Finding & Fixing Mic / Audio Hum in an Omni VI+

From the Ten-Tec Reflector January 1, 2006

It is common to get reports of slight over driving… too much processor, etc. Happens to us all at times. But what we all dread is that "what is that terrible noise?!" report. Which is exactly what I started receiving after rebuilding my station at our new QTH. About 10-20 percent of my audio was hum or worse. This was very perplexing to me as although my station makeup is somewhat complex (six working HF transceivers, two HF amps, auto and manual tuner, monitor scope, HEIL HM-10 boom mic, voice keyer, and numerous interconnects for voice, cw, multiple cw keyer sources, multimode terminal units, tncs, and soundcard-based digital modes) the new setup was essentially the same as what it had been at my last QTH, or so I thought. As it turns out I had made a few small changes and they came back to bite me… 'literally'.

I had three HUM sources to trace out:

  1. HUM when using microphones in SSB.
  2. Worse hum… actually two signals beating on each other when the Omni-VI+'s audio IN/OUT/PTT from the back panel were physically connected to my computer isolating interface while I was talking on SSB. Easy fix was to disconnect them but that was troublesome. Needed a better solution.
  3. My MFJ Voice Keyer 434 had become useless. Previously stored messages played nicely but connecting my HEIL to the 434 created terrible audio hum regardless of whether the MFJ was tuned on or off. Disconnecting the mic from the front of the MFJ did eliminate most of the hum when testing with prerecorded messages, but if the mic was connected…

[Please note that I had previously modified my HEIL cables so that the shield ground connection also went to the connector hood.]

  1. Incorrect Placement of 962 creates audio hum. So this next one is for the Ten-Tec Omni-VI+ reference book. Avoid placing your 962 power supply directly to the left of your Omni-VI+! Place it to the right, above, or below, but avoid being to close to the left side of your Omni. We don't need to argue it, just accept that in my environment this was found to be a very ugly truth. Understand that I am a very experienced RF engineer and that I assemble my station using best practices… clean, isolated AC power, effective distributed RF ground systems, ventilation, etc. So please NO lessons.
    How I discovered this placement issue. When setting up the new station I had space on my main desk for the 962 to be with the Omni-VI+ and since I am right handed I placed the power supply to the left. The same configuration I use for my 961 and Paragon. As it turned out this new placement generated the horrible audio hum I was seeking. Thinking it might have something to do with my Heil HM-10 boom mic or cables I tried other microphones and all except for my 705 had the terrible hum, and that was mostly because 705 is very hot thus requiring little mic gain. Nevertheless there was some hum there as well. When chasing down this mic circuit hum I changed out mic cables, reviewed grounds, looked for open grounds and potential loops, etc. I used a second receiver with headphones and a monitor scope to monitor my tests). I wasn't getting anywhere. Then I removed the Omni from the desk and removed the Transmit Audio board…. cleaned all the surfaces, cable connectors and reseated it. I also checked other ground connections/hardware in the Omni while I was there and found a lot of loose screws. I put the Omni back into position and the hum remained. Now all of my test transmissions were to a dummy load but there were a lot of coax cables between here and there. So I deiced to get real basic and pulled the Omni to the bench and connected a Drake dummy load directly, my bench Ten-Tec power supply (260) and a mic. Woah! No hum! I tried all my microphones. No hum. I set the Omni back on the desk with the 962 and HUM. I pulled the 962 off the bench and went through it thoroughly. Checking grounds and then measuring ripple percents under full load, etc. Looked good. Then I connected it to the Omni on the bench and there wasn't any hum…. but it was not next to the Omni but rather several feet away. I moved it closer and then to the left side of the Omni and there we had HUM. Move it no hum. So the 962 is now mounted to the right of my Omni-VI+ and that is that. Nothing in the manuals on placement issues.
  2. Avoid ground loops when interfacing rear panel connections to external devices (easier said than done). Honestly, I am still pondering this one. The computer interface I built has total isolations on all incoming and outgoing signal lines nevertheless and not caused me issues before but was NOW. This source of hum was also experienced on my Paragon which is my backup rig. The audio IN/OUT/PTT signals of the Paragon and the Omni-VI+ go to a 'plastic' box filled with relays than select/route hots and shields from one rig or the other to the soundcard interface. Supposedly full isolation once again. Anyway all I had to do to remove this source of hum for both rigs was disrupt the shield ground connection on the PTT shielded cable at the rear of the Paragon and the real of the Omni-VI+. Problem fixed for now. REMEMBER, when you transmit in SSB mode both front and rear audio connectors are hot. so if you are in PSK, for example, and you have a MIC connected what is not 'shorted out' when not in use, then you will have room audio transmitting with your PSK. Not good. Remove the mic if it doesn't short out on its own.
  3. MJF Voice Keyer 434 needs to get its power from an independent source and not share 12vdc with anything else in the station, including the rear panel 12vdc accessory connectors on the Omni-VI+ or 962 power supply. When I moved the 434 to its own wall wart all the noise/hum/distortion went completely away. It came back if I tried to connect to my station's shared battery buss or the Omni/962 12vdc connect points. Motto here… just use the provided wall wart.

Happy New Year Everyone.

Jerry, KG6TT

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