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IC-706MKIIG FM Problem?

Discussion in 'General Technical Questions and Answers' started by G1PPH, Apr 17, 2005.

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  1. G1PPH

    G1PPH Ham Member QRZ Page

    Hello.

    I've been using a IC-706MKIIG for a few weeks but today it started transmitting approximately 10KHz low when using FM. I looked around the net and found that this was a known issue with the first model of 706 but would like to know if anyone with later models has had the same problem.

    I've done a full reset on the rig and inspected the PCB for bad solder joints around the area suggested in the information for fixing this on the earlier model. So I just wondered if anyone knew anything to try before sending it off for repair.

    Thanks.
     
  2. kd5rpo

    kd5rpo Ham Member QRZ Page

    DON'T SEND IT IN FOR REPAIR! This is a common problem and very easy to fix.

    I had the same thing happen with my 706mk2G and fixed it about a month ago. I was having trouble for almost 2 years depending on temperature. My radio was going to the low side by up to 8kHz.

    Download the service manual if you have not done so. The very first item in the alignment section is the FM VCO adjustment. All you need is a digital voltmeter and a dummy load if possible.

    This radio uses a PLL circuit to make FM which is supposed to be locked to the main frequency reference that the rig uses for all modes. There is a variable capacitor C1022 which needs to be adjusted until the voltage across the varactor is in a normal range. This sets the free run freq. of the PLL oscillator. The voltage at test point CP 1011 needs to be at 1.8 volts at full power transmit in FM mode on 10 meters.

    I believe that earlier 706s needed to be set at 1.5 volts.

    The tiny trimmer cap needs a jeweler's screwdriver or very tiny alignment tool.

    Best of luck! Save your money.

    All 706 users should check this on their radios when it is convenient. The setting is a bit touchy and can drift with time. If you can only set it on 2 meters the voltage would read closer to 1.9 volts.


    IC-706mk2G service manual


    KD5RPO
     
  3. G1PPH

    G1PPH Ham Member QRZ Page

    It's working again now, the voltage was a long way out but adjusted easily so I'll keep an eye on it to make sure it's not a sign of a developing fault.

    Thanks for the help    [​IMG]
     
  4. KB2VXA

    KB2VXA Ham Member QRZ Page

    Hi Mr. Transistor (RPO)

    Thanks for the link, I didn't know DK had the full service manual, it's downloading now. Mine is a tad off the display and the error increases with frequency so now I should be able to correct it.
     
  5. kd5rpo

    kd5rpo Ham Member QRZ Page

    Hello VXA The original problem of this thread was about a radio that had a problem specific to FM mode. This is related to the FM VCO. I did not notice any increase of frequency error as the operating frequency was changed higher.

    Instead, it sounds as if you should check your master oscillator frequency (reference frequency adjust).
    This can be done as described in the owners manual in the section where they show you how to instal the optional CR-282 high stability crystal unit. The method they describe uses a precision frequency counter. If you do not own one you can use an alternate proceedure.

    Warning: the primary adjustment is to L601 which uses a ferite core with a very small slot. Using the wrong tool or applying too much force will crack the core!

    Without a frequency counter you need to remove the bottom cover screws of the radio and tune in WWV on 10 mHz with the ANF turned off. Warm up the radio for an hour. Connect a pair of headphones having good frequency response to the headphone jack and push the TS button on the front panel to enable display of frequency to the 1 Hz level.

    With bottom cover removed locate the little tin box covering the reference oscillator and remove the cover. You can now see the small canned inductor and see the top of the core in order to fit an alignment tool. There is a hole over this adjustment, but it is good to see it in order to make sure your tool fits OK. After you have done this once, just do it through the hole in the top cover of this circuit section.

    Switch the radio between USB and LSB while listening to the tone pulses of WWV. Adjust L601 until the tones have the same pitch on either sideband. There may be a shift of a few Hertz when the tin cover is installed rather than open. Put the radio together and heat cycle it a few times if you can. You will never get this exactly without a slight error, but the object is to minimize it. I check my radio about once a month on WWV without taking it apart. If I am within 7 Hertz that is close enough for me.
    Thats about all there is to it.

    Best of luck

    KD5RPO
     
  6. KB2VXA

    KB2VXA Ham Member QRZ Page

    Hi RPO and all,

    FYI, an RPO or rail post office is the mail car on a train where it's sorted en route.

    I rather figured the MO was just a bit off which is why I thanked you, the shop manual goes way beyond the owner's manual always. That's not the only reason, likely it's chock full of other useful information.

    The trick is that alignment tool but since you say "slot" I can probably whittle one like always when I don't have a store bought one of the proper size. WHEW! If it was a hex I'm boned, try to whittle one of those accurately.

    BTW, now it's a coin toss between using WWV to tweak my seldom used freq counter or the rig directly. (;->) The WW2 "B-29" phones don't go very low and neither does the 706 receive audio but I should be able to hear the breathing near zero beat. Many don't realize the "detector output" audio is anything but, it comes off after the DSP, found that out when I connected it to my stereo amp for nothing.
     
  7. kd5rpo

    kd5rpo Ham Member QRZ Page

    VXA, yes the core has a simple slot but did not fit my alignment tools. I had to make the one I used from a short length of carbon fiber left over from a broken fishing rod tip.

    As to 'RPO' I sometimes use the phonetics of Railway Post Office.


    KD5RPO
     
  8. K5SEP

    K5SEP Ham Member QRZ Page

    Brian KD5RPO -

    Thank you for the information on the problem with the Icom IC-706MKIIG on FM! I have an original IC-706 I purchased when it first was released in 1995. While it has worked flawlessly for ten years, I will check on this issue with my radio and let you know on this topic by reply.

    Again thank you for your research and thoroughness on this issue Brian!

    Best of 73's,

    Bob Scupp WB5YYX [​IMG]
     
  9. K2DJ

    K2DJ Ham Member QRZ Page

    Thanks for the help. i fix my ic-706mk2g just as described. mine was transmitting 10khz below frequency show in display on vhf-uhf. the voltage at the test point was way far too low from 1.8 volts, like minivolts. adjusted the C1022 to increase the voltage at test point to 1.8v , now work perfect.

    Thanks everybody. :)
     
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