KPA100 Standoffs between heat sink and board

[Ben Hofmann K1NT]

I am slowly progressing on my KPA100.  I am at the
part where the board and heat sink are temporarily
mounted together to provide the proper spacing for
soldering the final transistors.

The five standoffs that go between the board and the
heat sink are all 3/16", however, the heat sink has a
machined depression around the hole for the standoff
that is directly behind the center of the rear panel
when assembled.  Unless I missed it, the manual states
I should not use a washer or lock washer on any of the
standoffs between the heat sink and the board either
on this initial assembly, or on the final assembly.
This being the case, there is a gap between the board
and this particular standoff.  When I add the
additional standoffs for mounting the shield later on,
this will either prevent me from snugging the standoff
down on this particular stud, or force me to warp the
board at this point in order to get it snug.  Surely
this cannot be the case.  While I haven't measured it,
the depth of the machined depression in the heat sink
would lead me to believe that a washer or lock washer
is intended to be used either between the heat sink
and the standoff, or between the standoff and the
board during final assembly.  Anyone have any insight
on this?

-Ben  K1NT
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[Ben Hofmann K1NT]

Anyone have any insight on this?

-Ben  K1NT



I am slowly progressing on my KPA100.  I am at the
part where the board and heat sink are temporarily
mounted together to provide the proper spacing for
soldering the final transistors.

The five standoffs that go between the board and the
heat sink are all 3/16", however, the heat sink has a
machined depression around the hole for the standoff
that is directly behind the center of the rear panel
when assembled.  Unless I missed it, the manual states
I should not use a washer or lock washer on any of the
standoffs between the heat sink and the board either
on this initial assembly, or on the final assembly.
This being the case, there is a gap between the board
and this particular standoff.  When I add the
additional standoffs for mounting the shield later on,
this will either prevent me from snugging the standoff
down on this particular stud, or force me to warp the
board at this point in order to get it snug.  Surely
this cannot be the case.  While I haven't measured it,
the depth of the machined depression in the heat sink
would lead me to believe that a washer or lock washer
is intended to be used either between the heat sink
and the standoff, or between the standoff and the
board during final assembly.  Anyone have any insight
on this?

-Ben  K1NT


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[Don Wilhelm-3]

Ben,

Every KPA100 heatsink is milled in that area to remove part of the raised
platform that the PA transistors mount on and still allow for the standoff.
Just how much below the lower adjacent surface (large lower area) has your
heatsink been milled?  In most cases, it is only a tiny bit (1/64 inch or
less), and adding a washer would make the top of the standoff too high.  The
KPA00 board will tolerate some flex with no harm, particularly in the area
you are speaking of.

If your heatsink has been milled much deeper than that, you should contact
Scott or Brian at Elecraft and discuss the situation.  Perhaps a digital
photo would help.

73,
Don W3FPR

> -----Original Message-----
>
> I am slowly progressing on my KPA100.  I am at the
> part where the board and heat sink are temporarily
> mounted together to provide the proper spacing for
> soldering the final transistors.
>
> The five standoffs that go between the board and the
> heat sink are all 3/16", however, the heat sink has a
> machined depression around the hole for the standoff
> that is directly behind the center of the rear panel
> when assembled.  Unless I missed it, the manual states
> I should not use a washer or lock washer on any of the
> standoffs between the heat sink and the board either
> on this initial assembly, or on the final assembly.
> This being the case, there is a gap between the board
> and this particular standoff.  When I add the
> additional standoffs for mounting the shield later on,
> this will either prevent me from snugging the standoff
> down on this particular stud, or force me to warp the
> board at this point in order to get it snug.  Surely
> this cannot be the case.  While I haven't measured it,
> the depth of the machined depression in the heat sink
> would lead me to believe that a washer or lock washer
> is intended to be used either between the heat sink
> and the standoff, or between the standoff and the
> board during final assembly.  Anyone have any insight
> on this?
>
> -Ben  K1NT
>

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[Eric Swartz - WA6HHQ]

Guys -

There is no problem here.  The recessed area on the KPA100 heatsink is
actually exactly where it should be with regards to the height of the
other standoff locations around the edge of the heatsink. The recessed
area is will definitely vary from heatsink to heatsink. The reason it
may be recessed more on some heatsinks than others is due the normal
bowing of the heatsink during the extrusion process. The machining
actually gets the mounting point back to where it should be. :-)

The heatsink on the KPA100 is milled on a precision CNC (computer
numerically controlled)  machine. Every KPA100 heatsink is milled just
like all the others. (The milling process on every heatsink is
referenced to the same corner on a standard fixture.) If a heatsink is
bowed slightly more than others the CNC machine will mill this point
down to match where it should be with regards to the others.

Also, the larger pc boards like the one on the KPA100 are frequently
bowed slightly. Pulling them flat a small amount when screwing them down
will not cause any problems.

Please do not add any washers in this recessed area.

73, Eric   WA6HHQ
Elecraft

Don Wilhelm wrote:

> Ben,
>
> Every KPA100 heatsink is milled in that area to remove part of the raised
> platform that the PA transistors mount on and still allow for the standoff.
> Just how much below the lower adjacent surface (large lower area) has your
> heatsink been milled?  In most cases, it is only a tiny bit (1/64 inch or
> less), and adding a washer would make the top of the standoff too high.  The
> KPA00 board will tolerate some flex with no harm, particularly in the area
> you are speaking of.
>
> If your heatsink has been milled much deeper than that, you should contact
> Scott or Brian at Elecraft and discuss the situation.  Perhaps a digital
> photo would help.
>
> 73,
> Don W3FPR
>
>  
>> -----Original Message-----
>>
>> I am slowly progressing on my KPA100.  I am at the
>> part where the board and heat sink are temporarily
>> mounted together to provide the proper spacing for
>> soldering the final transistors.
>>
>> The five standoffs that go between the board and the
>> heat sink are all 3/16", however, the heat sink has a
>> machined depression around the hole for the standoff
>> that is directly behind the center of the rear panel
>> when assembled.  Unless I missed it, the manual states
>> I should not use a washer or lock washer on any of the
>> standoffs between the heat sink and the board either
>> on this initial assembly, or on the final assembly.
>> This being the case, there is a gap between the board
>> and this particular standoff.  When I add the
>> additional standoffs for mounting the shield later on,
>> this will either prevent me from snugging the standoff
>> down on this particular stud, or force me to warp the
>> board at this point in order to get it snug.  Surely
>> this cannot be the case.  While I haven't measured it,
>> the depth of the machined depression in the heat sink
>> would lead me to believe that a washer or lock washer
>> is intended to be used either between the heat sink
>> and the standoff, or between the standoff and the
>> board during final assembly.  Anyone have any insight
>> on this?
>>
>> -Ben  K1NT
>>
>>    

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