Sudden K2 PA Failure

[Bill - K6WLM]

Hello Elecrafters,
 
I recently finished construction of my first K2. 
 
I was very careful about ESD precautions during construction.  While performing the transmitter alignment it was never in transmit for more than just a few seconds.  The PA transistors and heat sink were all properly installed.  The rig had correct output power on all bands.  It was being operated from an Astron 4 amp supply with about 13.5 volts output.The transmitter was terminated into a known good commercial quality 50 ohm load.  At no time did the heat sink get more than slightly warm.  In other words the PA transistors were not abused in any way.
 
I have not yet even made the first QSO with this rig.  This morning when I turned it on to listen around the bands it came on for a few seconds and then failed.
 
Some clever and careful trouble shooting revealed that  Q7 PA transistor is shorted.
 
I have some spare 2SC1969 transistors but they are from a different manufacture.  Should I use these or get new ones from Elecraft?  According to some previous posts there may be a problem with other transistor brands.
 
Is this type of K2 failure common or just a fluke?  I have never had any kind of PA failure in any other  rig that I have owned over the years.
 
Thanks for any thoughts that you may want to pass along.
 
73, Bill - K6WLM
 
 
 


     
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[Don Wilhelm-4]

Bill,

You may use the 2SC1969 transistors you have, they should be just fine.
Also replace Q11 and Q13.

The most likely thing that has happened is that the bias on the PA
transistors increased - either because of a fault in Q11 or Q13 or
because one of the PA transistors developed collector to base leakage
which will drastically increase the bias current and destroy the PA
transistors.

73,
Don W3FPR

Bill Miner wrote:

> Hello Elecrafters,
>  
> I recently finished construction of my first K2.  
>  
> I was very careful about ESD precautions during construction.  While performing the transmitter alignment it was never in transmit for more than just a few seconds.  The PA transistors and heat sink were all properly installed.  The rig had correct output power on all bands.  It was being operated from an Astron 4 amp supply with about 13.5 volts output.The transmitter was terminated into a known good commercial quality 50 ohm load.  At no time did the heat sink get more than slightly warm.  In other words the PA transistors were not abused in any way.
>  
> I have not yet even made the first QSO with this rig.  This morning when I turned it on to listen around the bands it came on for a few seconds and then failed.
>  
> Some clever and careful trouble shooting revealed that  Q7 PA transistor is shorted.
>  
> I have some spare 2SC1969 transistors but they are from a different manufacture.  Should I use these or get new ones from Elecraft?  According to some previous posts there may be a problem with other transistor brands.
>  
> Is this type of K2 failure common or just a fluke?  I have never had any kind of PA failure in any other  rig that I have owned over the years.
>  
> Thanks for any thoughts that you may want to pass along.
>  
> 73, Bill - K6WLM
>  
>  
>

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