You start by finding out the make and model. Then you work out exactly how to describe the problem.
Then you do a Google search or two for that make, model and problem.
Then you find a dedicated forum for equipment from that manufacturer and see if there are any specialists who know.
Then you ask in a general forum like this, in the hope that a wider audience may throw something up.
Google is your friend. USE IT. Don't expect others to do it for you.