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.]
I. 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.
II 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.
III. 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.
73,
Jerry, KG6TT
Fairfield, CA
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