Answering my own question: Yum Yum was founded in 1971 and expanded fairly rapidly, so I would certainly remember having seen them around. Whether I ever had their donuts is another question. I see they bought up the Winchell's chain, and according to Wikipedia there are some 126 Yum Yums and Winchells around the greater L.A. area now.
Most trips home included a stop at Winchell's after leaving the airport & before my final destination. Always it was for a crumb donut, and a few other options. Over the years the donuts became inconsistent and eventually not worth stopping for 
Winchell's was the go-to all around town then, or so it seemed. Then once I discovered Stan's in Westwood, nothing else would do. Except that when I had moved to Redondo Beach and wasn't around Westwood so often, I "discovered" Dunkin' Donuts which had a really nice eat-in place in a prime location in Torrance. It was on my way to work, so I'd stop in occasionally and enjoy the donuts and coffee at the counter or in a booth. I thought their donuts were fine then, even though they're ubiquitous and "okay" nowadays. But the great thing was the coffee -- served in a regular ceramic mug the way coffee was meant to be served, and it was delicious. No one (especially young people, but even people who are old enough to know better) believes me now, but that's how they got their reputation for the coffee. Yes, kids, we used to drink from a vessel in which you could actually taste it, as opposed to tasting the plastic lid and styrofoam of today's cups -- which even runneth over if you are not careful.
