Not if they're only in "fair working condition." Seems like you'd be buying a headache trying to find the short or the burnt out bulbs.
The Magic Box commercial grade Christmas light tester isolates twisted bulbs, empty sockets and bad fuses by forcing electrical current through a non-working set. You can start repairing your defective lights instead of trashing them. It makes 'dollars and sense' to easily repair a set as opposed to throwing it away. Essential for lighting professionals and crews, this little box will pay for itself in no time.NOTE: To reduce possibility of electrical shock when using this machine, avoid holding the string of lights in your hands while testing.