KPA100 Issue
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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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Dave,
It is correct that the re-tightening be done after the initial assembly (after 10 to 20 hours of transmit operation). It should also be done after the thermal pads are changed. It should only have to be done once. It takes both time and temperature for the pads to conform to all the irregularities of the heat sink and the transistors, but once that happens it should be good for the life of the pads. For those who had not remembered - yes, do it, the life of your PA transistors depends on it. 73, Don W3FPR > -----Original Message----- > > 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 > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.17.8/649 - Release Date: 1/23/2007 8:40 PM _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by kd1na
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: <[hidden email]> To: "David Robertson" <[hidden email]>; "Elecraft" <[hidden email]> 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" <[hidden email]> > To: "Elecraft" <[hidden email]> > 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 >> >> _______________________________________________ >> Elecraft mailing list >> Post to: [hidden email] >> You must be a subscriber to post to the list. >> Subscriber Info (Addr. Change, sub, unsub etc.): >> http://mailman.qth.net/mailman/listinfo/elecraft >> >> Help: http://mailman.qth.net/subscribers.htm >> Elecraft web page: http://www.elecraft.com >> >> -- >> This email has been verified as Virus free >> Virus Protection and more available at http://www.plus.net >> >> >> -- >> No virus found in this incoming message. >> Checked by AVG Free Edition. >> Version: 7.5.432 / Virus Database: 268.17.10/651 - Release Date: >> 24/01/2007 18:48 >> >> > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Folks - Please do not replace the thermal pads with heatsink compound on
the KPA100. As designed, the material on the pads flows slightly as they are heated up during transmit for the first several times, filling in the small gaps between the transistors and the heatsink surface. That is why we note that your should tighten the screws after your first several hours transmitting. Once the initial flow has occurred and you have tightened the screws there should be no need to tighten them significantly in the future. 73, Eric WA6HHQ Elecraft Dave & Jeanne Robertson wrote: > 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. > > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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