This is the only piece of information I found on this organ on the web.
It’s an Italian made combo organ with 7 preset solo sounds, a few pre programmed beats, and an accompaniment section with bass and chords elements. I found it in the Haifa flee market and paid about $20 for it.
It had problem with the C key, which didn’t work in any of the octaves.
Organs work different than analog synthesizers in that they use what’s called a top octave generator. Instead of using voltage control to determine the oscillator’s pitch, a chip puts out all the frequencies in the top octave range of the organ, and each of these frequencies is then divided down creating the rest of the octaves. Also, instead of using a VCA to turn the notes on and off, the keyboard acts as a set of switches, where each key allows the corresponding note to be mixed in.
Having that in mind the prime suspects for the missing Cs were the divider ICs. To test this I switched them around and sure enough a different key was missing in all the octaves. Switching out the defective IC with another one fixed the problem. The frequency divider used in this organ is the SAA1005P. I just checked ebay, and the only one currently for sale is $50.. jesus.. lucky I had one lying around in my ICs box.
For a while the organ rested in my closet as I have an Oberheim OB-3 which I always use when I need organ sounds. But reading more about organ architecture and realizing that all the old string machines were based on organ technology I decided to add a few mods to the solo section and make it a little more useful.
The modifications are: external control for the original internal vibrato, separate outputs for the ‘rhythm/accompaniment’ section and the ‘solo’ section, a fully controllable variable mode filter section (Thomas Henry’s VCF-1), and an LFO (with both square and triangle waves) to modulate the filter.
TH’s VCF works very nicely with organ sounds apparently! The LFO wide range makes it possible to come up with some seriously modulated sounds – it’s an altogether different instrument now, and I can still get all the original sounds out of a separate output. I’m considering adding a similar mod to the accompaniment section and adding control to the on-board drum sounds. But maybe that’s going overboard.. dunno.
Here is a short demonstration of the mods in action. I wish I had a section in the video with the original sounds and drums/accompaniments in it. If I mod it further I’ll make sure to include those as well!