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This is a direct copy of the original ARP 3620 electronics so requires a similar keyboard arrangement - 4 octave (49-key) with two independent contacts per key, with the CV bus contact bent slightly to make momentarily before the gate bus contact (to prevent the previous note triggering in error on a subsequent keypress). You could use a 3-octave by placing a 1K2 precision resistor at one end of the CV resistor chain to make up the total 4k8 (48 x 100R).
I used a new keyboard assembly from Kimber Allen in the UK with twin gold contacts, and I know at least a couple of others building this have ordered the same. It is a little pricey though at £296 + VAT.
Other choices would be an old keyboard pulled from a scrap organ, and again Kimber Allen keyboards are a very popular choice for those too - they have been making organ parts since 1928.
I've attached the information pack which includes the original service manual etc. The schematic drawings shows how the key contacts should be wired, but if in doubt please ask.
Groupbuy for Panel and pcb:
Documents:
Attachments patterns Documents
Building:
you need the PCB and Panel from muffwiggler groupbuy, a keyboard, case, and parts from BOM (resistors, capaciators, semi)
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