I scoured the internet, and have even been in touch with the gurus who have been doing radio repair for a long time. Until now, there hasn't been any good circuit guides made available for the Hitachi KM-1520ZC that I'm aware of.
I took a chance on an eBay purchase of a 1972 KM-1520H radio manual (made for cars from Detroit, apparently) and it paid off. The circuit appears identical - at least as far as my amateur self got into it to identify a problematic trace in my own stereo. The faceplate seems to be the main difference between the models. I'd be interested to see if anyone else identifies any other differences.
I scanned and processed it, hopefully someone can find it helpful at some point as I did.
https://objects-us-east-1.dream.io/dox/KM-1520H.pdf
EDIT: Capacitor C38 is 3.3uF/25V on my radio - as opposed to 10uF/25v spec'd in this manual (for those doing electrolytic capacitor replacements)
