Fixing high resistance in a straight key

Discussion in 'Straight Keys - CW Enthusiasts' started by KG7WGX, May 9, 2019.

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  1. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Last summer I bought a fairly nice straight key. I was using it with a code practice oscillator and didn't notice this problem until I used it with a KX3. It had high key-down resistance, around 10 ohms or higher, and the resistance decreased if I pushed on the key's knob a little harder than usual. This caused a horrible "chirp" in the sidetone.

    After disassembly, I found the ball bearings were sliding in the trunnion blocks, and there was a setscrew for each bearing. The bearings stopped sliding after I tightened the setscrews.

    This got the resistance down to 2 ohms or so, but the KX3 still had a slight sidetone "chirp" when I used this key. There was no chirp at all with the Bunnell Triumph. Something was still wrong.

    I measured 0.5 ohms across the ball bearings and 1.0 ohms from the moving contact (at the end of an adjustment screw on the lever) to the lever itself.

    I drilled and tapped holes in the lever and base for two 6-32 button head screws and installed a "Kent strap" to jumper around the bearings. This lowered lever to base resistance to almost zero. It is completely invisible from the front and barely visible from the back, if you look between the trunnion blocks.

    I also ran a 8-32 tap through the hole for the moving contact to remove any possible paint contamination, and this lowered the resistance from contact to lever to almost zero, too.

    Final key-down resistance measurement after adding the strap and chasing the threads = 0.1 ohms, and the KX3 no longer chirps.

    I thought I would share this, not as a condemnation of the key, but as an example of how some basic troubleshooting can sometimes solve a problem.

    Of course, you may have to disassemble the thing. I still send my watches and cameras to a professional repair shop, but this straight key was just a fancy on/off switch. ;)
    KA0HCP likes this.
  2. W0AEW

    W0AEW Ham Member QRZ Page

    I was going to suggest keying down on some paper and sliding the paper around to clean the contacts but....
  3. W5LZ

    W5LZ Ham Member QRZ Page

    ... so basically, you're saying to just clean the contacts. Hmmm... makes sense to me!

    Now,.. how you go about that cleaning could get real interesting!
  4. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    The story was already long enough, but I did measure the resistance across the contacts as the first step and got a consistent 0.1 ohms.

    It was the wired path that showed high resistance, and that included the moving contact's threads, the bearings, the bearing setscrews and the trunnion block mounting screws. Each one a point of possible high resistance.

    I find it somewhat humorous that a 100 year old key had cleaner keying than a modern one with ball bearings.
  5. W5LZ

    W5LZ Ham Member QRZ Page

    Mechanically, you want minimum contact to reduce friction. Electrically, you want the least resistance, so maximum contact. For the usual/typical straight key you average the two until it feels comfortable to YOU.
    How do you tell when you get it right? It'll feel 'right'!
  6. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    contacts should be made with cold-pressed Latinum.
  7. G0GSR

    G0GSR Ham Member QRZ Page

    I had to do exactly the same with my 1950's Post Office pattern key. But mine already had two tapped holes!


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