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I'm currently preparing to recap a SE/30. First time for me so I'm taking some precautions. Now most electrolytics go from a voltage rail to ground, in fact nearly all of them. However there are a couple of electrolytics in the audio circuit as well. C3 and C4 are directly in the audio path so they're DC blocking caps (or coupling caps?). C5 goes from VREF for both sound chips to ground. I faintly recall that tantalum caps are not often used in the audio path so perhaps it's a better idea to stick to electrolytics here?   What's got me stumped at the moment is C6 vs C24. C6 is 1u electrolytic, C24 is 0.1 ceramic. C6 goes from SND-R-A/D to ground, C24 from SND-L-A/D. So both essentially have the same function. So why the difference between those values?

I'm not completely sure but the cap next to U22 (partly obscured by the SIMM) looks suspicious. The PCB around it has that "dusty/oily" look that I associate with a leaking electrolytic cap. Of course this also could be "normal" dust...  

Thanks very much. I’ll check all that out.

Usually, when it comes to this, it was just whatever you can get that is cheapest that will work.

Electrolytics vs. tantalums: That's an interesting question. I'm planning to recap a SE/30 and a IIsi in the near future and I'm also somewhat torn here. I've seen tantalums fail in vintage musical instruments occasionally but not regularly. Macs from the early 90's clearly used both electrolytics and tantalums (as is evident in the aforementioned IIsi); is there a similar failure pattern with those tantalums? I'm not aware of this.   So why did Apple then use both electrolytics and tantalums in that timeframe? The following is a bit of speculation: As with many other components there was some evolution in the capacitor field. Anyone having recapped musical instruments and amps from the 70's or earlier will have found that replacement caps are almost always physically smaller than the components originally installed. Caps definitely got significantly smaller over time. I find it possible/plausible that in the early 90's tantalums were larger than their electrolytic counterparts, especially when rated for a higher voltage. I think it took until the late 90's before high capacity tantalums in small sizes became widely available (but I might be wrong about this).   On the other hand tantalums have slightly different electric features (I believe), so perhaps Apple made the choice depending on the circuit?