MS-101 questions?

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MS-101 questions?

Post by guest » Sun Feb 09, 2020 5:33 pm

someone is lending me an MS-101, so of course the first thing i did was open it up. ill have it on my desk for a few days, so if anyone has any questions about the internals, feel free to ask. a few things i noticed:

1. there is only 1 poly cap inside, and its on the LFO integrator (any ideas why?? i cant see the extra cost helping in any way here).
2. almost all opamps are replaced with 4580s. those have very large bias currents, so arent used in the LFO and a few other places.
3. the V3340 is on a daugther board. i have to imagine they were having failures on those parts, and wanted to individually test them before putting them in the synth.
4. the VCF uses JFETs (MMBF4393) followed by the 13700 buffers, similar to some roland filters but not the IR3109.
5. the VCF expo converter is a PNP pair (DMMT3906) with a PNP cascode follower to reduce early effect related problems - although this adds beta related offset issues.
6. the thermistor for the VCF is no where near the matched pair.
openmusiclabs.com

KSS
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Re: MS-101 questions?

Post by KSS » Sun Feb 09, 2020 7:57 pm

guest wrote:
Sun Feb 09, 2020 5:33 pm
someone is lending me an MS-101, so of course the first thing i did was open it up. ill have it on my desk for a few days, so if anyone has any questions about the internals, feel free to ask. a few things i noticed:

1. there is only 1 poly cap inside, and its on the LFO integrator (any ideas why?? i cant see the extra cost helping in any way here).
2. almost all opamps are replaced with 4580s. those have very large bias currents, so arent used in the LFO and a few other places.
3. the V3340 is on a daugther board. i have to imagine they were having failures on those parts, and wanted to individually test them before putting them in the synth.
4. the VCF uses JFETs (MMBF4393) followed by the 13700 buffers, similar to some roland filters but not the IR3109.
5. the VCF expo converter is a PNP pair (DMMT3906) with a PNP cascode follower to reduce early effect related problems - although this adds beta related offset issues.
6. the thermistor for the VCF is no where near the matched pair.
guest, this is great. to hear these details from soneone who knows what they're talking about is wonderful.
Photos are always useful.
1) did it solve a PCB layout problem? reading "poly" as polystyrene, and therefore large and able to leap tall.. If "poly" instead means box film, microphonics would be the only thing I could think of. LFO having more resonant potential to be affected?
3) same as 1) could have solved a layout problem too. TTSH for ref.
6) somehow not surprised.

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Re: MS-101 questions?

Post by guest » Sun Feb 09, 2020 8:23 pm

yeah, its a box film cap. i feel like the LFO is probably one of the least sensitive waveforms in a synth. the 3340 is just floating on a small daughterboard right above an empty part of the pcb with a small outline of the 3340.
openmusiclabs.com

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