[. . . ] [. . . ] Older Vivitar 283 and some studio flash's have significantly higher sync voltages and should not be used. If you are unsure of your flash's sync voltage, you may try this test for the presence of high voltage: 1. Note, the flash will fire when the screwdriver touches the terminals, so shield your eyes to observe the spark. Most flashes designed in the last 10-15 years do not employ high voltage sync circuits. Rev A [. . . ] [. . . ]