Sounds from the K250 largely, using 7x oversampling and floating point to gain more sonic depth. Purely Digital with specialized chips with "arnold" on labels that perform the 'engine' work. It's good to note that the original K1000 lacked after touch but the "SE" version added that. Also the PX, HX, SX, GX, expander modules were available to add pro, horn, string and guitar sounds in separate racks. SX and PX were combine to make "AX" acoustic expander eventually. And an internal expansion was available for each also I believe. But anyway eventually the K1000 SE II units came out with version 5 EPROM and they supported the more advanced controller features found in the later pro 76 and K1200 which contained the K/PXB ROM sound expansion. THe K1200 just added more keys and user RAM over the pro 76 from what I can find.
"Arnold" chips have been known to have socket problems. Earlier black sockets replaced with brown ones. I recently worked on one where someone bent the pins and that led to some real interesting delay effects. Bizarre. I could probably put a switch in to recreate it and sell it as a 'circuit bent' thing on ebay :-). BE FOREWARNED that there can be some difficult to remove chips with solder sucker etc. Heat gun is the only thing I had that worked on the 1000PX. I was getting a short on the 5V to ground of around 25 ohms.
Kevin Lightner (Synthfool) had Arnold chips. I have a lot of the other parts for these including a pot and knobs that are close enough but not exact. I believe. Round the edges :-).
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Just in case this information ever vanishes I'll back it up here: Module and Keyboard Evolution Identification This list includes all the modules and keyboards (and the various upgrades) that we know about. The parameters p1 through p4 are the hex values one will get if they do a MIDI system exclusive request for the product information. (See the Kurzweil 1000 Series Developer Information in the manuals page.) Some of these are not documented anywhere but here! Each of these could have had two or more versions of the operating system (OS) - fixing bugs or changing features. p1 p2 p3 p4 Unit $15 K150 $19 K250 or 250RMX $64 $01 $00 $00 1000PX (8 button Professional Expander - The first module marketed) $64 $01 $00 $01 PXA (8 button with A Block upgrade - Upgrades were sold by Kurzweil) $64 $01 $01 $02 PXB (8 button PXA with B Block upgrade) $64 $01 $01 $01 PX Plus (12 button with PXA sounds) $64 $01 $00 $02 PX Plus w/B Block (12 button PXA with PXB sounds upgrade) $64 $01 $02 $00 1000SX (8 button Strings Expander) $64 $01 $02 $01 1000SXA (8 button with A Block string upgrade) $64 $01 $03 $00 1000HX (8 button Horn Expander) $64 $01 $?? $?? 1000HXA (8 button with A Block horn upgrade) $64 $01 $04 $00 1000GX (8 button Guitar Expander) $64 $01 $05 $00 AX Plus (12 button Acoustic Expander with SX + HX sounds) $64 $01 $?? $?? AX Plus (12 button AX Plus + SXA and HXA upgrades) $64 $01 $05 $02 1200 Pro (12 button Pro - Not sure what module gives this response? ) $64 $01 $07 $00 1200 Pro 1 (12 button PX with all upgrades and new OS features) $64 $01 $08 $00 1200 Pro 2 (12 button SX + HX with all upgrades and new OS features) $64 $01 $09 $00 1200 Pro 3 (12 button GX + HX with all upgrades and new OS features) $64 $02 $00 $00 K1000 Keyboard (PX Sounds - The first keyboard marketed) $64 $02 $01 $00 K1000 SE Keyboard (Added After touch) $64 $02 $01 $01 K1000 SE/Ext Keyboard (PXA Sounds) $64 $02 $01 $02 K1000 Pro76 (76 key Keyboard with Pro 1 sounds and 48Kb RAM) $64 $02 $06 $00 K1200 Professional (88 key Keyboard with Pro 1 sounds and 48Kb RAM) $64 $03 $01 $01 1000EX $64 $04 $01 $00 EGP