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 :-).
Another interesting thing that is undocumented in the manual near as I can see, is that when you are in SAW mode on an oscillator, the pw slider becomes a SUB AMOUNT. If it's in the middle you get your root waveform only. Either up or down..it gives a sub octave amount! Gee...that's a pretty significant feature to go undocumented. :-)
Service Tips:
As noted in service manual, one of the biggest issues people have is using the tape interface. I keep forgetting how so I'm going to document one workable way. They recommend going to cassette (lower function "C") and turning off 3 and 4, which turns it from digital to audio (recommended) and from high to low level (not recommended). Leave light 4 on in other words if you are using a sound blaster live like me or probably most other sound cards out there. Turn your volumes all the way up on wave and output in the computer's audio controls. Get a cable from your speaker out to the 'in' sync of the polaris and you should have success after hitting 'upper functions 12' or 'load all'. After you get done loading programs you hit it again and run the file with sequences in it.
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, and 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. Shipping is cheap when you removet he panel. I can test it here without the whole synth. 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.
A service manual is very helpful on these units. If missing voices though, hey, try a 'tune all' :-). Sometimes they're SO far out of bounds that you can't hear them. ALSO the manual was weak on tape dump verify. #11 just flashes if all is going well. It sounded like you were supposed to have it tracking through the patch LED's but that only happens if you do an actual load.
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.
NOTE: Regarding CEM3372 VCF's. We had a recent bit of experience trying to get these to work in Doug Terrebone's Prophet 600 and in one I had here. None of the stock I had would sound the same (not to mention that two I got from chipforbrains some time back were totally doa... bummer. Thought those were NOS!) They lacked the whispy overtones with Resonance all the way up in particular. THey were all "C" revision. The "B" rev's were in the P600's but Doug said he finally got in an AX80 that had Rev "C's" and those worked fine! Go figure! Anyway just beware that there is some variation in these chips.
I'd previously gotten some confused information about the reversal of designated trimmers. That doesn't seem to be true. Anyway I've had several recent repairs on these nearing the end of 2009. A couple of the strangest ever. Ryan Powers witnessed something that was quite interesting. This is why I love having people around when I work. We first did the membrane switch ribbon mod on all four cracked cables. We got the insulation off it appeared well. Three of the cables worked fine but the left side 7 connector one had 5 bad lines!! I pulled it after determining that it wasn't just dirty switches which I've never seen on these actually. And recleaned it a bit more. GREAT! NOW NONE OF THEM 7 WORK!! We kept recleaning it for an hour or something. Totally insane. None ever worked again. I Told Ryan "Sometimes..I have to pray for things. It's..just me...." Well I prayed and felt led to swab it lightly one more time and surface scrape it one more time and put it back in. ALL 7 WORKED AND NEVER FAILED AGAIN!
The second problem which I'd tried to address was that the ADR Release slider was acting like a channel volume! Now that function, as we know, resides on the assignables slider. And I thought..mmmayybe the 4051 is somehow..bridged there? I replaced it and it worked! Until..Ryan came back when it immediately made a fool of me and started acting exactly as it had been when he'd left for a bit. Now the assignables slider had a broken shaft but I checked the resistance measurements and it was actually working fine. But I prayed again and felt led to simply replace that slider and put things back together. It worked fine and never manifested that again. Does ANYONE have a theory as to what was causing that? Please...don't say 'supernatural beings'. I already have surmised that this is the most likely cause :-)
Tonight I worked on a Polaris II. It has a problem of 3374 number 2 having one VCO off tune by about a half step! All others would tune-all just fine. I replaced the vco chip..no effect. I replaced the comparator since the thing uses the square waves generated by it to tune I believe. Still fouled up. I'd cleaned the socket and I'm wondering if something didn't eventually help a connection or bad solder joint because I hit tune-all again and finally they all just tuned. Probably should redo the solder on that one. There was a little rust on a couple comparator pins near where they join with the plastic package.
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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