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FLEX-6400: 60Hz Hum with Isolation Transformer. Resolved, But Why?

gtilford Member
I "fixed" a 60Hz hum problem with my shiny new FLEX-6400, but I don't understand why what I did resolved the problem ....

As a standard practice, anytime I send a powered source (like a computer, or my FLEX) to my mixer I always use an isolation transformer. I want to stop ground loops before they happen. This is what I did when setting up my FLEX. As I increased the volume I noticed a pronounced 60Hz hum. I did the usual swapping around cables and I verified that the problem followed the FLEX. I powered down the FLEX and still had the hum. I started disconnecting inputs, like the power supply, coax, Ethernet and even the system ground for the FLEX and I still had the hum. At this point *nothing* is connected to the FLEX except the line isolated path to my mixer. I unplugged the cable from the back of the FLEX the hum went away. Plugged it back and the hum was there. I also tried swapping out the line isolation transfer with a known good unit and I had the same results.

After scratching my head a bit I decided to try the connection to the mixer without an isolation transformer and no hum! So, I have a problem with hum with an isolation transformer, but none without. Anyone have ideas on what is going on? I want to write this one down.

73, Greg, KM5GT


  • Geoff AB6BT
    Geoff AB6BT Member ✭✭✭

    Any chance of a drawing showing exactly how it was wired with and without the transformer(s)?

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited January 2

    I agree with Geoff. This sounds like what is called a Pin 1 problem.

  • Ken Wells
    Ken Wells Community Manager admin

    Depending upon what you had connected to the mixer, you may have had a Pin 1 problem, or you may have had a ground loop but were isolating the wrong input or output with the isolation transformer.


    the windings on the transformer or it's lines may have been picking up AC hum from a nearby power transformer.

  • Erika - KØDD
    Erika - KØDD Member ✭✭✭

    Sometimes you need to pull every plug out of your station and start building from Box A and Box B and test with every piece after hooking things up. You can never have enough GROUND... I finally drilled a hole in the side of my house and ran a 4ga from the radios' rack right out the wall down to the tower base and over to a tower leg. The tower has 3 ground rods driven in below and inside the concrete and another outside the concrete. I have the original ground rod coming off the end of the rack through the floor right down through the foundation to another ground rod ten feet away from the tower... I still get RF in the shack on occasion. AC hum is a very elusive issue I fought with this one and my last station mysteriously when it felt like having hum 14 years ago IC-7800 fed through the back panel from an Ivory 5051 tube compressor EQ etc... Thinking back that could have been a tube filament in the processor.

    Another problem is cranking up the power and having your router / switch shutting down your computer link. (occasional problem here) and I lose my EA4TX remote box for my coax switch with the computer flaking out the com port to the box. I just say, "I hate Ham Radio", wipe my tears, and tear it all apart again so I can do it all differently. The last shack I had separated Coaxes, from audio, from AC POWER, and DC Power using metal troughs at the back edge of the desk for shielding. Good luck, Erika DD

  • gtilford
    gtilford Member
    Thanks for the comments and suggestions. To answer Geoff's request, the final test configuration could not have been simpler. It was:

    FLEX6400<->Isolation Transformer<->Shielded 3-Wire Audio Cable<->1/8" stereo to 1/4" TR splitter<->Behringer<->JBL Powered Speakers

    Nothing, and I mean nothing else was connected to the FLEX. It wasn't even powered. With the audio cable disconnected and dangling in the air there was no hum. It only occurred when I completed the connection to the disconnected, unpowered FLEX.

    I use the exact same cable/line isolation configuration to connect my computer to my mixer. No hum.

    And Erika, yes, I learned the grounding and bonding lesson the hard way a few years ago. I now have 4 ground rounds, the last is tied into my shop ground (where my shack is located) and I use a single-point copper ground bus inside of the shack. It really helps eliminate a lot of those odd, random problems (like my transmitter turning off in the middle of a QSO, which it did), and it lowers the noise floor.
  • Geoff AB6BT
    Geoff AB6BT Member ✭✭✭

    Hi Greg,

    The information you provided does not answer the question of exactly how the components were wired.

    Without an explicit schematic of all the interconnections I'm afraid I can't offer any assistance.

    I'd like to help but...

  • Jim  KJ3P
    Jim KJ3P Member ✭✭✭

    If the isolation xfmr interrupts the ground/shield of unbalanced audio, then that might be the problem. If this is the case, everything from the xfmr through to the Flex becomes a fine pickup for ambient hum. Such isolation xfmrs work well for balanced audio, but often not dependably for unbalanced.

    --jim KJ3P

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