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Don't break the mini knobs

A few days ago I received a Freak module for repair/guarantee. The problem was that the mini knobs (attenuverters) were a bit loose and one of them was stuck. After removing the panel and inspecting the module, it turns out that all the mini knobs were broken.


The mini knobs used for my modules are Song Huei potentiometers which I buy from a reputable UK based store. I have used these potentiometers in many of my personal projects and never had an issue with them.



After looking at the broken potentiometers under the microscope I could see the crack in the shaft.



Broken shaft with a crack

In order to determine how much force needs to be applied to break it, I needed to do some Mythbusters science. Fortunately, I have a control board that I can sacrifice in the name of science.


Control board that we will break in the name of science

Test number one: let my two year old play with the board for a few hours. I just gave him a few instructions about the game. He needed to control the ship by turning knobs. When the game ended, I inspected the potentiometers and they were all fine.


Test number two: pretend that I'm Alessandro Cortini and I'm playing a Nine Inch Nails song in my Vultchla synthesizer. The result of this test was satisfactory. I turned energetically the potentiometers and they managed to survive.


Test number three: pull the potentiometer to the side as hard as I can. It takes quite some abuse. Initially, I pulled with the force of my hand only. This even made the PCB bend as shown in the picture below. After that test, the potentiometer still worked. I kept pushing harder using my thumb and resting the PCB on a table. Then I could feel that something inside the potentiometer broke. Surprisingly, the potentiometer still worked but it felt a bit loose. If I apply some of my extra kilograms directly to the shaft, it breaks.


Pulling the shaft as hard as I can with one hand. You can see that the PCB bends. The potentiometer survived.

Test number four: use pliers and break the thing. This one was easy. With the force that can be applied with the pliers the shafts are relatively easy to break and render them useless.


The best way of breaking it is by using the pliers




My conclusions are the following:

  • It does take some extra force to break the shafts. More than it is applied in a regular use of the module.

  • If you like to headbang while turning the attenuverters, you could accidentally apply too many kilograms to the shaft an deform the potentiometer. You could headbang while turning the Cutoff, Resonance and Drive potentiometers, those are sturdy, but you can damage the PCB.

  • Broken shafts will not be covered by the guarantee of the module. If you accidentally break them, you will be charged a fee to fix the control board. Depending on the location of the potentiomenter, it may be easy to replace, but sometimes it is better to replace the whole control board.


Please take care of your module and get it off the hands of mini knob haters.


In any case, I'll consider removing these type of potentiometers in future designs.





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©2019 by Leonardo Laguna Ruiz - Vult DSP.