KX1 Fire Sale
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Hello all:
I had disassembled my previously operational KX1 (SN854) in anticipation of adding the KXB3080 option. However I realized that it would be some time before I could get around to it, and I felt like operating. So I put it back together, powered it up, and released the smoke. Examination showed that a short section of V+ PCB trace just after the power switch had vaporized. PCB traces make excellent fusible links. I measured the resistance to ground from the far end of the trace. It was 1025 Ohms. By now I had a pretty good idea what was going on. I repaired the trace, and measured the same resistance at the power connector. So far so good. I powered it up outside of the case and all is well. Now I'm real sure what's going on. I reassembled it into the case, and measured the resistance at the power connector; 0.4 Ohms. Q6 was shorted to the case. I backed off the screw for Q6 1/8 of a turn and the short goes away. I examined the plastic spacer and it does not seem deformed. The insulating pad also seems fine. I did not torque the screw to 20 foot pounds. I don't know what to do next. All suggestions appreciated. Thanks, Paul N8NOV Houston _______________________________________________ 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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Paul,
Get the nylon screw. If you already have the nylon screw, then examine the area on the case around the countersink hole (top and bottom) for any burrs or pieces of metal that may be tightening against the transistor tab when you tighten the screw. 73, Don W3FPR > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]]On Behalf Of Paul T. Rubin > Sent: Friday, September 08, 2006 9:06 AM > To: [hidden email] > Subject: [Elecraft] KX1 Fire Sale > > > Hello all: > > > > I had disassembled my previously operational KX1 (SN854) in > anticipation of > adding the KXB3080 option. However I realized that it would be some time > before I could get around to it, and I felt like operating. So I put it > back together, powered it up, and released the smoke. Examination showed > that a short section of V+ PCB trace just after the power switch had > vaporized. PCB traces make excellent fusible links. I measured the > resistance to ground from the far end of the trace. It was 1025 Ohms. By > now I had a pretty good idea what was going on. I repaired the trace, and > measured the same resistance at the power connector. So far so good. I > powered it up outside of the case and all is well. Now I'm real > sure what's > going on. I reassembled it into the case, and measured the resistance at > the power connector; 0.4 Ohms. Q6 was shorted to the case. I > backed off the > screw for Q6 1/8 of a turn and the short goes away. I examined > the plastic > spacer and it does not seem deformed. The insulating pad also seems fine. > I did not torque the screw to 20 foot pounds. I don't know what > to do next. > All suggestions appreciated. > > > > Thanks, > > > > Paul N8NOV > > Houston > > > > _______________________________________________ > 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 > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.1.405 / Virus Database: 268.12.1/440 - Release Date: 9/6/2006 > > _______________________________________________ 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 Paul T. Rubin
Hi, Paul!
Sounds like a tiny metal burr is shorting the case of Q6 to ground. These are almost always at the screw hole. Check the following: 1. Make sure the screw itself is plastic; Elecraft will send you a new one free if it isn't. 2. Pull off the insulated pad and use an Xacto knife from the inside of the cover to "circle" (swirl around at a 45 deg angle) the Q6 mounting hole and remove any burrs. 3. Using fine sandpaper, sand the area inside the case that the insulated pad covers to ensure it is perfectly flat, especially around the screw hole. Replace the insulated pad and reassemble. Very smart of you to use an ohm meter to alert you of potential smoke inhalation during power-up. Brian - W6FVI Paul T. Rubin wrote: >Hello all: > > > >I had disassembled my previously operational KX1 (SN854) in anticipation of >adding the KXB3080 option. However I realized that it would be some time >before I could get around to it, and I felt like operating. So I put it >back together, powered it up, and released the smoke. Examination showed >that a short section of V+ PCB trace just after the power switch had >vaporized. PCB traces make excellent fusible links. I measured the >resistance to ground from the far end of the trace. It was 1025 Ohms. By >now I had a pretty good idea what was going on. I repaired the trace, and >measured the same resistance at the power connector. So far so good. I >powered it up outside of the case and all is well. Now I'm real sure what's >going on. I reassembled it into the case, and measured the resistance at >the power connector; 0.4 Ohms. Q6 was shorted to the case. I backed off the >screw for Q6 1/8 of a turn and the short goes away. I examined the plastic >spacer and it does not seem deformed. The insulating pad also seems fine. >I did not torque the screw to 20 foot pounds. I don't know what to do next. >All suggestions appreciated. > > > >Thanks, > > > >Paul N8NOV > >Houston > > > >_______________________________________________ >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 > > > > _______________________________________________ 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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I'll add to Brian's comments that it does not always require a burr to
produce the short. In some KX1s either the head of the screw is a tiny bit smaller or the hole is a tiny bit larger, allowing the beveled screw head to reach through the hole in the cover and thermal-conducting pad and touch the metal tab on the transistor, causing a short. I fixed mine long, long ago by adding a second thermal-conducting pad. Still, I'm very careful to check for a short whenever I reassemble it. But in dozens of disassembly/reassembly operations it's never been shorted. Others, with slightly smaller case holes or screws with a slightly bigger head never had the problem. They were unable to create a short, even when trying. The nylon screw is the really foolproof solution. That's why they are supplied with all current KX1s. Ron AC7AC -----Original Message----- Hi, Paul! Sounds like a tiny metal burr is shorting the case of Q6 to ground. These are almost always at the screw hole. Check the following: 1. Make sure the screw itself is plastic; Elecraft will send you a new one free if it isn't. 2. Pull off the insulated pad and use an Xacto knife from the inside of the cover to "circle" (swirl around at a 45 deg angle) the Q6 mounting hole and remove any burrs. 3. Using fine sandpaper, sand the area inside the case that the insulated pad covers to ensure it is perfectly flat, especially around the screw hole. Replace the insulated pad and reassemble. Very smart of you to use an ohm meter to alert you of potential smoke inhalation during power-up. Brian - W6FVI _______________________________________________ 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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Thanks to all who responded.
The archive provided a wealth of information on this well-known problem. I spoke with Scott at Elecraft and he is sending me a nylon screw. He told me that there had been a Builder's Alert on this problem in the past, but he was also unable to find it on the web site. He told me it would be reposted there, which I imagine will be of help to people with older KX1's, especially second hand owners. As always Elecraft shows that it's commitment to its customers is unparalleled in the industry. Now where's the K3? Paul N8NOV Houston _______________________________________________ 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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