So did you dig up the circuit somewhere or work it out from scratch??
I never heard the NJD, as far as I know anyway. Is it based on one of those old sound effects chips that are like a mini synth in one unit? I found a wicked data sheet on one..you could pretty much get any sounds you wanted out of it by getting a few of the right components around it. Wanted to have a play around and see what I could come up with, but only found the chips from a japanese supplier at 12 notes a peice, minimum order 30x.