Circuit Overview:
The last effort of the ARP engineering team, this synth adds MIDI to the previous "Chroma" interface, and uses albeit lo-res interface, real sliders to control the data input. (Eg. the resonance is an 8 position switch. Others are better fortunately.) This machine uses an actual ADC instead of the comparator stepping used in many other analogs like Siel DK600 for example. Values are multiplexed in from the controls to be converted to digital values which the processor then directly compares to the value stored in the initial knob reads I guess when patch is loaded and if anything is moved then that value is immediately slapped into patch buffer which then is read to the DAC during the s+H cell recharge cycle for that particular item (eg. resonance). The CP has an ADR envelope for VCA but has a 5 slider envelope for VCF making this machine a little more interesting than typical ADSR x2 synths. The fantastic 'assignalbe' knob has two halves on many of the parameters including pitch bend which allows you to on the upper half select the standard bend amount. On the lower half though it's bend amount ONLY for the notes currently held. So you can use the pedal and hold a chord with it then solo and bend just the solo note for example. You can also set a 'glide' amount and when you hit a chord and hit the damper pedal the next notes will be monophonic stack of remaining oscillators with the glide! Fantastic performance synth with these features.
Anyway 6 voice 2 oscillator per voice from the 6 dual vco CEM3374's. Ring modulator is preset depth but really nice. Sync. Pulse widths can be swept out of bounds which creates a rather gnashy sound of unpleasant nature so watch for that. One might think something is wrong with the machine :-). I've never used the simple sequencer and in fact mine is fouled up or something. AFter working on it I get an error honk as though memory is full all the time though I've emptied it completely! When it honks the stop light quickly toggles to play I think or record and back. Strange. Doesn't affect the sound fortunately so I just turned the honk down and live with it for now.
Service Tips:
I've seen op amps fail in various places. Power supplies have been problems so always verify rails of course. The membrane switch panels are notorious for cracking, but also several switches may cease to function due to a break in the line somewhere. I'm an expert at fixing these problems and can almost guarantee success. It's not a trivial proceedure to teach someone and depending on the situation there are a few different methods which may need to be employed so just send it if you need it done. I just did my own unit and it took about 4 hours! Arrghh. That's by far the worst one so far. But they all work and will forever or until the plastic turns to dust now.
The later models beware. The pots listed in the manual to be trimmed for voltages are wrong...it's R4 and R15 I think on the power supply board. Also the DAC control is R45.
Parts:
I think I have pretty much every part electronically for these save the exact CPU. I imagine higher speed 286's would work. Shouldn't be..real hard to find one :-)
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