I bought a second hand Digitone (no warranty) for a really good price, but unfortunately the button number 3 was not always triggering and sometimes double triggering. Compressed air and even Contact cleaner spray did not fix the issue.
Opening the device, I realised how tricky it is to get to the inside of the mechanical-keyboard style switches (which I believe are these ones in black: https://deskthority.net/wiki/SteelSeries_QS1 ).
They have 2 latches on both sides which hold them closed and it’s really hard to get to when they are already mounted to the PCB.
I did manage to bend the tip of a sewing needle with small pliers into a little hook, which has just the right shape to get under the latch and open the switch.
Of course a lot of caution has to given to not damaging the small resistors and other components next to the switches. It’s a bit of a surgical procedure, but not too difficult with a bit of care.
In the background on this pic, you can see the button 3 already opened. I am just posing with the needle on another button to show the action.
Once, open i could easily clean the contacts with a small soft paintbrush and some residue-free contact cleaner, which i sprayed into a small cup to dip my brush into. On the next pic you can see the contact assembly on the left side of the switch. Of course be careful not to bend any part of it.
Putting the switch back together is really easy. It just snaps shut again.
The result was that its working flawlessly again!! yay!
So I hope this is helpful for anyone in the same situation 
Here is some more pics:
