During my last summit activations, I was getting poor reports, only to discover at the end that my antenna was detuned most the time - although I would tune it at the beginning of the activation.
Also, I noticed that while tuning the variable capacitors on the tuner, the tuned antenna becomes suddenly completely detuned (the LED indicator starts shining brightly) on certain knob positions.
I have been using a
ZM4 antenna tuner since the beginning of my SOTA activations. It is not the original kit, but a clone I built following the original schematics. After some researching, I realized that the shafts of the variable capacitors are probably misaligned, brushing the ATU housing occasionally.
 |
| My ZM4 with the knobs detached |
 |
| Closer look at the possible rubbing areas (painted blue) |
 |
After some drilling -
no more contact between the shaft and housing |
I guessed that this must be the case with my ATU, and after short inspection, I found out that the shafts are indeed dangerously close to the housing. Therefore I enlarged the holes of the variable capacitor shafts, hoping that this will cure the problem.
However, after some testing it turned out that the problem is still there. Just touching the knob would detune a perfectly tuned ATU. Why? Maybe the variable capacitors are damaged, maybe I overheated them during soldering? Before buying new capacitors, there was one last thing to check - maybe poor solder joints of the capacitors are the cause of intermittent loss of electrical contact? There was only one way to check this, so I disassembled again the ATU and resoldered all variable capacitor contacts.
 |
| The variable capacitors are soldered to the small PCB facing down . |
And this finally cured the problem - my ZM4 started working reliably again!
