Based just on past experience, 30 years is likely too old for most electrolytics. There is a large dependency on the initial quality of the seals, the purity of the Aluminium, and of the electrolyte. Twenty years, or less, is the most likely ballpark lifetime for decent quality devices.
I've had some that were not powered for a decade that were 45 years old and had leaked electrolyte through failed seals. In the same equipment, others showed no physical degradation. Some worked, some did not.
If there is no sign of leakage (no white stuff around seals), it is possible a few may still function. If they leaked, replace them.
For those without physical leakage, you can try reforming them. Over time the insulating layer on the electrodes dissolves back into the electrolyte, making them more like resistors than capacitors. This can cause them to overheat if they are in a circuit that can supply much current. If they are coupling capacitors between stages (not common) they can cause destruction of the tube in the following stage.
Reforming consists of applying a lower voltage for a period of time to start rebuilding the insulating layer. You then gradually increase the voltage to build a thicker insulating layer. This is not a 10 minute project, more like several days for really old capacitors.
If you want to do this with the capacitors still installed in the equipment you need a variable transformer to supply a lower line voltage, the most common brands here in the US are 'Variac' and 'Staco'. You start out setting it around 10% of line voltage and powering the equipment for a few days, then turn it up to 20% and let it run, etc., until you reach full line voltage or something fails.
Monitor the temperature of the electrolytics, if one gets warm go back to the lower voltage for a few days and try again. If one still gets warm, replace it.
Unfortunately, the above reforming is unlikely to be successful. It is a 'last resort' when trying to keep the original components.
Good Luck! and please keep us updated on your progress.
Cheers,
Tom
p.s. The other approach is to purchase new capacitors, cut open the old ones, and mount the new ones in the old cases. This keeps the 'look' rather antique but solves the problem of old, dead capacitors. If you take this approach, wear a dust mask because the dust from the dried-out caps is rather irritating, and you probably want to do it outdoors, not in the kitchen!
