Just to bring things back to the original topic, here's a close-up of the off-the-shelf Holtek HT37B90 MCU from the SA-46 (first released 2010, still sold today).
Mine has an embossed datestamp of 2019-2020, so definitely not the original production run, but they probably all use the same MCU.
So that one is Holtek too. I own a Casio SA-76 with similar CPU.
CPU = "HT37B90, CCTECH CD-1, 94V-0 [9 .6 ], MX818-MA4M A" (36 pin COB)
The COB module layout itself differs, so the external pin names are in different places.
Interesting is that of SA-46 and SA-76 exist each in a 2nd version (SA-47,SA-77?) which AFAIK only differs in the sound of the separate "piano/organ" button (has "piano/harmonium" for the India market?).
I finally ordered a 4-channel digital oscilloscope (Siglent SDS1104X-E, in unhacked state 100MHz) before the euro becomes worthless. Will this be a sufficient tool to analyze 1980th/90th keyboard soundchips?
(I also got a 16 channel logic analyzer module for my laptop. Let's hope that the now coming nuclear war won't fry everything of semiconducors by the high altitude EMP nor incinerate me together with the city.)