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Author Topic: Hum on AM transmit  (Read 624 times)
KD4AC
Member

Posts: 81




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« on: May 28, 2007, 11:08:22 AM »

I don't know if this is the proper forum for this or not.  I noticed the other day that my transmitted AM carrier on my 746 has an AC hum.  It gets louder and softer as I touch various pieces of equipment (i.e. SteppIR controller, power meter, power supply or RigBlaster).  I thought maybe the noise was caused by the connection between my PC, RigBlaster and radio.  I disconnected everything from the radio with to no avail.  The radio has a DC ground from the chassis to the ground on a power outlet, as does the power supply.  Disconnecting them makes no difference.  I had the power supply plugged into a UPS that the PC is also plugged in to.  I unplugged it and put it in a plug on the wall...also with no effect.  I suspect that the noise is coming from the power supply but I can't think of a way to test that without buying a battery to use as another power source.  I'd also like to find out if it's just a problem with AM or if the hum is on all modes.  And if it is the power supply, which, by the way, is a Diamond GZV4000, how can I get rid of the noise?
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K7KBN
Member

Posts: 2805




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« Reply #1 on: May 28, 2007, 12:35:20 PM »

Maybe a poor ground connection inside the microphone or one of the other items in the audio chain.  Maybe a bit of RF in the shack (common mode)...hard to tell.

Shouldn't be difficult to tell if the hum is present in other modes: operate on the other modes and have someone listen.
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73
Pat K7KBN
CWO4 USNR Ret.
KE3WD
Member

Posts: 5689




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« Reply #2 on: June 10, 2007, 07:25:05 PM »

>>The radio has a DC ground from the chassis to the ground on a power outlet, as does the power supply.

Quite likely an AC Ground Loop.  

If rig and PS are at two different ground potentials, the (very) slight difference is not so slight at mic signal levels.  What you are hearing is quite possibly the zero voltage point of the Audio Freq mic preamp changing in time to the AC frequency.    

Try putting a two-prong adaptor between PS AC plug and wall to break the loop.  If the hum goes away, you have found the problem.  BUT this should only be a TEMPORARY test to isolate the problem, you need that AC safety ground for protection.  

If the hum does go away for the short test, time to rethink that grounding setup.  Using the third prong of the AC for your RF ground is nasty business and can be a cause of TVI/RFI problems, too.  

Websearch how to strap all your gear to a common grounding point at the back of the operating table, like a piece of copper plate or 1/2" or better copper pipe piece, then connect that to a good solid earth ground that is not run through the AC wiring.  Keep the powered gear on the three prong safety ground, though.  

HTH and 73,

.


 
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