RE: [Elecraft] Question about K2's Q7 and Q8 Final Amp
A question on this subject... I had the same problem, after I installed the 100 watt amp and put everything back together. I solved it with new pads. The only symptom was a failure to develop output power. It was awhile ago. It may be that the thing refused to power-up at all, I just don't remember. There was never any smoke, no loss of the finals. When I loosened the screws holding Q7 and Q8, everything worked. After I replaced the pads, I was able to properly tighten the screws with no problem. I have assumed there is protective circuitry so that a short to ground would not blow anything. Was I just lucky? I don't have the schematics in front of me, so I can't look to see for myself. My K2 lives a few states away. Thanks for any insights. 73 de Brian W3BW -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Terry Bassett Sent: Monday, October 04, 2004 10:39 To: elecraft@mailman.qth.net Subject: Re: [Elecraft] Question about K2's Q7 and Q8 Final Amp On Monday 04 October 2004 08:16, [EMAIL PROTECTED] wrote: > Hi, > > My K2 has been working fine but I decided to fix it anyway...hi. > The K2 manual warns that whenever the heat sink is > removed and replaced you should verify that the collectors of Q7 and Q8 are > not shorted to ground. > > Tom, WB2QDG > K2 1103 Hello Tom, The way I blew one of mine was due to a very tiny fragment of metal shaving getting installed along with the heat transfer pad of Q7, I took only a very short time to let the smoke out. This happened quite some time back, so I can't remember what I was doing or trying to accomplish, but I'd say if you've done anything to Q7/Q8 and can't find the short to ground. It "might" be something like what I did. Believe me it was a tiny, tiny fragment, but when Q7 was tightened down it made contact and did its damage. Just a bit of my experience that you can check and maybe gain from. 73, Terry KA9TXE ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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: Elecraft@mailman.qth.net 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: [Elecraft] Question about K2's Q7 and Q8 Final Amp
On Monday 04 October 2004 08:16, [EMAIL PROTECTED] wrote: > Hi, > > My K2 has been working fine but I decided to fix it anyway...hi. > The K2 manual warns that whenever the heat sink is > removed and replaced you should verify that the collectors of Q7 and Q8 are > not shorted to ground. > > Tom, WB2QDG > K2 1103 Hello Tom, The way I blew one of mine was due to a very tiny fragment of metal shaving getting installed along with the heat transfer pad of Q7, I took only a very short time to let the smoke out. This happened quite some time back, so I can't remember what I was doing or trying to accomplish, but I'd say if you've done anything to Q7/Q8 and can't find the short to ground. It "might" be something like what I did. Believe me it was a tiny, tiny fragment, but when Q7 was tightened down it made contact and did its damage. Just a bit of my experience that you can check and maybe gain from. 73, Terry KA9TXE ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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: [Elecraft] Question about K2's Q7 and Q8 Final Amp
Tom, Quite rare, but if you have a short with the heatsink off, look for a soldering problem with T4 or T4 might have an internal short (unlikely, but possible). You might try lifting the 1 and 2 terminals of T4 to further isolate the collectors and confirm that T4 is the culprit. Ask yourself 'What did I change?' - you said the K2 was working before you replaced the transformers, so it has to be associated with something that was touched in the process. Since you said you are using the 'buzzer' of your DMM, it amy be informative to see what the actual resistance is - the collectors normally show 500 to 600 ohms to ground, and if your DMM buzzes with that value, that would be misleading. It is much more likely to have a short only when the heat sink is mounted. That is most commonly caused by a small bit of aluminum becoming lodged in the thermal pads. The bits of aluminum come about when the toothed lockwashers are loosened. SOLUTION - replace the thermal pads and be certain there are no bits of aluminum still clinging to the transistors or the underside of the heat sink. As a further precaution, I usually do not use the toothed lockwashers for locking the heat sink screws and nuts, I use the split lockwashers instead because they are not as likely to leave those bits of aluminum behind when they are removed. 73, Don W3FPR - Original Message - My K2 has been working fine but I decided to fix it anyway...hi. Well actually I was a little short on the power output on 10 meters. I was getting about 7 watts out on 10 meters and Gary Suggested that I replace T2 and T1. Instead of winding them myself, I contacted 'The Toroid Guy' and he sent me two beauties (along with a T-4) - the guy is great, BTW. So yesterday I installed the new T1 and T2 (I did not yet install T4) and replaced the heat sink. The K2 manual warns that whenever the heat sink is removed and replaced you should verify that the collectors of Q7 and Q8 are not shorted to ground. With the heatsink attached I check and sure enough, there was a short between the collector (middle pin) and ground (the ground wire on the RF board) - I used the continuity buzzer on my DMM for this. I removed the heatsink and all of the hardware which connects Q7 and Q8 to the heatsink and then again checked for the sort. It was still there. In looking at the schematic, the section of the final which contains Q7 and Q8 seems pretty isolated from everything else, with just T3 and T4 seemingly getting into the action. Try as a I might I couldn't locate any shorts. Using the schematic, I started at the collectors of the transistors and worked to the right of the schematic. The only options I have presently connected are the Noise Blanker and the SSB module, both of which don't seem to be involved with the finals. Obviously the problem must be in the area that I was working on. If anyone has any suggestions in tracking this down, I appreciate your help Tom, WB2QDG K2 1103 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
