I checked out the specs, not to mention the batteries weren't included! Cheap! Batteries must be included in the price. Poor judgment of suppliers. Update, after checking the tester on the oscilloscope, I can see the signals it outputs and they should do their job. But when testing 28-year-old Neison valves, the "rattle" is so faint that it can hardly be heard directly at the valves. My guess is that if the valves are buried in a box with a lid, they couldn't be found. Possibly dependent on the valve type. Worth a try.