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ALL flash units have deterioration of the power capacitor, and all flash units -- regardless of battery powered portable or AC powered studio flash --need to have the capacitors 're-formed' at about 3 mo. or 6 mo. intervals (per manufacturer recommendation).
Also per manufacturer recommendation is the precise amount of time...
* my Metz flashes say to power on for 15 minutes every 3 months.
* My Dynalite studio flash says "When the power pack is not in use for one month or more, it is wise to periodically plug the power pack in and turn it ON for a couple of hours."
* Instructions for my 45 year old Honeywell (flash gone, still have owner manual) say to power up for 1 hour on AC, then flash 4-5 times, and do this once per month.
Simply turn on power, leave flash like that for recommended amount of time, then turn off.