Do poypropylene capacitors deteriorate over 20 years?

[Dr.H]

I've bought a 20 year old ribbon speaker (Apogee Scintilla) and noticed that the midrange is quite dark.

Should the Sprague polypropylene caps (25 of them!) be replaced given their age?

Do polyprops perform more poorly as they age?

[SY]

Polypropylene doesn't deteriorate when the caps are sealed. If they're encapsulated, chances are they're fine. If they're the wound type, see if it looks like the outer windings (cover) are delaminating.

Likely places to look for your problem are bias voltage, connection to the diaphragm, the diaphragm itself, and any dirt/residue causing leakage paths between diaphragm and stator. Mylar, in particular, can brittle up after many years of exposure to light and the elements.

[Dr.H]

Thanks Sy,
I just realised that I have not really compared the two speakers, to see if perhaps one sounds slightly different. Also the angle of tilt of the speaker could be playing a role.
Finally as you say, it's worth checking, after so much time, for quality of connections, including crimped connections.

[JarreYuri]

Apogee never made any electrostatic loudspeaker.

[john dozier]

The Apogee's are full range ribbons and go down to 1ohm. Thus they are quite difficult t drive. I would look to your amplifier for any SQ problems. Regards BTW to my knowledge Apogee only used hermetically sealed caps so I doubt this is the source of your problems.