Icom 756 Pro2 stops receiving when warmed up

Discussion in 'Radio Circuits, Repair & Performance' started by WA4MNT, Apr 28, 2019.

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  1. WA4MNT

    WA4MNT Ham Member QRZ Page


    I have a Icom 756 Pro2 that stops receiving when it warms up ~30min. It stops instantly when it happens, but will recover when shut down and cools off. I have looked on the internet for similar problems with this model, but have not found anything. Is there someone that has experienced this and can point me in a direction to repair. All other functions seem as they should. You can contact me directly as well at wa4mnt@gmail.com.

    Thanks and 73,

  2. K9YLI

    K9YLI Ham Member QRZ Page

    run with covers off to start with. see if it changes the time to fail..
    then get canned air and use upside down.. very cold stuff.
    when it fails give a shot of cold air a small area at a time.
    narrow down area that fails.. inspect with bright light and magnifier..
    look for solder joints or cracked land patterns or board.
    use schematic to take wild guess at defective part. look for fat electrolytics,
    KA0HCP likes this.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Is it the receiver or the audio section?
    -Have you checked audio output with both internal speaker, external speaker and headphones?
    -Does the Signal Strength Meter continue to move or does it start and stop in correlation with the loss of reception?
  4. WA4MNT

    WA4MNT Ham Member QRZ Page

    It's not just the audio. S meter quits too...
    KA0HCP likes this.
  5. WA4MNT

    WA4MNT Ham Member QRZ Page

    It's definitely heat related. With all the covers off it just takes longer to occur.
  6. AA5CT

    AA5CT Ham Member QRZ Page

    I have _always_ run with a low-speed muffin fan behind my ProI/ProIII radios ... just as a matter of practice. Just sayin BTW.

    Just forces a low amount of air through the rig, keeps that area up in front from getting warm. Maybe I'll extend its life a little.
  7. WA4MNT

    WA4MNT Ham Member QRZ Page

    A little more information I have learned digging around on the control board. I have located the group of components that are causing the receiver to cease. They are centered around Q25 on the control board and supply the switching to RL4 on the control board. When I warm these components only, shown in the photo, RL4 energizes, and the receiver cuts off. Cool them off and the receiver returns to normal. All the connections around these smt components look good. I am considering the shotgun approach and replacing Q25, D22, and D23.


    area of concern.JPG
  8. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    I think those are known to be ESD sensitive. Failure of these is also known to cause the oscillation on 6m. Happened to my PROIII.

  9. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    Your proposed action seems reasonable. I wouldn’t consider this be “a shot gun approach”.
    Let us know how you make out.

  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    What are you heating the parts with ?

    Q12 and Q13 would be suspect if the relay is switching.

    You can check the voltage to find out.
    WA6RE likes this.

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