I have several Harmony remotes but the one I prefer, from those that I have, is the Harmony 300. Probably because it is the simplest.
I made an assumption that the fault was just a dirty contact. It made little difference what the fault was because if it was anything else I would be neither able to diagnose it nor able to fix it.
Taking the remote apart is not too difficult once you know how.
There are four obvious screws under the battery cover and two hidden under a sticker at the infra-red end. I needed to use the blade of a box cutter as a lever to prise the sticker off.
The sides are held together with plastic clips. I used a mobile phone opening tool to slide between the seam and a small flat screwdriver to separate the clips. They all came apart easily with a little levering.
Once the two halves are separated the red plastic lens can also be lifted off to leave the circuit board fully exposed. That is held down with three screws.
Now you have access to the contacts and the inside of the buttons.
I cleaned both sides with some foam anti-static electrical contact cleaner and paper towel. I've had the spray of cleaner in my shed for years so I was surprised it still worked. I think we packed it when we moved house so it is over 10 years old!
Once dry I put it back together. The only tricky bit is making sure the rubber keys line up exactly. There are some tiny guide pins that must be in place.
The volume now works.