). Yes there is a balum at the feed point of the antenna, its a TH5 yagi. I acquired the IC-706MKIIG from a local ham who had the same issue. So I can safely says its not my antenna.

    Even when I have an antenna tuner in-line, the IC-706MKIIG goes into oscillation when keyed on 15 meters. Once it goes into oscillation, cannot tune the tuner, for the radio is transmitting on every frequency in the spectrum, as shown on my Tek 496 Spectrum Analyzer. On a 50 ohm non-inductive dummy load it works perfect.

    I called ICOM today and talked with a technician. His comment was a bad ground on either the "FILTER" or "PLL" boards, or maybe both. The reason why its works OK on the dummy load, he said because the SWR is below 2:1. Anything higher will cause the RF output circuit to oscillate. I have removed the FILTER board and cleaned all the ground contact points, however this did not help. Guess next will be to do the same to the PLL board, however its buried into the radio so it will be some work.

    Thomas NE7X...
  12. NE7X

    NE7X Ham Member QRZ Page

    I add this Un/Un balum in-line with the HF output coax cable. It STOPPED the oscillation on 15 meter transmit. When I take it out of line, the transmitter goes into oscillation on 15 meters.
    I picked this up at a local ham swap for $1.00.
    Has SO-239 connectors on each end.
    Its 50 ohms in / 50 ohms out (1:1) Un/Un
    What is inside is a piece of 50 ohm coax between the two SO-239 connectors,
    with a series of 10 torrid cores which go around the coax cable.
    You could make one by using maybe 10 of the snap-on torrid cores.
    Just snap them around the HF coax cable in series next to each other.

    It’s a shame I need to do this to make the IC-706MKIIG working on 15 meters.

    73s ‹(•¿•)› Thomas NE7X…
  13. AA6P

    AA6P Ham Member QRZ Page

    I experienced this problem in 2008 and found the information at the website referenced below.


    At that time, Icom offered to fix the radio at no charge if it was mailed to them. My understanding was that they would go through the steps listed in Tech Bulletin #928.

    That could represent quite a bit of work at no charge assuming that they include cleaning the ground spring connections under the PLL board. I don't know if that offer still stands. If anyone does intend to return their radio to correct this problem, it would be worth asking.

  14. AA6P

    AA6P Ham Member QRZ Page

    I just reviewed a few additional forum posts and articles, and it appears that both the IC-706MKII and IC-706MKIIG can have the RF Feedback instability problem on 15M/17M. However, the symptoms appear to be a little different between the two radios.

    One individual with a MKII discovered that the grounds on the final amplifier board were a critical point. Cleaning all ground contacts and a copper ground strip along the back edge of the board and tightening all mounting screws resolved the problem.

    My IC-706MKIIG is usable on 15M/17M with one specific antenna but the problem returns if I lengthen the feed line. It seems less stable now that the ground is very dry. I have noticed that the SWR changes between wet and dry conditions.

    My theory is that units with the 15M/17M problem on are the edge of instability and a "fix" that appears to work may be just barely fixing the problem for the moment. This makes the problem very frustrating to resolve. It also appears that every radio with this problem may have its own unique characteristics as a result of different components, tolerances, and the effectiveness of each ground connection.

    I may need to get under the PLL board to clean the ground spring contacts. Has anyone actually tried that? One thought was to loosen the mounting screws and try to slide a strip of clean paper under the ground spring contacts to burnish and clean the surfaces.

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