Add fine tunning on a cheap voltage regulated DC power supply

Atten-APS-1502D
Atten TPR 3003T

Time ago i bought the ATTEN TPR 3003T regulated power supply.
That device had constant voltage and constant current mode.
Offered voltage and current tuning by using two potentiometers a coarse one and a fine tune one.
It was a 30V / 3A general purpose power supply.
Recently i came across with Atten-APS-1502D. It was a voltage power supply (15V/2A) at a very attractive price.
I didn't spent further time and i bought that one too since i was a happy customer :) .
When it arrived i noticed that the voltage selection would be really hard to set since it didn't offered a pair of coarse/fine tune potentiometers like the 3003T but only a global potentiometer.

I must admit that i didn't knew about that when i was buying it.
When i tested it, my thoughts become reality, since the voltage selection was way too hard to set.
The potentiometer was not only single turn but really small that make things even harder.
That was it ! I had bought the classic old school power supply without even knowing it !  :-(
Here is some ideas of how to fix this fine tune problem...

10 Turns potentiometer

The most classic approach is to open the power supply and change the single turn potentiometer with a multi-turn potentiometer.
This was the option that i was going to do but i did not had any multi-turn potentiometer and the warranty of the power supply would be gone.
Can this problem be fixed without even opening the case ?
Well the answer is yes !

Measuring potentiometer button diameter

I took a look at the diameter of the potentiometer button.
As you can see it was only 1cm.
The perimeter C of a circle is C=3,14*d where d is the diameter.
As you can understand a much higher perimeter C would help as to select better the desired voltage.

6,5cm button wheel

I happened to have this huge 6,5cm button wheel around !
So the perimeter now would be 6,5 times bigger. :mrgreen:
It was like changing the potentiometer with a 5 turns one or so.
Not bad !
The big plastic button wheel had a hole that the small potesiometer wheel of the power supply would fit.