{"id":1240,"date":"2021-01-23T16:58:00","date_gmt":"2021-01-23T05:58:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2021\/01\/23\/klh-model-twenty-one-ii-af-amplifiers-too-hot-why\/"},"modified":"2025-07-21T11:06:50","modified_gmt":"2025-07-21T01:06:50","slug":"klh-model-twenty-one-ii-af-amplifiers-too-hot-why","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2021\/01\/23\/klh-model-twenty-one-ii-af-amplifiers-too-hot-why\/","title":{"rendered":"KLH Model Twenty-One II AF Amplifiers Too Hot. Why?"},"content":{"rendered":"

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I’ve been working on this nice old FM receiver that Rogier PA1ZZ sent me. When I first tried it, it sounded terrible. I thought it might have been the speaker, but the speaker is fine. There was clearly something wrong in the AF amplifier. Schematic above. Click on it for a better view. <\/b><\/span><\/p>\n

I ended up replacing the complementary pair of output transistors (Q6 and Q7) . The original had house brand designations — I wasn’t sure what to replace them with, so I just used a TIP29C and a TIP30C. With these transistors in there, the receiver sounds good. But the heat sink on the transistors is getting way too hot. I think the AF amplifier is now pulling about 1.4 amps, which is too much.<\/b><\/span><\/p>\n

Another thing I did: I thought Q4 and Q5 might have been bad, so I replaced them with a 2n3904 and a 2n3906.<\/span>

Why do you guys think the heat sink is getting so hot? What should I do? The supply on this receiver is 25V DC.<\/span><\/b>
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I’ve been working on this nice old FM receiver that Rogier PA1ZZ sent me. When I first tried it, it sounded terrible. I thought it might have been the speaker, but the speaker is fine. There was clearly something wrong in the AF amplifier. Schematic above. Click on it for a better view. I ended … Continue reading “KLH Model Twenty-One II AF Amplifiers Too Hot. Why?”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":1241,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1240","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorised"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/1240","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=1240"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/1240\/revisions"}],"predecessor-version":[{"id":1242,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/1240\/revisions\/1242"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/1241"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=1240"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=1240"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=1240"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}