Odd TX hum problem with IC-27E

Discussion in 'Radio Circuits, Repair & Performance' started by M5RJC, Jul 26, 2020.

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  1. M5RJC

    M5RJC XML Subscriber QRZ Page

    Hi all

    Just looking for ideas here... I have an old IC-27E (Europe version but near identical to the US IC-27A) that I'd like to use for local ragchews. It works great overall - it's sensitive on receive and outputs full power with nice modulation.

    Just one problem : it has a hum (100Hz ?) on transmit. It's an FM hum at about 10% deviation (as I see it on my SDR), but it only happens on the high power setting (25W). On 5W the hum is gone. There is no hum on receive.

    I established that the hum is not from the mic/audio side because if I turn down the mod level adjust pot (just before the modulator) to zero, there is no audio modulation but I still have the hum. So the hum must be in the PLL/modulator/RF side somewhere.

    Strangest of all is that the hum is sensitive to the supply voltage, I checked and the same thing happens with different PSU's so it's not a PSU issue as such. At 13.8V (my bench power distrubition) from a linear PSU I have the hum : at 14.4V (still well within 13.8V +- 15% spec for the rig) the hum disappears.

    At this point I'm stumped - my workaround is to run the rig off a separate switchmode supply at the higher voltage, but I'd love to know what's going on here !

    Open to any ideas !

    Best 73

  2. G4COE

    G4COE Ham Member QRZ Page

    Voltage sensitive hum, same on other psu's but power related.... strange that one is

    Not RF feedback or RF getting into the power supply is it?

    Before getting the 'can opener' out and looking for swollen electrolytic caps., they may also have a domed top or leaking gunge lets do a couple of test first eh?

    Try it on a dummy load and then try it on a car battery, lets see if we can eliminate these first, that's what I would be doing.

  3. WR2E

    WR2E Ham Member QRZ Page

    I ran a 27A mobile for many years. One day the morning drive crew I used to chat with on simplex reported a hum on my signal. It had been great for years and years before that.

    I retired that rig shortly afterwards because the volume pot opened up and no audio out and impossible to source a replacement, so it never got fixed.

    This was not a Mic cable or A.C. Or alternator whine. The guys on the net said it sounded like I had a PL tone turned on. I'm sorry I don't have a solution for ya, just wanted to say ' me too '

    I suspect a bad ground inside, or an audio stage oscillating.
    Last edited: Jul 28, 2020
  4. M5RJC

    M5RJC XML Subscriber QRZ Page

    Thanks for the suggestions Dave, but no dice so far.
    Looking to borrow a spectrum analyzer from a pal so I can trace through the stages and figure out where it's getting introduced.


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