[Elecraft] K2 Finals - Chronic Blow-Outs

[Don Wilhelm]

Ray,

You apparently have some problem that is destroying your K2 PA 
transistors.  What I am saying is that the dead PA transistors is the 
symptom but not the cause of failure.

When the PA transistors are removed, measure the voltage at the solder 
pad for the PA transistor base during a TUNE.  If it is greater than 
0.63 volts, replace Q11 and Q13.  You might just replace Q11 and Q13 
"just because".

When you replace the PA transistors, do you use the Elecraft K2PAKIT or 
do you purchase the transistors from another source.  The Elecraft 
K2PAKIT has transistors that are matched, and that is an important 
parameter.

Do you have the KAT2, KPA100, or KAT100 installed?  If so, check the 
wattmeter diodes.  Those diodes often become damaged by nearby lightning 
(do not leave the antenna connected when not operating), If those diodes 
are damaged, the K2 will go to uncontrolled maximum power output, and 
the usual result is that the base K2 PA transistors are overloaded and 
will sacrifice themselves.

73,
Don W3FPR

On 10/13/2012 7:29 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.
>