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DIY: converting a Roland 100M low-pass filter to high-pass

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As a public service, I'm making these images available since the original host (100m.org) seems to not have them any more. I take no responsibility for this mod going wrong, I'm just presenting the information.

A look at the circuit and a view of the changes on the board layout.

Photo-image of the board with the mods done.

Basically, this is a map for converting one Roland System 100M Low-pass filter (i.e., one-half of a 121 module) into a high-pass filter. I've done this mod to one side of one of my 121s and can verify that it does work, and sounds pretty good, if you like that sort of thing.

CAVEAT: We have had one field report concerning this mod and certain 121 boards: If you open up your 121 in anticipation of doing this mod and discover that (1) your 121 filter board has a green solder mask on the back, and (2) there's already a wire link where the mod here shows replacing R36 with a wire link, you will very likely have very little resonance once the filter is a High-Pass, and the resonance control will do nearly nothing. So, I guess not everyone can take advantage of this mod. Be aware before hacking away!

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