KPA100 mechanical assembly question
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Goo evening all,
I am on page 37 of my KPA100 build, getting ready to solder the finals on the board. After fitting the circuit board to the heat sink, I noticed that the bodies of transistors Q3 and Q4 prevent the circuit board from resting on pedestal standoff shoulder beside Q3. The transistors are flush with the bottom of the circuit board, but Q3 and Q4 are ~.095 inch thick, and the shoulder is only .075 inch higher than the pedestal. I realize that the transistors need to be able to be snugged to the heat sink for thermal transfer, but that standoff should make contact with the ground plane of the board. The manual mentions several times not to use washers between the sink and the board, but shouldn't there be something there? I looked forward in the manual, but did not find find anything that addressed this. Is this a non issue? Thanks, Pete wd4lst Pete Axson WD4LST 17901 NE 18th Ave Citra, FL 32113 _______________________________________________ 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 |
RE: KPA100 mechanical assembly question
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Pete,
The heat sink assembly is an extrusion, and may have some warp to it. The assembly is referenced on a CNC machine to cut the positions of the threaded rods that have standoffs around them to the same height. The two bosses on the transistor mounting platform are not considered in this procedure. The only parameter is that the bosses not be too high (low is OK). Yes, this is a NON-ISSUE. Follow the instructions in the manual. Q3 and Q4 are mounted first, then the board is attached to the heat sink, and the standoffs are attached finger tight. Now solder the PA transistors in place. When you are done, the transistors will determine the height of the board above the heat sink. There is no instruction to suggest that the small bosses on the heat-sink should control the height of the board. Those two bosses may not actually contact the board, they are there only to provide more threads to satisfactorily hold the threaded rods that mount the standoffs (these two threaded rods have no short standoffs to lock the threaded rods). Eric Swartz made a statement about this condition a month or two ago. If you are interested, you can find it in the archives. The real concern in milling the heatsink is that the bosses are not too high (which would cause inadequate contact of the transistors to the heat sink). Less than the distance from the board to the heat sink is OK. If the board bends a small amount when the upper standoffs are mounted finger tight, that again is of no concern, the board will flex a bit. Do not attempt to torque the standoffs down to make contact with the heat sink bosses. Follow the instructions in the manual and all will be OK. 73, Don W3FPR > -----Original Message----- > > Goo evening all, > I am on page 37 of my KPA100 build, getting ready to solder the > finals on the board. > After fitting the circuit board to the heat sink, I noticed that > the bodies of transistors > Q3 and Q4 prevent the circuit board from resting on pedestal > standoff shoulder beside Q3. > The transistors are flush with the bottom of the circuit board, > but Q3 and Q4 are > ~.095 inch thick, and the shoulder is only .075 inch higher than > the pedestal. > I realize that the transistors need to be able to be snugged to > the heat sink > for thermal transfer, but that standoff should make contact with > the ground plane of the > board. The manual mentions several times not to use washers > between the sink and the board, > but shouldn't there be something there? I looked forward in the > manual, but did not find > find anything that addressed this. Is this a non issue? > Thanks, > Pete > wd4lst > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.446 / Virus Database: 268.18.8/714 - Release Date: 3/8/2007 10:58 AM _______________________________________________ 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 wd4lst
Don,
Many thanks for the response. I just wanted to double check on that. I guess what threw me was the previous step to remove the anodized coating. "Locate the two machined standoffs on the pedestal, labeled S in Figure 29. Lay the sanding block across the top surfaces of both standoffs. Keeping the block parallel to the pedestal at all times, carefully sand off just enough of the black finish to expose the top surfaces of the standoffs. Do not remove any further material, since this may affect proper spacing of the power transistors." That combined with the absence of solder mask around the mounting holes in the board caused me to second guess what was required. I did not want the finals to start flying due to a missing ground. Thanks again. I will proceed as if I knew what I were doing :) -Pete > > From: "Don Wilhelm" <[hidden email]> > Date: 2007/03/08 Thu PM 11:24:25 CST > To: <[hidden email]>, > <[hidden email]> > Subject: RE: [Elecraft] KPA100 mechanical assembly question > > Pete, > > The heat sink assembly is an extrusion, and may have some warp to it. The > assembly is referenced on a CNC machine to cut the positions of the threaded > rods that have standoffs around them to the same height. The two bosses on > the transistor mounting platform are not considered in this procedure. The > only parameter is that the bosses not be too high (low is OK). > > Yes, this is a NON-ISSUE. Follow the instructions in the manual. Q3 and Q4 > are mounted first > > > Pete Axson WD4LST 17901 NE 18th Ave Citra, FL 32113 _______________________________________________ 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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