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K3 KAT3 High SWR Issue Solved

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Ed G

K3 KAT3 High SWR Issue Solved

I recently posted about a K3 40 meter SWR anomaly, where the K3 SWR reading on the front panel meter tended to increase as I tuned up higher in the band. I first observed the problem while using my Steppir yagi, which retunes itself so that I should have seen a low SWR across the entire 40 meter band. I saw a 3:1 SWR on the K3 meter at the high end of 40 meters, while at the same time I was reading a much lower SWR on my inline LP-100A. Further investigation showed that this situation also occurred when transmitting into a known-good dummy load. I was seeing about 1.1:1 at 7.001 MHz, and this rose to 3:1 at 7.299 MHz. When the high SWR was being sensed by the K3, output power was also being folded back. For example, I would have about 36 watts output when the K3 was set to 50 watts at 7.299 MHz. This anomaly also affected 40 meter operation with the KPA1500, as I observed what appeared to be power overshoot and experienced the need to frequently readjust drive power on 40 meters.
A fair amount of digging in the email archives found a couple posts which touched on an issue with the 8.215 MHz trap (C10 and L10) on the KAT3 tuner board. Keith WE6R at Elecraft then suggested removing C10 and L10, and adding a shorting wire in place of L10. This mod does indeed appear to fix the false SWR issue – at least it did in my K3. I now have flat 1.1:1 SWR readings into a dummy load on 40 meters at all power levels.
The trap is not included in the design of the KAT3A, so K3S rigs with the KAT3A should not see this SWR problem. Any K3 users who have retrofitted the KAT3A tuner into their K3 should not see this anomaly. The problematic trap, from comments by Keith, may be due to some lots of capacitors which for reasons unknown to me have a high failure rate. It may be prudent for K3 owners with the KAT3 tuner board (S/Ns under 10,000 or vicinity) to check SWR readings on 40 meters using a dummy load to ascertain whether they need to apply the KAT3 trap fix.
How to check your K3: Although I do not know how many KAT3 tuners are affected by the trap issue, I do feel it is the kind of problem that some users may have but just have not noticed. It is fairly easy to check for; just connect the K3 to a dummy load and observe the SWR as shown on the K3 meter at several random frequencies across the 40 meter band. Of course (assuming a stable 50 ohm dummy load) you should see a fairly flat 1:1 or near to it. But if the SWR rises as you approach 7.3 MHz, and you are using the older KAT3 tuner board, you most likely have the trap problem. Upon visual inspection, you may see evidence of burning on the trap components. Although my trap components looked fine, I apparently had some internal failure in C10. The faulty trap may also exist in the older KANT3 module (the trap parts would have different part designations) but I don’t have a KANT3 and have not researched this enough to comment further.
Trap fix: As Keith suggested, remove C10 and L10 on the KAT3 board, and install a jumper where L10 was. I cut out C10, and also cut the leads on L10 while leaving enough length to strip some enamel coating and solder the leads together, thereby creating the jumper. To remove the KAT3 board, use proper wrist strap procedures. First remove the K3 top cover and one rear panel corner screw near the ANT connectors on the rear panel, and unplug the coax from ANT1 and ANT 2 connectors going to the KAT3 board. The KAT3 board can then be carefully pulled out for the trap mod. I did not have to remove the subreceiver or the K3 side panel.
Results: For me, I now have a flat SWR reading across the 40 meter band when using the K3 internal SWR meter. Power output is now stable and fixed with no foldback, and I am not experiencing any KPA1500 drive faults.
Thanks to Keith at Elecraft for his assistance on this issue. Please correct me Keith if I have not documented this issue accurately.
--Ed, N3CW--

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K9MA

Re: K3 KAT3 High SWR Issue Solved

On 1/23/2019 09:41, Ed G wrote:
> A fair amount of digging in the email archives found a couple posts which touched on an issue with the 8.215 MHz trap (C10 and L10) on the KAT3 tuner board.

Although my older K3 doesn't seem to have a problem, I wonder why that
trap was not required in the KAT3A. It was there, presumably to improve
IF rejection, so I'd expect removing it without some other change to the
K3 would be unwise.

73,

Scott K9MA

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Scott K9MA

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Don Wilhelm

Re: K3 KAT3 High SWR Issue Solved

Scott,

It was added to the initial K3s, but later found not to be necessary,
and if the trap broke down, it would cause trouble. So the KAR3A does
not have it.

73,
Don W3FPR

On 1/23/2019 10:49 AM, K9MA wrote:

> On 1/23/2019 09:41, Ed G wrote:
>> A fair amount of digging in the email archives found a couple posts
>> which touched on an issue with the 8.215 MHz trap (C10 and L10) on the
>> KAT3 tuner board.
>
> Although my older K3 doesn't seem to have a problem, I wonder why that
> trap was not required in the KAT3A. It was there, presumably to improve
> IF rejection, so I'd expect removing it without some other change to the
> K3 would be unwise.
>

Mark Musick

Re: K3 KAT3 High SWR Issue Solved

Don,
Is the trap in all KAT3s up through those installed in K3s with 9000 or just the early K3s below some serial number?

Mark, WB9CIF

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Don Wilhelm
Sent: Wednesday, January 23, 2019 18:48
To: K9MA <[hidden email]>; [hidden email]
Subject: Re: [Elecraft] K3 KAT3 High SWR Issue Solved

Scott,

It was added to the initial K3s, but later found not to be necessary, and if the trap broke down, it would cause trouble. So the KAR3A does not have it.

73,
Don W3FPR

On 1/23/2019 10:49 AM, K9MA wrote:

> On 1/23/2019 09:41, Ed G wrote:
>> A fair amount of digging in the email archives found a couple posts
>> which touched on an issue with the 8.215 MHz trap (C10 and L10) on
>> the
>> KAT3 tuner board.
>
> Although my older K3 doesn't seem to have a problem, I wonder why that
> trap was not required in the KAT3A. It was there, presumably to
> improve IF rejection, so I'd expect removing it without some other
> change to the
> K3 would be unwise.
>

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Don Wilhelm

Re: K3 KAT3 High SWR Issue Solved

Mark,

I am not certain when they stopped installing the trap in the KAT3, but
I know it is not in the KAT3A.

If you pull your KAT3 out for any reason, eliminate the trap.

73,
Don W3FPR

On 1/23/2019 2:07 PM, Mark Musick wrote:

> Don,
> Is the trap in all KAT3s up through those installed in K3s with 9000 or just the early K3s below some serial number?
>
> Mark, WB9CIF
>
> -----Original Message-----
> From: [hidden email] <[hidden email]> On Behalf Of Don Wilhelm
> Sent: Wednesday, January 23, 2019 18:48
> To: K9MA <[hidden email]>; [hidden email]
> Subject: Re: [Elecraft] K3 KAT3 High SWR Issue Solved
>
> Scott,
>
> It was added to the initial K3s, but later found not to be necessary, and if the trap broke down, it would cause trouble. So the KAR3A does not have it.
>
> 73,
> Don W3FPR
>
> On 1/23/2019 10:49 AM, K9MA wrote:
>> On 1/23/2019 09:41, Ed G wrote:
>>> A fair amount of digging in the email archives found a couple posts
>>> which touched on an issue with the 8.215 MHz trap (C10 and L10) on
>>> the
>>> KAT3 tuner board.
>> Although my older K3 doesn't seem to have a problem, I wonder why that
>> trap was not required in the KAT3A. It was there, presumably to
>> improve IF rejection, so I'd expect removing it without some other
>> change to the
>> K3 would be unwise.
>>
> ______________________________________________________________
> Elecraft mailing list
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> Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email]
>

KeithWE6R

Re: K3 KAT3 High SWR Issue Solved

Administrator

Here is the history on the 8.215 trap on the KAT3 KANT3 boards.
Overview; There is an 8.215 trap on the RF board, the trap on the
KAT/KANT3 boards was an additional trap that was later found to be way
overkill.
On the KAT3 ATU, it is on the antenna side of the tuner, so it is
subject to high SWR and can heat and burn.

On the KANT3 you cannot cannot run with high SWR or the radio will back
off, so not a problem on these boards.

Early ATUs had a small coil L10 and single pink/brown capacitor C10.
The cap can fail internally invisibly or catastrophically!
The coil can get burnt.

We went to a beefier coil and 2 small blue capacitors around 2010.
It is rare, but these beefier components can also fail.

_**In repairs,_ if it has the older weak components, we just pull the
coil and insert jumper (no need to pull the capacitor as it is
effectively now shorted).
If it has the newer beefier components, we leave it alone unless burnt.

The new KAT3"A" does not even have this trap, it came out with the K3"S".
Keith WE6R

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