When a customer complains, it's important to be sympathetic, but businesslike as well. Do apologise, of course, and then, taking a personal interest in the way the equipment will be used. Suggest a newer, more recent model rather than simply replacing the original one. This is often a good solution, because it not only deals with the complaint, but also the client feels they've got away with a bit extra to make up for the inconvenience. I don't think all complaints can be dealt with in the same way. Basically, you've got to solve the immediate problem to try to keep the customer's confidence. Sometimes guaranteeing your product for a longer period can be a good way to do this. Fix what's wrong as quickly as possible and show you're prepared to do so a second time if necessary. Point out that the customer won't have any worries, at least not until they need an upgrade. It's a good idea to let the customer know why there's been a problem. It shows you are prepared to be honest, and you might win a bit of sympathy. They realise that things don't always go right for the supplier either. But the most satisfactory thing to do is to offer a refund. Then, of course, they are more likely to deal with you again when they need a replacement.