[Elecraft] KPA100 Issue

[Dave & Jeanne Robertson]

Thanks for your reply Geoff.

I think that the reason the screws loosen isn't because of twisting out, but 
because the heat sink material that is placed between the power transistors 
and the heat sink is cold flowing.

Many plastics will, under pressure, actually flow a bit and change it's 
shape ( in this case becoming thinner). After a period of time the plastic 
assumes it's final new shape.

Using locktite (especially the permanent type) will make it very challenging 
to remove the screws when needed.

Another idea would be using heat sink compound (a form of heat conducting 
paste) instead of the pads provided by Elecraft. The mounting bracket of the 
power transistors is insulated from the transistor itself by a heat 
conducting oxide so no insulating material is needed for mounting. Just 
mounting the transistor to the heat sink will not provide enough thermo 
conductivity between the transistor and heat sink. That is why Elecraft 
provided the thermo conductive pads. If heat sink compound is used the 
screws will remain tight because there is no lingering cold flow properties 
in the compound.

73
Dave KD1NA

----- Original Message ----- 
From: <g3naq at geoffgrayer.force9.co.uk>
To: "David Robertson" <darjar at comcast.net>; "Elecraft" 
<elecraft at mailman.qth.net>
Sent: Thursday, January 25, 2007 11:11 AM
Subject: Re: [Elecraft] KPA100 Issue


> Hi David;
>
> I can confirm your experience - and add a bit more.
>
> I too had occasion to remove the KPA100 shield, and found he PA and driver 
> fixing screws I had tightened (but not overtightened!) quite loose.
>
> I re-tightened them, but this time overdid it with one of the studs 
> tightened with an octagonal wrench, which twisted off. Fortunately a spare 
> had been included with the kit, and it was not too difficult to remove and 
> replace it.
>
> I wonder if a spot of 'Locktite' (cyanoacrylite 'superglue') applied 
> during construction would be a good idea?
>
> Geoff G3NAQ (ex AE4PI)
>
>
> ----- Original Message ----- 
> From: "David Robertson" <darjar at comcast.net>
> To: "Elecraft" <elecraft at mailman.qth.net>
> Sent: Thursday, January 25, 2007 1:31 PM
> Subject: [Elecraft] KPA100 Issue
>
>
>> To All,
>>
>> Though there is some mention about tightening the screws that mount the 
>> final power transistors in the manual, it can be easily missed.
>>
>> When I first built the KPA100 I made sure the transistor hear sink 
>> mounting screws were tight.  After operating the rig for about a week 
>> (and heat cycling the heat sink) I had an opportunity to open up the K2 
>> to show a friend. At that time something in my mind reminded me to 
>> recheck the final transistor heat sink mounting screws (please remember 
>> the screws were carefully tightened upon assembly) and to my surprise I 
>> could easily tighten the screws 1/2 turn or more! It seems the heat 
>> conducting material cold flows a bit between the final transistors and 
>> the heat sink.
>> So for those who have a KPA100 it would be prudent to check those final 
>> transistor heat sink mounting screws every so often. If you don't you 
>> could lose your finals.
>>
>> 73
>>
>> Dave KD1NA
>>
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>