The DS-2 is an enhancement of DS-1 with extra LFO and a polyphonic string divide down synth. First synths I know of to have what are labelled "DCO's". I owned a DS-1 which is quite rare and sold it to Doug at Synthark. All serial numbers I've seen have been double digit save one that was 120 something I think. On the DS's there's a wire from each key interfaced to a mother board on the left as you see in the internal photo. The cards on that have mostly common digital chips (40xx, 45xx, and 74LSxxx and so on) and a few op amps. Along with some 4016 analog switch chips. The front most card is power supply filters, second one contains the two 74LS221 DCO circuits w/ tune trimmers. (Note: The DS2 adds a poly section which has a third oscillator of the same design. This board under the keyboard is a very basic divide down organ and it's output is mixed into the VCF input when the other Two Oscillators and noise and an external which could be hooked up if one desires) These two DCO signals (which are really analog controlled digital oscillators) 'clock' various circuits. But let's go to the back of the card bay first. Here is where a struck note gets translated into a 6 bit code that can represent the 44 notes on the keyboard.
The 6 bit code is then 'translated' (using PROM chips programmed to give appropriate scaling) to a 12 Bit code. With glide off this value then immediately appears on the input of the "digital multipler" which converts this data to a clock signals to drive the waveform generation sections. Selected waveforms are generated using 4520 counter chips and proportioning resistors. ie. we're adding square waves that are octaves to approximate the various waveforms available. For a view of these waveforms watch my ds2 video. (Starting at 9 minutes in for that section). NOTE that I resolved the issue (bad 800K precision resistor) and indeed there should be no less than 15 steps perceived in the waveforms since they are produced by 4 bit counters.
The glide switch on, we have the introduction of a counter and a bit comparator which work together with a clock to step up and down as needed the 12 Bit value that had existed on the output of the counter towards the destination of the new note's 12 bit code. The 'clock' for this process was an a lovely innovative piece of design. A DAC circuit generates an analog voltage from the 12 bit value first. This dac output is routed (The line labelled TA..perhaps an Italian can tell me why?) to the keyboard switch presenting voltages there of about -3/4Volt for the lowest F to -8.2V for the high C. With the keyboard switch off a fixed ~5.7V reference is selected into the VCF frequency control circuit. And the DAC output also goes to the GLIDE slider and the current from this source drives a flip flop circuit scaled to give a nice analog portamento emulation I suppose. Rather than just a linear rate of glide it varies with the DAC output and this clock increments the counter til it reaches the target voltage as signalled by the big comparator circuit.
Once these waveforms are generated the DS2's circuitry looks much like any other subractive synthesizer. It uses 3080 OTA's in the VCF and a couple 3086 transistor arrays and a dual transistor and op amps in the vca. Standing off the main analog board there we have a few more 3080's used in the Delays for the LFO's to give a nice smoothing dimension for that aspect even! no cheesy switching on suddenly of modulations! Class act all the way. Playing the simple organ through the filter with the right settings is something special to me.
One thing totally omitted from the documentation near as I find, is the 'in' switch and how the range knobs are hooked up. VCF and VCF ADSR outs are from a whiteish and blue wire in the right side of the motherboard that the sliders are on for ADSR's. pins 3 and 4 going front to back respectively. THey got to the non-common pins of the IN switch. The common pin of the switch goes to the VCA RANGE pot top lug. Wiper on the pot goes of course to the VCA control mod in on the back of the filter/amp board. (Left most 2 pins on that i/c) Now also the VCF ADSR out goes to the top of the VCF ADRS Range pot and the wiper of course goes to via the green wire to the front i/c on the filter/amp board. So when the in switch is to the left, VCA AND VCF are BOTH controlled by the VCF ADSR (through their respective range controls of course). When it's to the right (in) then they are controlled by the separate sets of sliders.
Ok, first my DS-1 unit had an error. Well make that two errors that were the same. They did not put any pull down resistors on the "Sawtooth" (labelled SEGA in diagram) lines from the switches. Dark grey and red wires. You may add 100K resistors to ground whereever it is convenient to keep the thing from delaying 10 seconds when you switch say from saw to triangle wave..and then going though this phase where the noise combined with the 4069 chips logic threshold just switches back and forth between the two waves rapidly! I can't believe they released the synth that way!
OTherwise, the DS-1 is really built fairly well. I had no bad interconnects like in my performer. The DS-2 I now own went through a lot of corrosion I think (judging from the panel that i touched up...lots of rust on). I had some interconnect issues AND of course the notorious 'through hole' posts that corrode and cause things to cut out erratically etc. And I did have to clean some shellac looking stuff off some of the pins in the cards that have pin row interconnects. And all chips are socketed and pins on some of them tend to get black build up on them. A thorough cleaning and I had a working synth mostly! They use the type of sockets where it's easy to misallign a chip from front to back so that pins don't quite touch the contacts in the sockets. Watch out as you clean them. SOLDER JOB IS EXCELLENT in mine. LOts of hard wiring but all kept out of the way. A joy to work on compared to many synths. TO ACCESS, simply pull the two side screws and the three that are right behind the keys on the front panel. Do NOT pull the ones that go into the wood! The whole front with the wood sides hinges up for nice access! Then there are two screws underneath that allow you to pull the keyboard up. Don't pull up on the keys. Often they'll come loose as the glue is getting old and brittle. But goop adhesive works great to hold them down again if you have problems.
OH and another problem with my DS-1 was that the 1458 failed which is located on the D/A board. That caused the thing to just latch the first note hit when glide is on and basically remove keyboard control somehow. Anyway, just a heads up in case you see that problem.
CARD LOCATIONS:(this will save you a ton of time with schematics in hand. Some schemos seem to be marked as though they are two VERSIONs. This is not the case. First "Version" is analog stuff. hehe.)
DWG (Drawing...) 6- EG's. 2 ea. of this circuit on board attached to slider board. (error on 4016.
Pin 3 should be pin2 going to ground! Also a fixed resistor is attached to real pin 3 in series with release pot. Attack is upper center. Decay pot to the right and 5K one is sustain.)
DWG9 - LFO. Fc is apparently the frequency control. Don't bother looking for waveform switch dwg.
DWG3 - Filter, source mixer. LOcated on main board, bottom center.
DWG4 - VCA, output. Also located on main board. My output amp marked 1458, right rear most.
DWG11- LFO delay. This is the little boards that stand off from the main board by card cage.
DWG13- Noise generator. This is under the key movement. Remove bottom two screws and tilt keys up.
DWG14- Power supply. 4 screws hold down shield. Reseat chip while you're in there.
SCHEMA-A -Keyboard coder. Slot 7 (rearmost in card cage)
SCHEMA-B -Code conversion Slot 6
SCHEMA-C -DCO's. Slot 2. Transistors BC286 on mine.
SCHEMA-D -Digital Mult. - Slot 3 and Slot 4 (two identical boards both drawn on this schemo)
SCHEMA-E1, E2- Waveform Generation. Back center of unit with interconnected board. That's where I chose to add my 100K resistors noted above. Be sure to verify you have the right red and dark grey wires.
SCHEMA-F -DAC. SLot 5. BC308B transistors in mine not 208, and BC1738 by "K" lower right. K goes to code conversion board btw.
Slot 1 contains a card with just connections coming in from glide pot, etc. And some filter caps.
INFORMATION SPECIFIC TO DS-2 (but mostly identical to DS-1 above it appears):
Here is a listing of board locations/schematic labels from Giovanni Mocerino! Thanks!
P-431 Keyboard coder (schema A - slot 7)
P-432 Code converter (schema B - slot 6)
P-433 DAC converter (schema F - slot 5)
P-434 Digital multipliers (schema D - slot 3 and 4)
P-435 Master oscillators (schema C - slot 2)
P-436 Waveform generators (schema E1, E2 -Left rear)
---- And I add
I just worked on one of these. Lots of bad chips but this one had a lot of water exposure it appears. Sliders took a lot of cleaning and a few pots were frozen including the bender. Had to free them up with penetrating oil. The LFO board was so badly corroded it took many hours to repair. All new sockets but the chips all worked on it. These are well worthwhile machines though!
Also be forewarned that the digital stuff is NOT INCLUDED in the service manual I have! (With the exception of a schematic of one side of the main oscillator board; a circuit that appears shared under the keyboard for the string section also I might add. That schemo is on the block diagram btw.) Giovanni also drew out the P-431 section nicely and I'll make a shrunken/nicely readable copy of it available for download here!
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Semiconductors: and misc parts:
Lots of discrete parts but some oddball ones. Ask if you need a specific part.
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