Abba Gold: Track 5 Lay - All Your Love On Me
I've just had a listen to the song (they were a great, great pop band - don't let anybody tell you otherwise).
The effect sounds to me like a vocal sample that's having the pitch changed downward - somewhat like a glissando. As the pitch alters, the samples become more and more coarse. It's not an analogue effect, my guess is it's digital with a touch of reverb or echo in there too perhaps?
Maybe it's an early version of the pitch corrector made infamous by Cher...
There are enough Abba fan sites, perhaps you could post the question over there?
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BluesMeister