Flex 6700 humming issue

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  • Updated 2 months ago
I have strong hum coming from my Bose Companion 2 Series III.

After playing with the setup I come to the following conclusion: hum is coming from Flex radio 6700. 

The hum stops only when I turn off my Flex. The hum is just reduced to a certain extent If I just disconnect audio cable from PC or/and Flex. Obviously audio cables act as antennas but real source is - Flex.

Tried with different cables, different power supplies for both radio and Bose, different computers, etc.

Any idea?

Many thanks in advance,

Tom, 9A4T
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Tom

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Posted 2 months ago

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KD0RC

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Sounds like it could be a ground loop.  How are you powering the Bose speakers?  Are you using the Bose switching supply, or powering from the same 13.8 V source that powers the radio?  If you have a 1:1 audio transformer that you can use to isolate the audio output, that might be something to try.

73,
Len, KD0RC
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Tom

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I tried original Bose PS and two different external PS including one that provides DC for Flex.

Isolation transformer I don't have at hand, but hum is here even when Flex is not connected to the Bose speaker.

Hum is eliminated only when Flex is turned off.
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KD0RC

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Are the Bose speakers physically close to the Flex?  I am wondering if it is magnetic pick up of some kind.
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Tom

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Flex is sitting on my PA that is sitting on the wooden shelf, Bose is downstairs on the desk, maybe 1m and PA in between.
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KD0RC

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OK, so it sounds like a ground loop.  Put everything in normal operating mode so that you have the hum.  Now start disconnecting things until the hum quits (or gets worse).  That should point you in the direction of the ground loop.  Alternatively, you could start grounding things to see if that works.  You might want to start with unplugging the laptop power supply (i.e. run off internal battery).
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William Boutwell

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I had a similar problem with my 6700 and Bose speakers and found two issues: 1) the phono jack on the 6700 has very weak connections and the plug on the audio cable slips out VERY easily; 2) the phono jack on my Bose speakers (different model) is inset a little and phono plug doesn’t go in far enough to be fully seated. I had to trim some of the plastic/rubber of the edge of the connector to get it fully seated. Of course there could be other issues. Good luck.
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Tom

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True for weak connector on Flex, but I have hum even when Flex is not connected to the Bose, just running in the same room.
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Jim Gilliam

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You might try using the computer speaker and see if the hum goes away.
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Tom

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When PC only is connected to the Bose there is no ham providing Flex is not running. If Flex is running hum is in the speakers even if they are not connected to the Flex.

If speakers are wired to the Flex, hum is only stronger.
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Steve Sterling

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as said before, sounds like a ground loop.
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Tom

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It is almost 200 mV difference between Flex chassis and - connector at power supply. 
It seems a lot to me.