K3 output stage high SWR - fault-finding?

[g7vjr]

Hi guys,

I have a problem with my K3 reading 5.0:1 or more SWR on tune into a 50 ohm load. I was alerted to this problem when I could not get the same SWR my analyser was showing for a new antenna, and quickly  discovered the K3 had developed a fault on all bands.

The K3 shows a high SWR into a pure 50 ohm load or resonant antenna on both Ant 1 or Ant 2. I find that if I use an external SWR meter and connect the K3 to a dummy load through that, the SWR shows high on both the K3 and the SWR meter. I have checked the dummy load and patch lead. So, at a guess, the SWR reading on the K3 is correct and there is something wrong in the output stage of my K3.

I have the ATU module and find this is able to tune back to 1.0:1, but I'm pretty sure that's treating the symptom rather than the cause!

Any suggestions or advice on fault-finding this would be most welcome; sadly, I'm off the air!

Many thanks,
Michael G7VJR

K3/100 no. 1904
KXV3
KAT3
KRX3
KDVR3

[Joe Subich, W4TV-4]

> The K3 shows a high SWR into a pure 50 ohm load or resonant antenna
> on both Ant 1 or Ant 2. I find that if I use an external SWR meter
> and connect the K3 to a dummy load through that, the SWR shows high
> on both the K3 and the SWR meter.

An external SWR meter can not show a fault with the transceiver - only
with those devices and connections further downstream.

 > I have checked the dummy load and patch lead.

How have you checked the dummy load and patch lead?  What convinces you
that they are "good"?

> I have the ATU module and find this is able to tune back to 1.0:1,
> but I'm pretty sure that's treating the symptom rather than the
> cause!

If the KAT3 is able to reach a SWR of 1:1 there is nothing wrong with
either the KAT3 or the K3/KPA3.  The function of any "tuner" is to
"treat the symptom rather than the cause".  To treat the cause one
would need to modify the antenna/load/lines to resolve the impedance
mismatch.

73,

    ... Joe, W4TV



On 4/7/2012 12:23 PM, g7vjr wrote:

> Hi guys,
>
> I have a problem with my K3 reading 5.0:1 or more SWR on tune into a 50 ohm
> load. I was alerted to this problem when I could not get the same SWR my
> analyser was showing for a new antenna, and quickly  discovered the K3 had
> developed a fault on all bands.
>
> The K3 shows a high SWR into a pure 50 ohm load or resonant antenna on both
> Ant 1 or Ant 2. I find that if I use an external SWR meter and connect the
> K3 to a dummy load through that, the SWR shows high on both the K3 and the
> SWR meter. I have checked the dummy load and patch lead. So, at a guess, the
> SWR reading on the K3 is correct and there is something wrong in the output
> stage of my K3.
>
> I have the ATU module and find this is able to tune back to 1.0:1, but I'm
> pretty sure that's treating the symptom rather than the cause!
>
> Any suggestions or advice on fault-finding this would be most welcome;
> sadly, I'm off the air!
>
> Many thanks,
> Michael G7VJR
>
> K3/100 no. 1904
> KXV3
> KAT3
> KRX3
> KDVR3
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K3-output-stage-high-SWR-fault-finding-tp7446109p7446109.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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[Bruce Beford-2]

Michael,
Make sure the K3 ATU is in bypass mode, and check the SWR into your dummy
load. It sounds to me as if you have the ATU in line when you are making
this measurement.
GL,
Bruce, N1RX

> The K3 shows a high SWR into a pure 50 ohm load or resonant antenna on
> both Ant 1 or Ant 2.


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[g7vjr]

Hi Joe, Bruce,

> Make sure the K3 ATU is in bypass mode

Thanks for the tip. I had the same doubts but double checked for this. The ATU is bypassed.

> How have you checked the dummy load and patch lead?  What convinces you
> that they are "good"?

The usual stuff - continuity checks on the patch lead and a digital multimeter looking for 50 ohms on the dummy load.

> If the KAT3 is able to reach a SWR of 1:1 there is nothing wrong with
> either the KAT3 or the K3/KPA3.

Thanks Joe. I am exploring the K3 main board in a bit more detail and think I have a smoking gun; I can see that T4 (hidden under P70 to the ATU board) has been scorched. The laminations have burned away on the windings and I can see a burn on the primary turn inside. :-(

73
Michael G7VJR

[Bruce Beford-2]

The nose knows... follow the smoke.
Glad you found something.
GL with the repair.
-Bruce, N1RX

> I can see that T4 (hidden under P70 to the ATU board)
> has been scorched. The laminations have burned away on the windings and I
> can see a burn on the primary turn inside. :-(


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[Joe Subich, W4TV-4]

Michael,

> Thanks Joe. I am exploring the K3 main board in a bit more detail
> and think I have a smoking gun; I can see that T4 (hidden under P70
> to the ATU board) has been scorched. The laminations have burned away
> on the windings and I can see a burn on the primary turn inside. :-(

T4 looks to be the voltage transformer in the internal SWR bridge.  If
the SWR bridge is bad, it will obviously lie to you (and the K3's CPU).

However, the outboard SWR meter should show a 1:1 or very close to it
into the "correct" dummy load *regardless* of what the K3's SWR meter
is showing.  The K3 will reduce power output in response to the "high"
SWR that it sees due to the bad bridge ... and running full power with
a mistuned KAT3 is probably not wise!

73,

    ... Joe, W4TV


On 4/7/2012 1:03 PM, g7vjr wrote:

> Hi Joe, Bruce,
>
>> Make sure the K3 ATU is in bypass mode
>
> Thanks for the tip. I had the same doubts but double checked for this. The
> ATU is bypassed.
>
>> How have you checked the dummy load and patch lead?  What convinces you
>> that they are "good"?
>
> The usual stuff - continuity checks on the patch lead and a digital
> multimeter looking for 50 ohms on the dummy load.
>
>> If the KAT3 is able to reach a SWR of 1:1 there is nothing wrong with
>> either the KAT3 or the K3/KPA3.
>
> Thanks Joe. I am exploring the K3 main board in a bit more detail and think
> I have a smoking gun; I can see that T4 (hidden under P70 to the ATU board)
> has been scorched. The laminations have burned away on the windings and I
> can see a burn on the primary turn inside. :-(
>
> 73
> Michael G7VJR
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K3-output-stage-high-SWR-fault-finding-tp7446109p7446195.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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[g7vjr]

Hi Joe,

I agree, and I can't understand why the external SWR reading is also wrong.  Once I have reassembled the K3 I will do some more tests to see if I can get further with that anomaly.

Many thanks to you and Bruce for you input.

Best wishes
Michael G7VJR