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Icom 756pro III got zapped

Discussion in 'Ham Equipment' started by 357, May 26, 2018.

  1. 357

    357 Walkin' the dog

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    Anyone familiar with Q25, D22, D23 on the CTRL unit?
    Mine got smoked by a close lightning strike.

    I repaired it but its only partially fixed.
    Apparently there are some shorted diodes on the RF-B unit.





    History: The radio wouldn't make power, only a 50w carrier on all modes.
    It did 100w with only 18db att. on oscillated at 0,6 and 12db, receive was 50% what it should be with 0 Att on the 24mhz up bands..

    After fixing the CTRL unit, it would only make power in 6, 12 or 18db att. oscillated 50w on 0db.
    Receive still low, nothing on the lower bands, no wwv, no broadcast stations at all, deaf.
    Digging further I found shorted diodes on the RF-B unit. Mainly D104, D851 but others are leaky too.
    (maybe just do them all?)

    Anyone dealt with this before?
    I've scoured the net I'm looking for some first hand knowledge.

    Thanks.

    IMG_3079 (2).jpg 97a9d7c00ebf0e90227937032ba5618f.jpg 2017060.jpg IMG_3h088.jpg
     
    #1 357, May 26, 2018
    Last edited: May 26, 2018

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Sounds like maybe the first RF amp is gone and possibly one or more bandpass filters. Check for open inductors in the different bandpass filters and make sure they are being switched when they should be.
     
  3. 357

    357 Walkin' the dog

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    Well I'm just gonna do all the diodes on the backside and a few on the front that look suspicious.

    RF-B BACKSIDE BOTTOM.jpg RF-B BACKSIDE TOP.jpg IMG_3092.JPG IMG_3091.JPG IMG_3090.JPG RF-B FRONTSIDE TOP.jpg RF-B FRONTSIDE BOTTOM.jpg IMG_3093.JPG
     
  4. 357

    357 Walkin' the dog

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    2017088.jpg 2017089.jpg
     
  5. 357

    357 Walkin' the dog

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    please excuse my language but.....FUCKING A IT WORKS I FIXED IT HALLELUJAH!!!!!

    [​IMG]
     
    PA770 and Shadetree Mechanic like this.
  6. 357

    357 Walkin' the dog

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    Heres the results.
    I melted a cap a bit but otherwise no problems.
    The last pictures are it transmitting with no oscillation, mission accomplished.



    IMG_3106.JPG IMG_3107.JPG IMG_3108.JPG IMG_3109.JPG IMG_3110.JPG IMG_3111.JPG IMG_3112.JPG IMG_3113.JPG IMG_3114.JPG IMG_3115.JPG
     
  7. 357

    357 Walkin' the dog

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    IMG_3116.JPG IMG_3117.JPG IMG_3118.JPG IMG_3119.JPG
     
  8. 357

    357 Walkin' the dog

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    special shout out in the video description....you know who you are.
    (nobody from here)
     
  9. Klondike Mike

    Klondike Mike Well-Known Member

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    Congratulations on your repair. You sure learn a lot about your radio when you go though repairs such as that.
     
    357 likes this.
  10. 357

    357 Walkin' the dog

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    thanks!
    While the pro 3 was down I've been drooling over the 7610...I was gonna sell my car and buy a POS to get one.
    Now I'm not jonesing so bad I'll wait.

    :cool:
     
  11. 357

    357 Walkin' the dog

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    Well the 756 was still having trouble.
    The low band, 0.3-1.5999, it was deaf.

    Everything else worked. I started chasing the schematic and determined I didn't replace D802 and figured it was popped.
    Under further inspection I seen this.


    IMG_3174.JPG

    That was after pulling the preamp board off to get to anything. A real chore with a soldering iron.

    I looked online and found a mod that allows full signal in the low band and it effectively jumpers out the burnt R801 so I thought thats half the battle right there.
    Now these chips are under the bpf board and I just did't want to chance removing it again.

    When I first read this mod I thought it said jump R802 and R803 to 0 ohms so I swapped in 0 ohm smd's and put it all back together and the low band was still dead.
    I fucked up and was supposed to bridge 802 and 803 and after going over it all again, tore back into it again today.

    I put in 2 new 100ohm's and Replace C802 that the mod said to remove also.

    Now this all was s u p e r difficult.

    Maybe Yvo and TRX and Mike have all the fancy equipment to make this easier but I dont so some stuff is crooked and difficult to get to.

    Some plastic got nicked but in the end a miracle happened and the radio works top notch now.

    Dont try this without flux. The flux is the secret to not vaporizing parts.
    I basically hold the part down with a small screwdriver and tack them down with the iron and lots of flux.

    It was pure luck I was able to fix this radio.




    IMG_31u86.jpg IMG_3185.JPG IMG_3175.JPG IMG_3171.JPG IMG_3172.JPG IMG_3173.JPG
     
    #11 357, Jun 26, 2018
    Last edited: Jun 26, 2018
    Shadetree Mechanic likes this.
  12. 357

    357 Walkin' the dog

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    All finished. Dont forget this is on a Imax for the antenna.
    In the end D802 was fine and it was R801 that smoked and killed the MW band.
    Then I almost killed it more and then got it working in the end plus the MW "mod".

    Time for the papers and scissors, its a wrap!


    IMG_3190.JPG IMG_3191.JPG

    IMG_3193.JPG IMG_3194.JPG 2017008.jpg
     
    #12 357, Jun 26, 2018
    Last edited: Jun 26, 2018
  13. 357

    357 Walkin' the dog

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    Oh and I relocated C802 here insead of removing it as in the mod:

    IMG63185.jpg
     
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  14. 357

    357 Walkin' the dog

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    d3d273a250.jpg
     
  15. 357

    357 Walkin' the dog

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    Seems to be ok
     

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