The PCB was sold as 'for parts only' but described as 'fully working'.  First task was to find out if it was working, and if it would run without the rest of the boards connected. 

The motherboard contains an H8S CPU and the TMS320C55 DSP, along with RAM, ADC, DAC and ports for everything except microphone, headphones and SD card. 

The DC jack appears to be non standard, so I soldered a salvaged 12V 2A power supply directly to the board. 

Connecting the USB and powering up, the board is recognised as an ALSA MIDI client and a mass storage device. The SD card is missing as it is on a separate board with the mic input and headphone output.

aconnect -l
...
client 36: 'KP3' [type=kernel,card=5]    0 'KP3 KP3 _ S  

[37051.005966] usb 3-3: new full-speed USB device number 11 using xhci_hcd
[37051.163089] usb 3-3: New USB device found, idVendor=0944, idProduct=0020, bcdDevice= 1.00
[37051.163092] usb 3-3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[37051.163093] usb 3-3: Product: KP3
[37051.163094] usb 3-3: Manufacturer: KORG INC.
[37051.177248] usb-storage 3-3:1.1: USB Mass Storage device detected
[37051.177359] scsi host7: usb-storage 3-3:1.1
[37052.181804] scsi 7:0:0:0: Direct-Access     GENERIC  KORG KP3 R/W     0010 PQ: 0 ANSI: 2
[37052.182632] sd 7:0:0:0: Power-on or device reset occurred
[37052.183447] sd 7:0:0:0: [sdi] Media removed, stopped polling
[37052.184288] sd 7:0:0:0: [sdi] Attached SCSI removable disk

Awesome, it's alive.  I connected audio input and output hoping for audio passthrough.  Nothing.