Look who dropped by for a play date!
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, but maybe I’ll add one or two.
Here’s a medley of mods:
VCO Sync, Filter Overdrive, Accent Decay (EQ &Reverb added):
rubberduck and accent decay
glide, zero glide time, accent decay:
VCO hard sync, external filter FM:
hard sync, manual slide, filter stuff:
Disassembly
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.
The top panel should come off easily,and now you see two PCBs, 777-BASSIX Analog Board and 777-BASSIX Sequencer.
CIRCUITRY
This is obviously and unashamedly a monster TB303 with extended functions, so for reference, first of all, here is the 303 schematic. In some pics below, my yellow part labels refer to that schematic. As there isn‘t any FR777 schematic publicly available, and most part labels are underneath the components, so I‘m 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‘s start…
VCOs
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:
Exponential converter Q76 and IC11a of the 303 are replicated here, and so is the switch circuit comprising Q24, Q25, Q27. 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).
What’s different is that the FET buffer at the output side of the SAW (Q28 on the 303) is replaced by an op-amp buffer, and that Q8 of the 303 (which derives the particular square wave from the saw) is replaced by a MC14584BCP hex Inverter Schmitt Trigger (two inverter cells per square derivation). 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.
Both sub oscillators are generated with a SCL4013BE dual Flip Flop IC, and everything is mixed with JRC/TL072 dual OPAS (both brands are used) before entering the filter.
Manual Glide Button: Wire a momentary on pushbutton between ground and Pin 6 of the TC4066 IC at the VCO section.
ZERO GLIDE TIME (Flickinger): that’s a mod described in the FR modification suggestions. Lift the terminal of R186 (100k) that is not connected to Q41 and insert an A/B switch that toggles that lose resistor terminal between its former place and ground. Notes with glide programmed will not glide but be gated 100%. This only makes sense when running VCA on GATE, not env. Manual glide mod overrides this though.
VCO Hardsync (VCO2 from VCO1): Yep, can do! Connect a 1k resistor to the SAW buffer output of VCO1 (Pin1 of the opa that replaces Q28 of the TB303), and connect the cathode of a switching diode (1N4148) to the base of Q6 (FR777 part number, this equals Q25 on the TB303). Connect anode of diode and resistor to an on/off switch. Because VCO B can be modulated by an envelope, this mod simply makes sense 🙂
Also try engaging cross modulation when sync is on – you get totally new spectra from your VCOs… Here is some external CV to VCO2 (synced to VCO1) while VCO1 remains silent:
Pre-filter Overdrive (classic Devilfish stuff): I wired an A1M pot and 10k resistor in series and put these across the 220k resistor that’s connected to C41 (filter input capacitor). I like this mod because it really f…cks up the sound but when you hit it just right, the saw bites hard. Mind you, there is an input level knob already but that’s rather gentle in terms of effect.
VCF
Now, this VCF is quite something! Total 303 at the core (with it’s alleged 3 poles I don’t get into this debate) and then some added poles for much more lower low-passing (such bass, very no high frequency! – sorry, In fact, I think is is such a rad tweak to the original circuit!).
I need to trace this a bit more (esp. frequency control and that CV warp modulator thing), but the basics are there. Filter audio in is with a OPA buffer and through the usual 1uf cap, with an extra level control for the oscillator (and noise) mix sum. The 3pole/7 pole switch, I think, simply selects which pole outputs are forwarded to the differential amplifier (Q21 on the 303). When the 7-Pole option is on, you get a steeper cutoff and stronger resonance. As for the latter, you also have your classic increased resonance modification on a dedicated potentiometer (Tb303 R97 mod).
Increased Env Mod amount (Flickinger/Devilfish): Classic mod here too. I reduced R131 (just underneath Q14 and left of the env mod pot) from 220k to 150k.
Increase Accent to filter amount (Flickinger): Decrease R130 (100k, right hand of Q48, underneath 3pole/7pole switch) to 47K.
EXTERNAL CV to Frequency Control: the person sending me their 777 stated issues with external CV IN to cutoff control; there’s a section in the manual mentioning CV in to FCV, but on this unit at least, the CV in socket is for the VCOs only. At least I simply could not find how CV would be forwarded to the CVTYPE/CVAMOUNT section. What I did in the end was to rewire the switched CV IN socket (normalled either to GND or +6V per jumper) to the sequencer output and the FCV section input to the TIP pin of the CV in socket. This way, the CV out keeps playing the sequencer notes but, depending if a cable is plugged into CV in or not, the filter frequency is modulated per sequencer or external CV. This also means that the VCOs keep playing sequencer notes even when you switch them to CV in first, but as soon as you plug an external CV cable, that connection is interrupted. IMO that’s the most flexible and easiest to remember way…
VCA
The VCA is a simple LM3080 OTA (compare obsolete CA3080, or modern ALFA 3080 replacement). Good enough, I think, and the characteristics when overdriven are sweet. I don’t think I’ll mod anything here.
ENVELOPES
There are three envelopes, all classic TB303 style, one for VCF, one for VCA (if gate is not desired), one for pitch to VCO2. While the latter two are fine as are, and offer extended options (VCA env), I wanted to have minimum decay time variable on a pot for the filter envelope. This makes playing with accent times nice and allows for notes to disappear completely. FR suggest repplacing R78 with a 50pot, but that’s to tame for me. I replaced R78 with a 500k pot and bridged R77 (22k), so that when my pot is at zero, accent is zero/minimum decay is zero. When I pull the pot fully up, accent can be longer than the normal envelope. For part locations see pics in VCF section.
Manual Accent Pushbutton: Wire a momentary on pushbutton between ground and this resistor terminal here shown in the pic.
Rubberduck: I found this mod when toying with the TD3 back then and in the meantime it even found its way into some commercial products by proper companies and stuff. What it does is that it introduces slew to the filter envelope amount output. When this mod is engaged and the ENV AMOUNT pot is at lower to medium settings, you get whopp and quack sounds. This is even more apparent if you switch between accented and unaccented notes. Pulling ENV AMOUNT up then, makes teh effect gradually disappear.
Simply solder the anode of a 10uf cap to the middle pin of ENV AMOUNT potentiometer. Then put an ON/OFF switch between the cathode of that capacitor and ground.
Right, maybe more later… In the meantime some pre filter overdrive and my rubber duck filter slew thing from Td3 days – there are some clicks when I engage the mods…
