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

gtilford

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

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

Mike-VA3MW

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

https://www.nutsvolts.com/magazine/article/September2015_HamWorkbench

Ken Wells

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.

OR.

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

Erika - KØDD

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

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

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

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