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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

    It does receive without Q25. I took it out and re-soldered it. Thought maybe I missed a leg or something else.
  2. WA4MNT

    WA4MNT Ham Member QRZ Page

    My inexpensive tester says the original Q25 was bad.
  3. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    I feel confident that you have found the problem. Replace Q25 with the correct part and retest. This time don't apply external heat, just let it run normally.
    Let us know how the final testing goes ... if you have time.
  4. WA4MNT

    WA4MNT Ham Member QRZ Page

    Well there's the problem. I'm just getting ready to go to Dayton Hamvention, and then 4 months in Canada. Nothing like jumping in at the last minute and trying to fix something. There won't be room to take the pro2. So ine transistor will hneed to wait until I return. Obviously the $.40 transistors from Mouser will be here by then. I feel the same as you that this may in fact be the problem. I really don't know what else would have made that transistor go bad. A good long burn in will make me feel better. I will stay in touch. You can get me anytime at I can't thank you enough for all you have done getting me pointed in the right direction. This is my main rig besides a bunch of various qrp gear.

    Best 73

  5. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    You did the hard part by locating it in the area of Q25.
    Have a good trip.
  6. WA4MNT

    WA4MNT Ham Member QRZ Page

    Just wanted to let you know, when I got home from Canada a couple of days ago, I replaced the transistor and all is well. No more heat shutdown. Thank you again for all you help and encouragement.

    Best 73,

    ken - wa4mnt
    W1TRY, K9ASE and KA0HCP like this.
  7. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    Glad to hear it
  8. N0CEL

    N0CEL Ham Member QRZ Page

    I agree 100% with James, AA5CT, on the "lack of cooling" in regards to the Icom 756 (all flavours!) I, too, had the "Thermal Issue w/ loss of RX". And I also ran my 756 w/o the top and bottom covers, yet, the Control Unit components, that were mentioned above in this thread, still cut out occasionally, much to my chagrin. :( MY work-around was this: One, I left the inner top cover plenum (plate) OFF (on purpose, just to test). Two, I never have used the "internal" speaker - on ANY of my previous transceivers, so I took the two hold-down screws out, unplugged the speaker, and put it up for safe keeping. Since I, too, also added a small 12 vdc "Muffin - style" fan onto the rear of the chassis, this allowed for MUCH better internal cooling - overall! In conclusion, it is this old man's opinion that this could have and should have, been addressed in the original design if this specific transceiver, since the chassis appears to be one large heat sink, which is fine, unless one does not keep in mind Icom's lack of ventilation louvers, etc. I hope this helps any future amateur radio ops who may stumble across this thread. 73 Carl de NĂ˜CEL BTW, I still have some picture, taken from the Icom 756 Service Manual, that I doctored-up, with the explainations for my doing so. If interested, as we say around here, "My email is good on QRZ!" :cool:

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