[Elecraft] Need some help please

[R. Kevin Stover]

W3FPR - Don Wilhelm wrote:
> Kevin,
> 
>>From the resistance readings on Q7 and Q8 you appear to have shorted final
> transistors.  I'm not certain how it happened, they are quite hardy
> transistors.
> 
> If you havn't already, repeat the resistance measurements with the heat sink
> removed on the chance that you just have a short to the heat sink.  Then
> remove the transistors to confirm that they are indeed shorted - the board
> resistance readings should be restored to about normal if the finals are
> indeed the problem.
> 
> The 'too high' RF voltage readings that you obtained at the transmit mixer,
> buffer etc. are not a problem - that is the normal reaction of the K2 when
> it does not sense output power, it just increases the level in an attempt to
> boost the output higher.
> 
> You likely do not have a failing D9 - it just does not have any RF to
> detect, but having an extra on hand is not a big deal.
> 
> The real challenge will be to determine why they failed - there may be
> something in the KPA100 input that is not right, or you did not have the
> base K2 heat sink in contact with the tabs of Q7 and Q8, or ??, just guesses
> here - but excessive heat is normally the killer of solid state devices, the
> question is 'how did it get so hot'.
> 
> 73,
> Don W3FPR
> 
> 

Thanks for the reply Don,

I sent the same mail to Elecraft and got pretty much the same reply.
I have no idea how they got smoked. I've never transmitted with the rig 
without the heat sink, as far as I know. It could be that the finals 
never made very good contact with the heatsink. Maybe Elecraft should 
consider a spacer of some sort to make sure the tabs of those 
transisitors are flat and parallel with the heatsink before they get 
soldered.

Aftre taking the hardware apart this morning and lloking closely, the 
shoulder washer on Q7 seems to have been overheated and melted slightly. 
It didn't want to come out of the tab.

How much lead length should you have on the top side of the board when 
Q7 and Q8 are installed correctly?

Thanks.



-- 
R. Kevin Stover	ACØH

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