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The Crumar DS2 is an unexpected jewel in the rough of Italian synths. it has a unique Dual Digital Oscillator architecture with dual LFOs, a 3080 based 4pole filter, white and pink noise, big chunky knobs, a layout that focuses on modulation, and a poly section that sounds surprisingly good.
Manufacturing errors:
Somehow, many DS2 were assembled with incorrect potentiometers on the filter cutoff and resonance, severly compromising the performance of the filter.
Due to an error in the silkscreen design, some of the original knobs also only go up to 9, Which is a great shame.
The correct values are:
filter cutoff: A50k
Resonance:B100k
Installing these potentiometers will greatly improve the filter performance.
Mouser part numbers:
filter cutoff: 313-2440F-50K
resonance: 313-2441F-100K
These parts will also let the knobs go up to 9.5, not quite eleven, but much better nonetheless.
some other pot values for reference:
Poly Pitch: B2.2k
Pitch bend: dual B2.5k wire wound with a big long 6mm diameter shaft.
Here you can see the card stack for the Dual digital oscillators.
unlike many other DCOs these are not digital counters controlled by a cpu, the DS2 has no CPU. The oscillator is instead controlled by a trio of PROM chips that contain a lookup table whose contents are selected by the keyboard.
It also has a cute smily face, whats not to like!
It's quite busy inside, Certainly a good example of italian synth construction. the hinged Lid gives easy access for troubleshooting, but many of the boards are hard to remove for repair.
Some problems i've fixed in DS2s that i have repaired:
Dirty potentiometers: Some of the potentiometers are wire wound, so respond well to dissasembly and cleaning.
Broken solder joints: The DS2 is full of soldered wires, so this is pretty common. a particularly weak link are the connections on the pitch bend "nose" potentiometer, one DS2 had a missing oscilator due to this, and another developed bad connection halfway through the repair process, which was quite confusing.
Dead CMOS chips: The DS2 has a lot of Cmos logic chips and it's getting quite old, so don't be surprised if they die randomly. The DS2 also gets quite hot inside, so that doesn't help. luckily the DS2 has no custom chips inside, apart from the PROMs that containg the lookup tables for the Digital oscilators, the contents of which are helpfully printed in the Service manual.
Dead Opamps: two of the CA3080 opamps in the filter section
Capacitors: There is a 1uf 16v tantalum capacitor across a power rail in the PSU, waiting to pop one day, probably a good idea to replace it with a 1uf 16v electrolytic. Since it was made in the late 70s, it's probably a good idea to recap the PSU and filter/amp board just to be safe.
Filter Mod:
Theres a quirk with he design of the filter in the DS2, a capacitor in the last stage of the filter is 22pF rather than 150pF as it is on the other three stages, changing this 22pF capacitor to 150pF improves filter response, especialy at high resonance settings. this mod is sometimes refered to as a moog filter mod, but it's not really, it does make the filter similar to something like the farfisa soundmaker
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