Circuit Overview:
This is the same as the Siel Orchestra 1 synth, a divide down oscillator synth with analog filtering and a simple brass filter control.
Service Tips:
Same again as Siel Orchestra. The main things to check immediately are the interconnects. Simply pull up on the wires and they will slide right out of the pinch clips in those headers. These stiff wires are no good. BAD idea people!! :-) One had 4 of 6 broken on the ARP version I saw. You can truncate some of them but the short one I'd just replace as I did with a more modern cable. Strip a typical mackie ribbon cable back for instance a full inch. Then tin that whole length so you can reliably shove the length down into the interconnects. Once these are good most of the functions should work provided the power supplies are good (check where they're marked with voltages on the "F" board on bottom rear. ) and the TOS is outputting signal. Check for signal on the 1458 at the front left of the "F" board for stepped saw waveforms rather than pulling keyboard up if you don't have to. The unit I saw had a very weak brass signal. The diagrams I got are bad copies...some numbers smudged. SO it was a nightmare getting to the bottom of it. The MUTE=Brass Sustain on the ARP version also which is confusing at first. The Italians were apparently referring to muting the envelope :-). When that switch is on it should short the 120K resistor Rb28 (they put the board designator in the middle of the part designator note, so that one is on the "B" board which is on the back panel to the right.) There was a resistor from the trim pot on the B board marked 390K on the diagram that was 680K on the ARP version. I changed it and it gave me more headroom on the weak sound I was getting but eventually found the real trouble. A crack in the brass attack pot. Also I assume as with the Orchestra 2/SCI Prelude, it's capacitors may have drifted so that decay times vary quite a bit.I generally try to clip from the top so I don't have to pull the whole board if it's just one of them, and carefully quick solder onto the wire on top.( Pry as much wire out of cap as you can to do that)
Parts:
I have several scraps from Siel units and lots of the discrete parts.
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