Well considering it’s on that newer Family Gard detector you have & not on your earlier First Alert one, perhaps BRK encountered issues with their detectors sometimes experiencing circuitry-killing power surges, so they added that capacitor at some point to guard against such. As for mine, I never thought to look for that difference before now, but now that I have it seems like all but one of the “8-row” & Family Gard-style detectors I have (of which there are quite a few) have that capacitor (though its make & model (or at least color) often differ): said odd one out is a 1993 2nd-gen SA67D, & it actually has a spot on the board for a capacitor, but there’s nothing there: meanwhile the next oldest one I have, a 1996 SA67D, does have a capacitor in that spot, so I guess the change was made sometime in between 1993 & 1996.
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