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Author Topic: Headset RFI again  (Read 5252 times)
VA3CP
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Posts: 8




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« on: February 05, 2017, 05:08:14 PM »

HI all,

I have read the posts by others regarding RFI into headsets.  I am running into a problem here with a Yamaha CM-500 on a Kenwood TS-570s.

I have been using a pair of Koss HV1A headphones and never experienced any problem. I wanted to try a headset with the integrated microphone so bought the CM-500's.  Whenever I transmit, I hear some RF noise in the ears but there has never been a report of RF on the transmit audio.  This occurs on all bands when the power is above 20 watts.  This occurs even when the headset microphone is not connected to the radio. I tried multiple RF chokes to no avail. One choke was near the headphone jack, and another right at the top end of the cord.   Even wrapped the whole headset cord in aluminum foil and grounded it to the radio chassis, no fix. I get the RF in the ears on three different antennas.  I have tried a different power supply, removed all peripheral equipment from the radio and still have the problem. 

Borrowed an Icom 706MKIIG and there is no RF on any band, any power level using my Kenwood PS-50 or an Astron PS-35. This points to RF coming out of the TS-570s but if that were the case, why is there no problem with the Koss headphones. 

Any thoughts would be appreciated. 

Dave, VA3CP
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W9IQ
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Posts: 3032




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« Reply #1 on: February 05, 2017, 06:45:55 PM »

The CM-500 earphones are higher impedance (120 ohms) than a normal speaker or earphones. You may wish to experiment with swamping the higher impedance by paralleling some resistance with the headphones. Values in the 8-84 ohm range should be tried.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
VA3CP
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Posts: 8




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« Reply #2 on: February 05, 2017, 09:21:23 PM »

Glen,

I need to try some resistors with a lower value.  I cobbled up some 86 ohm resistors and it does knock the RF into the headset down quite a bit. 

Will let you know what happens with the lower value resistors. 

Dave, VA3CP
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VA3CP
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Posts: 8




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« Reply #3 on: February 06, 2017, 08:00:22 AM »

Glen,

More experimenting this morning.  I added some more 82 ohm resistors ( I typed 86 last night ) in parallel with each other to get the resister value down to 41 ohms.  This reduced the RF in the headset a little bit more.  Then I added another pair of paralleled 82 ohm resistors to bring the value down to 20 ohms each side.  I did not see any improvement from the 41 ohm setup. 

The "resistive" reading on each side of the headset is now at 17 ohms, down from 114 ohms.  If course, that is not the actual impedance. 

One thing to note, the RF feedback in the headset is not affected by the volume control on the radio. It is slightly less on 80m than on 10m. I am not sure if I will gain anything more by lowering the parallel resistor values below what I have now.  However, the recommended headphone impedance is from 4-32 ohms.  I should be somewhere within that range now. 

Sure would like to eliminate all of the feedback but I suppose I could live with what I have now. It is still somewhat distracting.

Dave, VA3CP
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KD0REQ
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« Reply #4 on: February 06, 2017, 09:39:50 AM »

you could also put the fabled magic of a .001 disc cap across the headphone leads.  I hear they even cure arthritis if you solder a ring of them (short leads, please) into a bracelet.  don't burn your wrist with the last connection.
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VA3CP
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« Reply #5 on: February 06, 2017, 11:25:18 AM »

I had thought of trying the bypass caps but I don't have any in stock.  Means a trip to the electronics store again probably this weekend. 

Thanks....Dave
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WB4SPT
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« Reply #6 on: February 07, 2017, 12:36:49 PM »

Does everything work well when using a dummy load?   
I had a terrible time trying the get a couple of aviation headsets to work.   That's when I discovered I needed a balun at my dipole. 
Surprising you have this issue with all 3 antennas though. 
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KE2KB
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« Reply #7 on: February 07, 2017, 02:57:21 PM »

What's your ground situation? A poor RF ground will result in the kind of problems you are describing.
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WB4SPT
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« Reply #8 on: February 08, 2017, 10:38:33 AM »

Quote
What's your ground situation? A poor RF ground will result in the kind of problems you are describing.

I'm going to propose something else.  If the issue is RF currents at the shack in unwanted places (headset, etc), you need to stop the currents.  Either zero RF ground, a shield, or stop the currents from occurring in the first place.  The first two choices are near impossible.  The 3rd choice works.
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VA3CP
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« Reply #9 on: February 08, 2017, 03:55:28 PM »

I am going to answer the last three posts here at the same time. 

I still get the RF noise in the headset when using a dummy load.  That makes me think it is coming out of the rig. 
Used an RF sniffer and see a little on the headset lead but not nearly as much as I see when testing with a borrowed Icom 706MKIIG.
There is no RF noise in the ears when using the Icom radio. As mentioned before, I have disconnected every peripheral from the TS570s and tried a different  power supply, no change.   

I have a piece of #8 ground wire that is 4 feet long going to an 8 foot ground rod that goes through the basement floor. It has been there quite a while and I do not have a megger to test how good the ground is.  There is a possibility that it may not be a zero RF ground any more.

My OCFD antenna has a 4:1 balun.  My 40m 1/4 wave vertical has 6 ferrite cores on the coax at the feed point.  The R5 vertical has an RF choke built into the matching network.  This RF choke core was replaced last winter because water got into it and cracked it. 

I would dearly like to eliminate RF currents from occurring in the first place, but as stated, a very difficult task to accomplish.
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WB4SPT
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« Reply #10 on: February 08, 2017, 04:57:56 PM »

OK,  still happens with a dummy load.   Won't be talking any more about antennas and toroids, then.  Grin

This "RF noise" in the ears, would be worth elaborating.  Since ears obviously can't hear RF, what does it sound like?  hum?  does it change when you move around the cable?  What is the RF sniffer?   Any sensitive rx will pick up the Tx carrier in the same room.  

I honestly don't think much of ground rods in the basement.  Is this intended to be a lightning current sink?  

The CM500 is a "full range" headset, according to Yamaha.  THat means it will be good at picking up LF noise, like maybe there is a noise generator inside the Kenwood that is more active when the DC load is higher.  Do you have a scope to look at this signal on the headphone jack?  Whatever this is, it's not getting muted on Tx.
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VA3CP
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« Reply #11 on: February 08, 2017, 06:56:40 PM »

The noise that I am hearing in the headset is my voice similar to what it sounds like when you are getting into computer speakers or a home stereo but significantly subdued. Moving the cord around makes no difference.  If I turn off the monitored sidetone when operating CW, I can hear a thumping in the headset. In AM mode, at 25w, I can hear feedback.  A dead carrier does nothing. 

The RF sniffer that I am using is actually a field strength function on an SWR meter.  I have the antenna reduced in length so that the meter needle moves in the lowest 1/4 of the scale.

I rounded up a couple more headsets and interestingly enough, one of them reacts the same as the CM-500's. 

Looking at the schematic, it appears that the headset jack ring is the audio out, the ring and tip are the return and are grounded through a 120 ohm resistor when the headset is plugged in. 

Unfortunately, I do not have a working scope.

I have tried removing the indoor ground wire as part of my testing.  The ground wire would be 25 ft long if I were to run it to the ground rods outside at the base of my pipe tower. 

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WB4SPT
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« Reply #12 on: February 08, 2017, 07:35:07 PM »

All indications are that the rig is putting some noise on the headphone jack when you modulate the carrier.  One thing you could do is simply put an ac voltmeter on that port, with or without a headset, to measure that voltage.  A scope or audio spectrum analyzer would be great tools here to get an idea of frequency content.   
I tend to believe the radio has an issue, either design or an actual fault.    Is there no sign of this when using the speaker?
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VA3CP
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« Reply #13 on: February 08, 2017, 08:15:45 PM »

I put a DVM in AC mode on the headphone jack.  When I key the mic, the AC voltage drops to 0.000 and when I whistle into the mic, it goes to 0.002.  In CW mode, 100 watts, key down, it reads 0.014.

When using the external speaker, there is absolutely nothing heard.  The internal and external speakers are fed through the NC contacts of the headphone jack.

It is a possibility that it is a design fault or a rig fault. I can get a TS-590SG over here and test it on my setup but it may take up to a week to do that.   
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VA3CP
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« Reply #14 on: February 10, 2017, 07:29:49 AM »

I made my way to the electronics store and picked up some parts.  I have fabricated an adapter which has a 15 ohm resistor and a .047uF capacitor from each side of the headset audio feed line to the headset return line.  I used this calculator to determine the capacitor value based on using a 15 ohm resistor and a frequency of 3.5mhz. I could have used a .033uF or .039uF capacitor but had to take what was available.   
http://www.learningaboutelectronics.com/Articles/Bypass-capacitor-calculator.php

About 95% of the problem is now gone and I can live with that.  I know this is a band-aid solution and does not eliminate the cause of the problem.  I will continue to experiment and see if I can solve this one completely. 

Thanks to everyone who had input on this thread. It is really appreciated.     

Dave, VA3CP
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