{"id":4457,"date":"2024-02-10T14:19:35","date_gmt":"2024-02-10T14:19:35","guid":{"rendered":"https:\/\/maffez.com\/?page_id=4457"},"modified":"2024-02-14T09:39:27","modified_gmt":"2024-02-14T09:39:27","slug":"future-retro-fr777","status":"publish","type":"page","link":"https:\/\/maffez.com\/?page_id=4457","title":{"rendered":"Future-Retro FR777"},"content":{"rendered":"\n

Look who dropped by for a play date!<\/p>\n\n\n

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I’m just gave this FR777 a little spring cleaning (and some mods). While I’m at it, I thought why not document some bits and pieces since there isn’t much info publicly available. FR have some cool modifications listed here already<\/a>, but maybe I’ll add one or two. <\/p>\n\n\n\n

Here’s a medley of mods: <\/p>\n\n\n\n

VCO Sync, Filter Overdrive, Accent Decay (EQ &Reverb added):<\/p>\n\n\n\n

Disassembly<\/strong><\/h2>\n\n\n\n

Easy: pull of the potentiometer caps, and unscrew the seven switch nuts first. Then take care of the four bigger screws on the backside bottom and another four on top. Also unscrew the spacer I marked yellow from the bottom case as this is in the way of the PSU connector (only if you need to pull out the analog board PCB though!). The other spacers can stay in. BTW as all potentiometers are mounted upside down, you need caps that fit this orientation if replacing them.<\/p>\n\n\n\n

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The top panel should come off easily,and now you see two PCBs, 777-BASSIX Analog Board and 777-BASSIX Sequencer.<\/p>\n\n\n

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CIRCUITRY<\/strong><\/h2>\n\n\n\n

This is obviously and unashamedly a monster TB303 with extended functions, so for reference, first of all, here is the 303 schematic<\/a>. In some pics below, my yellow part labels refer to that schematic. As there isn\u2018t any FR777 schematic publicly available, and most part labels are underneath the components, so I\u2018m not sure if I will manage to make a complete inventory. At any rate: we get two aVCOs, including subs, three envelopes, an extended filter, and many classic 303 mods as extra controls in a sweet package. So, let\u2018s start\u2026<\/p>\n\n\n\n

VCOs<\/strong><\/h2>\n\n\n\n

There’s a CA3140 OPAt buffering CV from the sequencer, and a TC4066 for switching in slide. External CV in is limited by diode clipping and then directly forwarded to S1 and S2 Cv select switches. Now, on to the VCO core:<\/p>\n\n\n

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Exponential converter<\/strong> Q76 and IC11a of the 303 are replicated<\/strong> here, and so is the switch circuit comprising Q24, Q25, Q27<\/strong>. For the dual package NPN there is an NTE42, and NPN\/PNPs are 2n3094\/2n3096, with the exception of the equivalent of Q24, which is a 2N4124 (sporting higher hfe specs, which is recommended for nice saw waves).<\/p>\n\n\n

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What’s different<\/strong> is that the FET buffer at the output side of the SAW (Q28 on the 303) is replaced by an op-amp buffer<\/strong>, and that Q8 of the 303 (which derives the particular square wave from the saw) is replaced by a MC14584BCP hex Inverter Schmitt Trigger<\/strong> (two inverter cells per square derivation<\/strong>). I don’t know yet if this means that you don’t get the peculiar 303 square wave the pulse width of which is dependent on VCO frequency. <\/p>\n\n\n

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Both sub oscillators<\/strong> are generated with a SCL4013BE dual Flip Flop IC<\/strong>, and everything is mixed with JRC\/TL072 dual OPAS (both brands are used) before entering the filter.<\/p>\n\n\n\n

Manual Glide Button<\/strong>: Wire a momentary on pushbutton between ground and Pin 6 of the TC4066 IC at the VCO section. <\/p>\n\n\n

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