I have an electone B405 and the effect you describe is a kind of chorus effect. It produces a warmer richer sound on the strings which otherwise sound thin and reedy.
All PSR keyboards featuring a DSP can emulate this effect, its called (believe it or not) Symphonic chorus and essentially produces pitch shifted versions of the sound and then mixes them together to produce a richer effect.
Infact theres even a harmony effect on the yamaha psr's, similar to the good ole melody on chord feature of the electones. You can add all sorts of chord and harmony effects to any voice, according to the chord you play in the accompaniment section.
Of course the sounds of modern keyboards is considerably different to the ole "organ" sounds of the 70's and 80's.
Some of the upmarket arrangers such as the PSR740, 8000, 9000 or 9000 pro have organ flutes, similar to the drawbars where you can combine various floot footages and attack sounds to produce a range of organ sounds.
But back to your strings, yes, you can achieve the symphonic chorus effect on most modern arranger keyboards.
Regards
Simon
SVPworld
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Simon G.K. Williams
simon@svpworld.com
Creative Music & Multimedia
http://www.svpworld.com________________________