[Elecraft] Q7 failure in K2

[Don Wilhelm]

George,

First, you do not have to replace the entire RF transmit chain, only the 
two PA output transistors (Q7 and Q8) and the 2 2N2222As that control 
the PA bias (Q11 and Q13).  Replace Q11 and Q13 first, and then measure 
the voltage on the PA transistor base solder pads during a TUNE - it 
should be between 0.61 and 0.64 volts.  If it is not in that range, do 
not install the PA transistors until that condition has been corrected 
or you will end up with a second set of failed PA transistors.

Yes, that static charge could have been a factor.  The real failure 
mechanism was either leakage in the Q7 PA transistor from the collector 
to the base which will drive a lot of current into the base and saturate 
the PA transistor.  It is not designed for that kind of switching 
service, so it will quickly explode with the high current and subsequent 
heat inside the transistor housing.
The other possibility is that Q11 or Q13 failed and increased the bias 
on Q7 and Q8 causing a swift increase in current through the PA 
transistors - with similar results.
OK, so that is how it happened, one can only speculate on the real root 
cause, but the static charge could have been a contributing factor.

Order the K2PAKIT from Elecraft - that kit has matched PA transistors.
Is this a K2/10 or is the KPA100 present?  If the KPA100 is present, I 
would suggest ordering 2 (or even 4) 1N5711 diodes (KPA100 D16 and 
D17).  Often with the KPA100 installed, static will destroy KPA100 D16 
and D17, and then the power tries to go to maximum because the K2 MCU 
thinks there is no power output - that attempt to drive at maximum power 
often damages one or both of the base K2 PA transistors.
Bottom line - if the KPA100 is installed, make certain the KPA100 
wattmeter diodes are good in addition to replacing the base K2 PA 
transistors.

73,
Don W3FPR

On 2/18/2012 3:17 PM, N9AUP wrote:
> After 2 weeks or so of happy operating of my newly assembled K2, Q7 exploded
> with a loud snap.  From what I read here, I'm going to have to replace the
> whole transmit transistor kit.  I did see that this failure has happened to
> others, so I'd really like to hear if those who had the experience were able
> to identify the cause of the problem.  Replacing the TX chain will be bad
> enough--I don't want to have to do it a second time.
>
> What's crazy here is that I wasn't in transmit mode when it happened.
> Actually, I had gone over to change taps on my ladder line tuner to change
> bands.  There was a considerable static build-up on the antenna and I got a
> small spark when I went to change a tap...and the Q7 let go at the same
> time.  That could have been a complete coincidence. . .but it still seems
> strange.  Any thoughts or experience regarding possible causes?
>
> thanks,
>
> George
> N9AUP
>
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