Elecraft K1 Excessive Current on Transmit

My K1 with the top cover removed

Several years I had a great time building an Elecraft K1, which I later took on honeymoon to the Drakensberg mountains. Last year I offered to lend it to my good friend Vidi ZS1EL to take with him on a visit to the USA. However while I was checking it out, it started drawing excessive current on transmit, so I wasn’t able to lend it to him after all. Fortunately Vidi borrowed an Elecraft K2 in the USA which he later purchased and brought back with him, so i guess things worked out after all. Since then fixing my K1 has been sitting on my “to do” list.

Last night I was calling CQ on 40m when Vidi called me with his K2. During our chat, he asked whether I had fixed the K1 yet, and very kindly offered me a spare 2SC1969 final transistor if necessary, as he has one left over after a repair to his K2. Now it just so happens that my wife and I are going on holiday to the Drakensberg later this month, and I had been idly thinking that I really should take a look at the K1, but hadn’t got around to it. So Vidi shook me out of my complacency, and this morning I retrieved the K1 from a box in the store room and had a look at it.

Thanks to this post by OZ9AEC I soon located the problem. Diode D19, a 1N4753 Zener which protects the final Q7 from excessive collector voltage caused by high VSWR, had blown, and was now acting like a 1.4 Ohm resistor, not a 36 V Zener. I replaced it this evening, and made a couple of test QSOs on 40m to Russia and the Ukraine on 5W, so I think the final transistor has survived.

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Tags: amateur radio, elecraft, qrp