Divide down tone generation machine. Typical use of electrolytic capacistors for an A-R envelope for each key. Typical dual OTA used in paraphonic filter circuit (ie. one filter shared by all notes and triggered eithen after all keys released and another key hit or with every key hit. In this case I think it's probably similar to OR400/Orch 2/SCI Prelude in this way, but the releases are truncated when new notes are hit making it better in some cases but less smooth sounding. This machine was also marketed as the ARP Quartet and the main thing to note is that the switches aren't labelled the same. Particularly troubling is the "mute" on the orchestra which is labelled rather 'Brass Sustain' on the Arp Quartet! I really don't understand this.
On my ARP unit the brass and piano can't be mixed. Brass is just defeated where those are mixed. However in multiple mixes I believe it changes the enveloping to have the brass included. But there seem to be a total of 11 meaningful combinations timbrally of the 4 buttons. ie. the full 7 combinations of the right three buttons plus brass alone plus brass and string plus brass and organ plus brass/organ/string. Of course the individual footages can also be altered within those mixes to give even more diversity as well as envelopes when the lights are lit indicating those are active.
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 have several scraps from Siel units and lots of the discrete parts.
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