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: <[EMAIL PROTECTED]>
To: "David Robertson" <[EMAIL PROTECTED]>; "Elecraft"
<[email protected]>
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" <[EMAIL PROTECTED]>
To: "Elecraft" <[email protected]>
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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