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Author Topic: RFI in FT-920 earphone circuit?  (Read 509 times)
K6RQR
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Posts: 367




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« on: November 29, 2017, 05:37:23 PM »

Hello All -
 I'm using a Yaesu FT-920 on 80 -15 meters. I had a doublet antenna which was tuned with a Dentron Super Tuner. When I got the FT-920 I decided to switch over to an Off-Center Fed dipole (90 feet by 45 feet) that was tuned by the internal tuner of the FT-920. I use about 35 feet of RG-8X coax that connects to a 4:1 current balun at the feedpoint. The internal The tuner works well and I get low SWR on all bands from 80-15 meters. I like to use a pair of stereo headphones that I have with my TV to hear some of the weaker signals better and here is where the problem arises. On 80 and 40 there is a bit of what sounds like some audio distortion in the headphones. It gets worse in the upper bands. When I try and transmit on 17 and 15 it is really bad in the headphones. I tried another pair of stereo headphones with possibly better shielding in the cord and it was the same. It sounds like the distortion is coming from the rig, in the headphone circuit. Would a pair of regular communications headphones be better for this?
 Has anybody experienced this problem? I'm wondering if it would be better to go back to the original doublet antenna fed with 450 ohm twinlead and a balanced antenna tuner? Any suggestions appreciated.

73,
Bruce  K6RQR
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VK6HP
Member

Posts: 181




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« Reply #1 on: November 29, 2017, 08:39:52 PM »

I'm not clear whether you are actively monitoring your transmitter output via an inbuilt monitoring facility, or whether you're simply leaving the headphones on your head when transmitting normally.

Either way, I would check very carefully that the transmitted audio was not actually distorted at various drive levels, perhaps by listening on an independent receiver, suitably terminated and with the RF gain throttled well back. It could indeed be RF feedback, via a number of possible mechanisms.  For example, it could be "RF in the shack" from common mode currents on the OCFD coax, or pickup simply by virtue of the antenna proximity.  If you have a common mode issue, it will also likely be degrading your receive performance with excess local noise pickup as well.

If your transmitted audio is OK, I guess your problem could be due to RF pickup via the headphones/audio amp.  Are the headphones the normal passive type, or are they fancier with inbuilt electronics? Have you had a look around the owners' forums for your radio to see if there are similar reports?  

With good choking techniques, it ought to be possible to suppress common mode effects sufficiently but, nevertheless, I'm a fan of symmetrical antennas, despite the fact that common mode current can still occur in particular installations. In your OCFD arrangement, I would look closely at the grounding of the coax where the feeder enters the shack and, if improvements there don't help, try a second in-line current choke (1:1), around the entry point as a starter.

If you are confident you don't have unreasonable levels of RF in the shack, and that the pickup is indeed due to the headset alone, you could try making an in-line choke by winding the cable a few times through a small HF toroid.  With a passive headset, you'd normally put the choke at the transceiver end. I'm assuming your radio already has some internal RF bypassing on the headphone output but there'd be no harm in adding some additional bypass capacitors, perhaps mounted in a metal 6.5mm (or whatever) headphone plug assembly.
« Last Edit: November 29, 2017, 08:48:35 PM by VK6HP » Logged
K6RQR
Member

Posts: 367




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« Reply #2 on: November 30, 2017, 09:26:22 AM »

Hello -
 Thank you for the excellent answer. Yes, the transmitted audio is OK as related by some local hams. I suspect that my changeover to the OCF dipole is what is doing this and I plan to go back to a balanced antenna like I had before. I did try the chokes on the headphone's cord and it  didn't work. This isn't a problem as I discovered that the FT-920 I just  bought has been modded so I can operate on 60 meters. I am now even more inclined to go back to a doublet tuned by a good balanced tuner as the OCF dipole will not operate on 60. Thanks again.

73,
Bruce  K6RQR
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WA3SKN
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Posts: 6517




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« Reply #3 on: November 30, 2017, 04:21:26 PM »

1. You have "RF in the shack".
2. Your stereo headphones are not handling it well.

Off center fed antennas are prone to this, and stereo headphones are usually prone to RF problems.
Regular comm phones are better shielded, but I doubt this would help.  And going back to a dipole probably would help.
Getting your antenna up , in the clear, and away from the shack would definately help.  Dipoles and balanced line are a great asset here!
Good luck with the project.  73s.

-Mike.
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K7KBN
Member

Posts: 3478




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« Reply #4 on: December 01, 2017, 07:11:00 AM »

Communications headphones generally have a higher impedance than stereo phones.  If you can find older military-style cans, they work fine and might help your situation.  You might have to turn the volume control a bit, but that's no problem.
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73
Pat K7KBN
CWO4 USNR Ret.
K6RQR
Member

Posts: 367




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« Reply #5 on: December 01, 2017, 06:12:49 PM »

Thanks for the answers. I just bought a tuner capable of balanced loads and I will replace the OCF dipole this weekend. I'm looking for a pair of communications type headphones.
 as well
73,
Bruce  K6RQR
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