[Elecraft] KPA100 stripped final's screw removal

[Don Brown]

Hi

I believe the screws loosen due to the thermal pads compressing not the 
screws backing off. The lock washers should prevent the screws from backing 
off and locktite may make the screws impossible to remove. Once the pads are 
fully compressed the screws should not loosen, but it may take a couple of 
times at 10-20 hour operation intervals to fully compress the pads. These 
pads are not insulators, they are for heat transfer only and they have a 
coating that slightly melts under high temperature. This coating fills all 
of the gaps under heat and compression and forms a good thermal path to the 
heatsink.

Sears sells a small kit of stripped screw removal tools. These tools bite 
into the head of a stripped screw making it easy to remove. Screwdrivers 
will ware out and start slipping in the slots. Mt favorite screwdriver is a 
magnetic hand tool that holds the bits for power drivers. These bits are 
much harder than most standard screwdrivers and can be easily swapped out if 
they are worn. The magnet also holds the screw for starting or removing and 
the handle of the screwdriver holds several different bits. The handle of 
this screwdriver locks or it can ratchet in either direction. Most large 
hardware stores carry these screwdrivers.

Don Brown
KD5NDB




----- Original Message ----- 
From: "Bob Evans" <k5wa at houston.rr.com>
To: <elecraft at mailman.qth.net>
Sent: Monday, November 21, 2005 7:40 PM
Subject: [Elecraft] KPA100 stripped final's screw removal


> Hi Folks,
>
>
>
> This may not be a very high tech solution, but it got me out of a 
> perceived
> pickle.  I had stripped one of the screws securing the final transistors 
> to
> the heat sink in one of my K2/100s.  I was thinking "how the heck am I 
> going
> to remove a stripped screw in such a tight space?".  A friend had 
> suggested
> super gluing a Phillips screwdriver into the stripped screw.  I tried it 
> and
> the super glue didn't hold.  Then I thought I could drill and tap the 
> screw
> out, but I couldn't find a tap that was small enough.  What finally worked
> was simply to drill into the center of the screw until the screw head
> detached from the screw shaft.  It took a 5/64" bit.  I had loosened all 
> the
> other screws on the board and the board simply lifted away from the heat
> sink when the screw head separated from the shaft.  With the board 
> removed,
> the screw shaft could easily be unscrewed from the heat sink and replaced
> with a new screw.  By the way, I turned the KPA100 board screw-head-down
> when I drilled so there wouldn't be much of a chance for metal splinters
> getting into the board.  It worked!
>
>
>
> I have always used a high quality #1 screwdriver when I tightened these
> screws, but it stripped out anyway.  I use my K2/100s exclusively for
> contesting and have been finding I need to re-tighten those screws after
> 20-30 hours of full bore operating (100 watts, lots of CQs).  Maybe a
> Lock-tight type product is needed to keep the screws from backing out 
> after
> thermal cycling, but some of you may have some advice whether this should 
> be
> done or not.  Anyway, in case you find your final's screw stripped, this
> method worked for me.
>
>
>
> P.S.  I played around in ARRL SS SSB this past weekend.  The K2 is a GREAT
> side band radio too!  I know, I know.I can get ejected from this reflector
> for such heresy.  ;-)
>
>
>
> 73,
>
> Bob K5WA
>
> K2s #4687 and #5119 in SO2R configuration
>
>
>
>
>
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