Magic Smoke

There’s two ways I learn best – studying other people’s mistakes, or making my own mistakes. Today I was the subject of the mistake. My Astron power supply has two meters on the front…one for voltage and one for amperage. I noticed a few weeks ago that the ammeter was reading way too high. I rechecked the voltage, and all was fine. My loads hadn’t changed, and my other ammeter wasn’t showing anything amiss. I cracked open the case and found the simple pot to adjust. Since I needed to put a load on the supply to set the ammeter I left the AC plugged in. Just as I was connecting up the load, I dropped a lead. Well crud, that wasn’t a nice snap or crackle. I blew the AC fuse, which was quickly replaced. I went on with the procedure, buttoned everything back up, and went to check out the radios. Unfortunately it wasn’t just a fuse that gave up its magic smoke. My Elecraft KX3 refused to power on. Sitting back a few minutes and thinking through what had happened, I realized I had just exposed my beloved radio to 120V across its DC input. I opened up the KX3 but didn’t see anything obvious that I was up to servicing – but I did sniff the board and caught a whiff of magic smoke. A few google searches, a visit to the yahoo group, and the KX3 is packed in a box readied for a trip back to Elecraft. Another mistake, another learning opportunity. And did you know that magic smoke comes from $100 bills? It must, because that’s what it costs when I have to have other people put it back in my radios.

Michael Brown, KG9DW, is a regular contributor to and writes from Illinois, USA. Contact him at [email protected].

3 Responses to “Magic Smoke”

  • mike VE3WDM:

    Good evening Michael, not a very good feeling for sure and we all have been there at one time or another. When I was building one of my K2’s to took my time to not reverse the polarity on the DC cord. Well my just finished K2 yes you have it was powered with reverse polarity! Even after being very careful not to do it. Hope the rig did not get damaged to bad. Do blog about the results once it’s back from Elecraft.

  • Howard vk3qa:

    A series diode is an inexpensive way to protect against wrong polarity and a 16 volt Zener and fuse is an easy way to protect against catastrophic over voltage. Also enable retention of $100 bills! Regards

  • Mike KG9DW:

    @Mike – thanks for the comment…glad I’m not the only one that messes up like this!

    @Howard – yes, a fuse would have been helpful, and your zener diode idea would be good as well. I’ll definitely put that on my to do list.

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