Many older flashes use a high voltage to trigger the flash.
These high voltages can "fry" many digital cameras.
For example, a 283 flash uses 600 volts for triggering.
However, the circuit on a Nikon D70 is built for only 12 volts.
The flash would damage the camera.
Check the flash triggering voltage requirements for your camera in the instruction manual.
Then, find out what voltage is used by your flash.
If you have the original manual for your flash, check for the voltage specifications there.
If in doubt . . .
2) Have the flash voltage voltage checked by a camera repair technician, with new batteries in your flash.