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Flex 6400 problems with Elecraft KAT500

Phil N3ZP
Phil N3ZP Member ✭✭

I have been using my Flex 6400 with an Elecraft KAT500 tuner and KPA500 amp. I have all the cables connected as per the KAT500 manual. I have been using GAP 20 Meter Mono and a Zero-Five 43 Ft. vertical Everything has worked fine for about 6 months but recently I get a Fault on the tuner and sometimes the amp also on certain bands even with a dummy load or on the GAP on 10 or 17 antenna with only a 2.3 SWR on some bands. I even converted my Zero-Five to a 33 Ft 40 meter vertical and still have a problem. When it happens it even has a Fault when in Bypass. When I get a fault it's after seeing low SWR on the meter in my Maestro on Tune but then goes high on transmit. I have checked or replaced all my coax cables and don't believe their is problem with my setup.

I have been in contact with Elecraft support but not getting any answers yet. Apparently due to stay-at-home orders in CA their repair activity is limited.

I did notice after a Fault, that with the USB able from the KAT500 to the Flex the high SWR problem is gone in Bypass. I am wondering if the communication between the Flex and tuner is giving the tuner a frequency change on transmit. I have not had any RF in the shack issues and everything is well grounded and bonded as per the N0AX book.

Has anyone else using this setup experienced anything like this? Right now I am very disappointed with Elecraft and wonder if I should have saved money and got an LDG AT-100Pro II.

Any comments appreciated.

Phil N3ZP


  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Phil, I have the same setup, and have not had any of these problems. One thing to check - do you have the internal tuner installed in your Flex? If so, try disabling it and see if that makes a difference. If that is not it, try taking the KAT 500 physically out of the circuit (not just in bypass mode) so that the amp only sees a dummy load or a known resonant antenna. That should help determine if the amp or tuner is having the issue. You might also try different antenna ports on the tuner to see if there is an issue there. You could also try the KAT 500 with the amp out of the circuit (again, not just in standby mode) to see how it behaves by itself. The goal is to isolate the gear that is causing the problem. All of this assumes that the feedlines, connectors antennas, and dummy loads are good.

    All the best in troubleshooting this issue.


    Len, KD0RC

  • Phil N3ZP
    Phil N3ZP Member ✭✭

    Hi Len,

    Thanks for your comments, I don't have an internal tuner so no issue there. I have been using the KPA500 connected directly to the Flex, leaving the tuner out of the circuit and have no problems where SWR is below 2:1. I even got some new cables and used a good dummy load and still see the problem. Early on I did swap antenna ports on the tuner with no change. I haven't really tried the tuner without the amp, primarily because I use the amp almost all the time and just reduce the input power as needed. I have the Flex power maxed out at 22 Watts so as not to accidentally overdrive the KPA500. I may try the tuner with my IC-7300 whichI have as a backup rig. The big difference there will be the lower power. I should probably also connect the tuner to the Flex without the amp in the circuit.


    Phil N3ZP

  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin

    FYI, SInce this post is related to using the FLEX-6000 with third-party hardware, I am moving this post the the Station Integration category where it will have higher visibility in the Community. The FLEX-6000 category is for hardware discussion or questions directly related to radio hardware itself.

  • Phil N3ZP
    Phil N3ZP Member ✭✭

    OK thanks Tim.

  • Patrick
    Patrick Member ✭✭✭

    Maybe I missed it, but have you tested into a good dummy load? First at the output of the radio and then at the end of the coax at the tuner output, then at the antenna end. Breaking up the problem into defined parts can help diagnose the problem. If there is a problem with either of the Elecraft devices, be assured that the Elecraft people will work with you on the problem if it is in one of the two devices. I also own the amp and tuner set, and had a similar problem caused by a bad component in the amp t/r switch.

  • Roger Hartel
    Roger Hartel Member ✭✭

    I have had a similar issue with remote ops with a 6600, a Maestro, and an LDG RT600 or a RT100. I use a Carolina Windom Short 40 antenna. The antenna is not resonant on 10, but the tuner will achieve an SWR of 1.4 to one. I tune at 5 watts, transmit at 100 watts. When transmit the tuner will start re tuning, and then will fault to a protection mode. Then it won't tune anything until I recycle power to the tuner.

    This phenomena is discussed in the RT600 manual, also on (SGC's) web site and maybe the Elecraft manual. So both LDG and SGC speak of the issue. LDG has a way to prevent re tuning, it is not convenient from 1200 miles away. Recycling power 3 times after tuning prevents re tuning, then manually initiating a tuning cycle takes the tuner out of that mode.

    So I am working on a remote way to control the tuner with web relays.

  • Phil N3ZP
    Phil N3ZP Member ✭✭

    Thanks for the replies. I have done additional testing with a dummy load and new cables still get a Fault with the tuner on all bands. I have been in contact with Elecraft support but still waiting for a response. I hope to be able to send it in for service soon.

  • Patrick
    Patrick Member ✭✭✭

    Phil, I like pass along a piece of advice, but not knowing how you set up the KAP & KAT 500’s. My set up was to set it up with the tx1 signal going to the KAT first then to the amp. I will assume you did that. This makes sure that the amp is not triggered while tuner is tuning. Also all control signals come from the Flex via usb to both the kap and the KAT. But the one thing I missed that eventually Brock the amp t/r switch was a setting in the amp. To protect it from inadvertent triggering at high power from the flex. I wish I could tell you what the setting is, but can not remember, it has been a while. But Flex can enlighten you on this setting. Good luck! It was a great amp, tuner combination.

  • Phil N3ZP
    Phil N3ZP Member ✭✭

    Hi Patrick. Thanks for your comment. I have the cables connected to the amp and tuner as per the KAT500 diagram for Flex 6XXX. Not sure about any settings other than the ones described in the Elecraft manuals. I do have a USB cable from both the tune and amp going to the Flex 6400 and the Flex recognizes them. When I tune, I have the amp in Standby and use Tune on the Flex at about 10 watts before I put the amp in Operate. When I go to Operate and transmit the tuner goes to Fault. Elecraft told me to tune at 25 watts but I have my Flex power set at 22 Watts max to not overdrive the KPA500. I did try a higher setting but didn't change anything. I was finally able to get a return authorization to send it back. Hopefully Elecraft will find a problem and fix it.

  • Harold Rosee
    Harold Rosee Member ✭✭


    When I first set up my KAT500 I put it in auto and then tuned it on all bands every 30 khz or so. That put all in memory for all bands. Then I run the tuner in manual mode. That way it switches to the correct memory but will not try and retune like some tuners do when they operate with high power. If I ever go to a new band I have worked I put it in auto to get the memories set and then go back to manual and leave it there.

    Once you set all the memories there is no need to tune. If for some reason something has got wrong in you system causing a high SWR the like from the tuner to the amp will shut the amp down.

    Hope this helps.



  • W1IZZ
    W1IZZ Member
    edited August 2020
    Phil--I used the KPA/KAT500 combo with a 6500 for about 4 years. The one time I had issues, I cleared the KAT500 tuning memories, re-trained it on all bands by tuning in auto every 50 KHz, returned it to manual, and it was then fine. I'm sure you trained the tuner in auto and then ran in in manual for normal operation?

    I found the KAT500 to be an excellent tuner, capable of tuning my ladder line doublet on all bands. I'm finding the same with the KPA1500's built-in tuner.

    Larry, W1IZZ
  • Phil N3ZP
    Phil N3ZP Member ✭✭

    Yes I did train the tuner also. It would be OK on one part of the band but I would switch from the phone part of a band to CW it would fault. It's been sent back to Elecraft. Hopefully they'll find a problem and I'll get it back in place in a few weeks.

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