hi there.....your problem is common with the 420 model when it first came onto the market..stannah improved it by replacing the spring on the carbon brush with a stronger one then later fitting two carbon brush housings[the holes for the second brush are already pre-drilled about 4 inches down from existing one]so if one failed the other one would pick up charge.the main cause is dust particals getting onto the copper strip known as the buzz bar[rail] and then getting picked up by the carbon brush as you rode the stairlift up/down,so if you clean the buzz bar you also need to clean of the carbon brush which is awkward to get to[remember to turn of the isolator switch back of carriage and the spur supply to charger before you attempt this] hope this will help,let us know.....ian..portsmouth uk.......
IAN, April 2012
