[Elecraft] K2 Finals - Chronic Blow-Outs

[Jim Wiley]

What is the high current alarm set at?   Are there the correct number of 
turns on T4? (check against the illustration in the assembly manual)   
Perhaps there is a problem with the bias system. Transistors Q11 and Q13 
should probably be changed as well as the finals if there is a problem 
with the PA stage.  You did not say what the set serial number is.  I am 
not sure if that might make a difference with troubleshooting help, but 
it could.


- Jim, KL7CC


On 10/13/2012 3:29 AM, w7asa wrote:
> This K2 was a bought from someone on the internet as a working unit & etc.
> Once received, it blew the finals in about 2 - 3 hours of QSO time.  I
> really like the K2 but it looks it's mine to solve.  I've done a lot of
> radoi building and RF troubleshooting down through the years, and have
> looked for all the usual suspects of shorts, debris shaken loose during
> shipment & etc.
>
> BTW: The condition of the board and mounting hardware for the finals and PA
> bias transistors STRONGLY indicates that they've been changed more than a
> few times *before* the rig reached me . . .   so , in all probability, I was
> probably the only one surprised by it's sudden demise.
>
> Here's what I am seeing:
>
> It has fried three sets of finals in two weeks - I changed them per the
> manual twice ( am a glutton for punishment )  and VERY carefully re-checked
> my work and metered for shorts each time. After =< 2 to 3 hours of 'normal'
> operation each time, the finals blew.  This happened on three separate bands
> with an SWR of 1.2:1 (on average) at 10 Watts indicated both on my SWR/power
> meter and the K2 digital display. I did not notice a current warning during
> the last two failures and now am not certain whether I did during the first
> failure.
>
> Most recently: - I had finished the most recent ragchew then switched to
> calling CQ using a Vibroplex (as always) and heard a small 'crack' of a
> transistor case rupturing when immediately the 3 Amp automotive fuse in my
> power cord blew. The smell told me where to look.
>
> This is a relatively simple circuit and it seems to me that there is not a
> lot to go wrong:  T4 with bad insulation ?  Something in the common parts of
> the LGP/Band switching but not band specific? I am mystified why it would
> run flawlessly for a few hours of CW then let the smoke out.
>
> Anybody have ideas? I have a ton of Elecraft accessory boards on the way and
> that's a lot of assessories for a radio that drops dead continuously.
>
>
>> Ray
> W7ASA ..._ ._
>
>
>
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