Flex 6700 humming issue

Tom

KD0RC

William Boutwell

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.

Tom

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.

Tom

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.

Jim Gilliam

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

Tom

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.

KD0RC

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

Tom

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.

Steve Sterling

as said before, sounds like a ground loop.

Tom

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

KD0RC

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).

Patrick

Install audio isolation transformer on output of the radio,  had the same problem, feeding powered speakers.  Ground loop through the AC.  Now super quiet, no hum or noise.

Stephen Thomason

Pat, what audio isolation transformer did you use and from where did you source it?

Ian1