Stupid but known error in KX2: ATU stopped working - transmit power halved on bypass

At the NA QSO Party on 05 November everything was still in order.
When I wanted to activate HB/BE-012 on November 8, I suddenly got a shock. The ATU of the KX2 did not adapt the antenna (the vertical with a radial) any more. The SWR was 24:1 on 40m. I did not get an adjustment with the ATU in any band. When I switched to bypass I got at least on 20m a just acceptable SWR.

The next summits on this day I made then with the tuned Endfed in the bypass of the ATU. But the power of the KX 2 was just 5 watts on 40m. At 10m it was only 1.5 watts.

The german Elecraft importer didn’t know any advice for me. He also discontinued his operation and referred me to the Elecraft representation in Bologna.

I was quite desperate and had no idea except that it was probably the drizzle at the end of the NA QSO party that caused the damage.

At some point I came across information on the internet that this type of fault was already known and that Walt @KK4DF had already repaired it. I contacted Walt and he gave me the right hint … and also the tip where I could get the spare parts. MANY THANKS FOR THIS!

The check actually revealed the U22 driver as the source of the error. Unfortunately it sits in such a way that one cannot measure directly at it. But you can get to R61 and R64 and so you have the comparison.

U22 is convenient for desoldering - directly at the edge.

I ordered the spare part here. It was in the mailbox in 3 days.

Everything is ok again and the next Summits can be activated with it.

73 Armin


Thanks Armin for the hint.

So this time one would have suspected the diodes D10.1 and 10.2 in the SWR bridge for nothing, hi.
Further confirmation that the surge protection elements (GDT1) at the input of the KXAT2 and KX2 are appropriately dimensioned.



Do you know what caused the failure? Was it moisture getting into the ATU or was it something else?

I’m not entirely sure… but I have had the device configuration in operation many times. What was different was the humidity… I was in the cloud and it had a fine drizzle.
At home I had opened everything and spread it out to dry… but it was probably already too late.
73 Armin

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Moisture as the cause of precisely this component defect in precisely this function on the RF board of the KX2 (not on the KXAT2 board) seems very unlikely to me - but I’m not related to Murphy after all, hi.

No, there are KX2 users who have been able to work regularly for hours “in the rain” and in very heavy fog for hours without any problems.

I encountered a slightly different, also very annoying problem with a colleague’s KX2:

  • Symptom: The KXAT2 sporadically does not complete the matching process and the KX2 no longer emits any transmission power.
  • The very simple reason for this … no, not moisture here either, hi … the plug of the RF connection cable to the BNC antenna socket was only about half plugged in and was crooked. Probably accidentally touched. So a small cause with annoying effects.


Hello Heinz

I also immediately suspected these connections! The to and from the ATU… the connections are so loose! I have bent the pins a bit, but I’m not happy with it yet.

73 Armin

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Kudos to Walt, KK4DF!

Andy, N4LAG

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I get really paranoid about moisture getting into the KX2. During winter, when I operate below zero, I always let the KX2 come back up to room temperature in its container before I open the container and expose the KX2 to the humidity in my flat. I have heard that water can condense on very cold electronics when they are brought suddenly into a warm humid environment. Maybe I’m just overcautious.
73 de Matt

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Hi all
High humidity is usually not a problem for electronics. Condensation, on the other hand, is a real problem and can lead to increased failure rates, but only when the electronic is in operation. Storage only is not a big problem. Ion migration (electrochemical migration) can occur when condensed air is present on an electronic circuit which is in operation. This can lead to short circuits or/and failure of components.
If you have packed your rig dry and moisture (e.g. from rain) is added later during operation, this is not a problem. Because of the power dissipation the device is always warmer than the environment, there is no condensation inside the device. A simple advice is to pack the device dry, and when operating in a humid environment (a rainy summit …) always leave your rig in operation.
73 de Arthur


Anyone know if Elecraft are planning a refresh of the KX2 and KX3? I’ve been surprised that the design has remained without update for so long, when even some simple changes (such as increasing the audio output of the KX3, or replacing the AA battery tray with a rechargable battery) would make a big difference to operators.



Not sure about a KX2 or KX3 refresh, but there is a fairly recent KX4 Dreams and Visions thread in the Elecraft KX User Group, and some comments from Wayne:

Nothing is in the works on a KX4 at present other than listening to customers, gathering ideas, and enthusiastic “what-if” sessions in the engineering lounge.

Support for the KX2 and KX3 is ongoing, including recent firmware updates for both. These updates are now at production status. The KX2 will also soon have an internal battery charger option (sorry for the delay).

We’re also experimenting with a variety of new accessories that will work with both rigs.

I doubt that unless you have a much higher tx-to-rx ratio than I do.

A few weeks ago, I participated in the Transatlantic S2S event in dense fog and heavy rain. My KX2 was inside a repurposed see-thru toilet bag with the bag lying on its side. I had the zip open to let the antenna feeder and headphones cable out and also so I could key the twin paddles on the KX2.

In no time, rain was on and running off the toilet bag but the rig remained dry inside. However, after half an hour of operating I noticed several patches of condensation on the inside of the KX2 display panel (so I decided it was time to quit) yet the outside of the KX2 case remained dry.

I think the condensation was due to high humidity rather than direct contact with rainwater. I doubt the heat generated from components would be enough to prevent water vapour from condensing in the low ambient temperature at the summit.


I don’t know why people think the next KX device will be a four. I saw the case for the next one more than a year ago, and it’s smaller, not bigger, than either the 2 or the 3. I got the strong impression that it won’t be called a KX4.

Elliott, K6EL
Sota MT


I wrote to Elecraft asking them for a KX0. It would be the ultimate in miniaturisation; maybe just 2 bands, no tuner, CW only. Like a competitor to the MTR3b, but even smaller. I guess that, whilst these radios are still popular, they don’t sell in large enough quantities for Elecraft. I’ll take whatever they’ve got in the works.
73 de OE6FEG


I’d love a self build Elecraft kit but I know that that would never happen these days. Almost all the modern devices are SMD and there’s no way that Elecraft would want to entertain all of the building woes that would inevitably result! I was too late to the party for all the cool rigs like KX1 and KD1JV ATS-4.

I would definitely be interested in a commercial CW only rig that was multiband and small. But again, I can’t see that happening, even though I suspect it would be popular with SOTA activators, the market is probably just around several hundred units at most, at a guess.

73, Colin

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If they offered the KX1 again I’d buy it. However, I envisaged the KX0 as a factory built rig where Elecraft used their manufacturing capabilities to achieve maximum miniaturisation. That said, Elecraft clearly understand that small size is part of the commodity. Whilst other radios are interesting, nothing is quite as small and well rounded as the KX2.
73 Matt


I think the KX2 is generally the best for me… only it should have the robustness of a Discovery TX-500… :wink:

A self-build with SMD would also be nothing for me. With the very small parts I can no longer cope because of the eyes. I have a special glasses made … and with the illuminated magnifier I can work… more or less…
I was glad that the U22 was a larger SMD component, and that it was so easy to get to.

I also often think that I am too late for many interesting devices… KX1, Mountain Topper,… and I keep looking at the 2nd hand market.

73 Armin

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Not if it is really really shiny :wink:


I had a similar problem to what was described by HB9BCB, where the connection between the transmitter and the Antenna Tuner Unit had come unplugged. Plugged it back in an all worked fine.
Frank K8FB


Thank you for this comment @HB9BCB ! My KX2 atu also stopped working & I checked this cable and found that the other plug got loose. I don’t have to cancel my weekend activation anymore.
73 Daro