I have just dug out my old Vivitar 2500 Flash to try with my Canon Powershot
SX10IS.

It was working and I was getting used to it and then it failed. Tried fresh
batteries and it is erratic. I hear it whining as it charges but the ready
light isn't coming on. If I press the manual button I can see a faint
flicker in the flash tube and the green light flickers.

Has anyone got any ideas about how these fail?