DCO machine with analog filter. The HZ-600 came first apparently to be followed by the versions with speakers included and auto-accompaniment feature. The SD (Spectrum Dynamic) synthesis is some kind of pulse additive scheme it appears offering dynamic interaction in the process. HT-700 has 4 octaves of smaller keys, HT-3000 full sized keys, and HT-6000 (released in'88) is 4 dco/voice 8 voice to take the most dco's record. And someone confirmed there are indeed 8 JRC2090 VCF's in there. (5V headroom on those, see link under modifications above for some details.) My little HT700 is charming! 8 voice poly and there appear to be two filters. One for chord and one for melody voicing. Haven't figured it all out yet but have messed with paramters and you can ..make some great sounds! 1st class machine! I fell into some compelling grooves using some of the patterns that really are good backbone tracks immediately. You'll be hearing from this bad toy :-).
I got one with a power supply that didn't fit the jack right. Those casio's use..what is it 1.7mm or something instead of 2.1mm. It'll act like the solder's broken but it's isn't :-). Those JRC2090's I can't find much info on. Imgine if they break it's a pain to find a new vcf possibly. Could be they use the typical design of the era with OTA's in series like the Korg Delta filter on one chip. But Casio likes to use their own parts it seems and when they break that's it. So I wouldn't count on it being economically feasable to repair these if anything bad happens. Some of their stuff will even blow a cpu if power gets inverted I found recently with a 1990 ish portable piano unit.
Let me know if there are common failing parts in these.
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