[Elecraft] K2 PA Failure

[Don Wilhelm]

  John,

I agree with Johnny and Wayne.  I have built and repaired a LOT of K2s, 
and in general the PA transistors are quite robust.  Due to an 
unfortunate set of circumstances about 2 years ago, the PA transistors 
were found to be quite variable in gain which placed stress on one of 
the PA transistor pairs.  Replacements (and new kits) are now being 
provided as pairs matched for hfe.

I would suggest that you obtain a new K2PAKIT from Elecraft and use 
those transistors in their original configuration.  If the solder pads 
to your PA transistors are damaged, repair them with wire laid in the 
same track as the original PC Board traces.  The traces are on the 
bottom side of the RF Board.

While you have the PA transistors removed from the board, do a TUNE and 
measure the DC voltage at the base solder pads.  If it is above 0.63 
volts (or lower than 0.60 volts) also replace Q11 and Q13 and 
remeasure.  If it is still out of bounds, check the resistor values 
associated with Q11 and Q13 (see the schematic).

73,
Don W3FPR

On 2/16/2011 7:48 PM, Johnny Siu wrote:
> Hello John,
>
> I built quite a number of K2 in the past.  The PA of K2 is fairly tough.  It is
> nothing demanding for that pair of PA transistors for an output of merely
> 10-15W.
>
> I don't think it is a good idea to build external PA board.  I do suggest you
> take a digital photo of the PCB section related to the PA.  Then send your photo
> to support at elecraft.com and seek their advice.
>
> K2 is a very mature product and all the production bugs have been
> overcome.  Elecraft knows it inside out and can give you all the required
> support.  The beauty of Elecraft product is that most of the repair can be done
> on the field and without the trouble of sending back for repair.
>
> You may wish to take this chance to investigate why the PA of your K2 keeps on
> failing.  It is a very unusual circumstances.
>
>   cheers,
>
>
> Johnny VR2XMC
>
>
>
>
> ________________________________
> 寄件人﹕ john petters<tjpost at traditional-jazz.com>
> 收件人﹕ Elecraft<elecraft at mailman.qth.net>
> 傳送日期﹕ 2011/2/17 (四) 4:15:25 AM
> 主題： Re: [Elecraft] K2 PA Failure
>
>
>
> -------
>
>
> Hi Folks,
> I have had another PA transistor failure in my 2003 vintage K2. The
> transistors have been replaced several times over the years and the
> board is now a mess.
> I'm planning to rebuild the PA on an external board with a proper heat
> sink mounted on the back panel of the K2. This will involve taking the
> secondary from the driver transformer on a piece of coax about 6 inches
> long.
>
> My query is what impedance is the secondary of T2 designed to work into
> and will I have a problem taking this to the external board?
>
> I've never liked the design of the mechanical fitting of the K2 PA and
> always regarded this as the Achilles Heel of the rig.
> Suggestions appreciated.
> 73
>
>