Nikko STA-5010. FM stereo demodulator is a UPC554C chip.

Circuit diagrams somewhere on www.covingtoninnovations.com/audio.

Thanks!

Hey, I've got a Nikko 5055 out in the shed. Are these things considered
respectable? I've thought about giving it away several times, but can't
quite make myself do it.

Yes, I think so. They weren't as widely sold as Pioneer or Marantz, but
they had a following. Someone sold a mint-in-box Nikko 5055 on eBay
recently.