Capacitor Life

Hi,

Have some 'audio grade' Polypropylene caps 3, 8 and 33 uf in a loudspeaker crossover unit, now some 15 years old.

I know Electros deteriorate over time, but do PP ?

Am refurbishing the cabinets, wonder if its worth replacing them or are they good for another 15 years ?

Nope

They will be OK.

Film capacitors I think only deteriorate by losing capacity if they are overvolted, so I would check that they are within tolerance and use them again.

Kubeek makes sense, if you overvolt a cap you can blow a hole through the insulator shorting layers, and thinking about it this would reduce the capacitance.
The caps most likely will last longer than the drivers.
One thing you can do is check for leakage, at the caps rated voltage, but for that you'll need a special gizmo, older tube era cap testers use a neon to check for leakage.
Thats assuming the caps are rated around 100v or more.

Doesn't need a special "gizmo", capacitor meters are Cheap on Amazon.
Don't expect calibrated values, But it Will spec the unit within reason (have one).
However, can't test them in circuit (duh).

On a bad day, a "good" multimeter resistance scale will show leakage.
If a cap doesn't show as "infinite", it may be a problem.

Most meters can't properly measure > 10 uF: charge time vs meter voltage becomes the issue.
Really Big value caps can be estimated by time x voltage rise calculation.
I mention Big, 'cause measuring the time is less sensitive.

I bought the cheap cap meter after having my Fluke give goofy numbers on higher values.
Good Hunting.. <<<)))

I referred to vintage cap testers as these can apply high voltages, this is probably necessary as crossover caps tend to be higher voltage rated.
You can use a high voltage source and a multimeter to do the job perfectly well.