[Elecraft] K3 output stage high SWR - fault-finding?

[Joe Subich, W4TV]

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
>
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