My experience is that the capacitors in many kinds of electronic flash units; strobes, studio units, speedlights, monolights, and one and two-piece older portables can become be deformed if left dormant and unused for long periods of time. It is best, therefore, if you want to keep them serviceable you have to do a simple procedure periodically.
Turn the unit on but DO NOT fire it for about an hour. Then fire it several times. When firing it, observe it and make certain the ready light is coming on regularly and does no extinguish intermittently between flashes. Do this procedure at full power on units that have fixed power settings. On thyristor controlled auto-flash models, cover the receptor in order to obtain full power. If all goes well, repeat the procedure at the various fixed power settings or hold the flash at various distance from a wall to activate the thyristor system and achieve various outputs. The recycling times should be uniform at each given power output. The higher the output, the longer the recycling time.
When doing this procedure at various fixed power settings, make sure the unit is fully recycled before changing power settings. On some older units, NOT doing that can burn out the power selector switch or dial.
Symptoms of capacitors that are deformed and are malfunctioning are: 1) the unit will not fire up at all-DEAD! 2) It will turn on but won't produce enough power to activate the ready light, 3) There could be a crackling or popping sound accompanied by smoke or the odor of burning plastic or phenolic material emanating from the unit. 4)The ready light may extinguish or flicker irregularly- not the normal regular blinking that some readr lights will do. 5) There could be a loud BANG when a deformed capacitor causes a short circuit across another capacitor in the circuit. 6) The flash tube will glow at a very high output and remain on rather than flashing- this indicating that the capacitors are completely inoperative and there is a flow, rather than a pulse, of high voltage directly to the lamp head or flash tube.
If any of the above symptoms occur, shut the unit off immediately. There are also other components that can dry up or burn out such as trigger coils or smaller capacitors in the trigger circuit.
The viability and economics of repair will depend on the original value of the unit and the condition of other parts such as high voltage connector cables, flash tubes, and lamp heads and the current availability of replacement parts.
I have not seen oil-filled capacitors in any unit manufactured after the mid-1950s. These units operated at extremely high voltages and capacitor with a comparatively low rating in microfarads.
At my studio, I have units that are still operating perfectly after 30 or more years of service. I use them regularly and fire the spares up as I explained above. Regular maintenance includes general cleaning, replacement of carbonized or blackened flash tubes and careful cleaning of all contacts, sockets and battery compartments.
I am not claiming to understand the chemistry or inner workings of electrolytic capacitors and if "forming" or "reforming" is not the proper nomenclature or correct technical word from an engineering point of view, I apologize. These procedures and descriptions of symptomatic indicators of malfunctioning capacitors are strictly based on experience. When a unit came into our shop with theses specific symptoms, the capacitors were usually at fault. We did have instrumentation, capacitors testers, to check suspect capacitors. On custom builds we could check out new capacitors and determine their exact values. In some cases on the older unit we were able to reform certain capacitors, others required replacement. The procedure I firstly outlined in more of a preventative measure.
Remember, older units should NEVER be directly connected to a digital camera unless the trigger voltage is checked and found to be well within the safe parameters determined by the camera manufacturers. Safe-Sync or Paramount protection devices should be used or the flash should be synchronized by radio. Even if the trigger voltage is considered safe, a flash unit suspected of any malfunction should never be connected to any camera. Short circuits or defective or burned out components can direct extremely high voltages and current into the camera- and the PHOTOGRAPHER !
A don't like to be an "alarmist" but when folks mention old dormant units, I always prefer to issue this warning. I've seen some pretty nasty damage caused by old strobes. I don't wanna see any of my photographer friends become a crispy-critter. Cameras are replaceable- burned out nerve endings, and other injuries caused by electrocution are harder to fix. Y'all remember the electric chair!?