{"id":5319,"date":"2026-04-12T08:31:47","date_gmt":"2026-04-12T08:31:47","guid":{"rendered":"https:\/\/maffez.com\/?page_id=5319"},"modified":"2026-04-12T08:31:47","modified_gmt":"2026-04-12T08:31:47","slug":"roland-alpha-juno-mods","status":"publish","type":"page","link":"https:\/\/maffez.com\/?page_id=5319","title":{"rendered":"Roland Alpha Juno mods"},"content":{"rendered":"\n

I’ll focus on two mods here in particular: Chorus adjustments and variable modulation depth, and self oscillating filters. Might add other bits later, but these seem the most interesting ones to me. <\/p>\n\n\n

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See also here: https:\/\/www.modwiggler.com\/forum\/viewtopic.php?t=299416<\/a><\/p>\n\n\n\n

Service Notes: https:\/\/www.synthxl.com\/roland-juno-2\/#google_vignette<\/a><\/p>\n\n\n\n

Making the VCF go into self-oscillation<\/strong><\/h2>\n\n\n\n

It’s common knowledge that most implementations of the IR3R05 do not go into self-oscillation. According to Rob Keeble, if memory serves, this was done in order to prevent clipping. While the VCFs in the JX-8p, MKS-50, Alpha Juno 1 & 2, or MKS-80 cannot self oscillate, some implementations can, for instance Rob’s wonderful Eurorack version of the JX VCF<\/a>. So, to me this looked more like a matter of scaling than general incapability, and yep, you can make em go “ringggg” easily. <\/p>\n\n\n

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A86 “Noise Shots” presets with reso and then self osc:<\/p>\n\n\n\n

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The control signal is common for all voices, passing R74 (470 ohms) and being smoothed by C44, before it is distributed to the G8108605 resistor arrays at each voice. The datasheet does not show the resistor values for these arrays, but I traced them here: <\/p>\n\n\n

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Relevant for us here is the resistance between Pins 11 and 2, which is the resonance CV path. If you put an 68k resistor in parallel with these two pins, your filter can now go well into self oscillation. Actually, I reduced the amount a bit by replacing R74 (the common resistor feeding all arrays with CV) with a 1k2 resistor, so I got self oscillation at resonance value 125, which I found the least intrusive way. <\/p>\n\n\n

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You will find that the reso thins out the passband of the filter massively, so why do this? In some cases, a second (sine) oscillator might be nice, and in some cases you might wanna do jungalist basses like this one here, innit…<\/p>\n\n\n\n