I’ve been curious for quite a while about the Bot, especially with this thread in the back of my mind, so eventually went for a second hand Cyclone Analogic TT-303 MK2. As far as I know, nobody offers commercial mods for the MKII, so here are some modification descriptions.
This page is an *open-source* DIY project, meant for 303 aficionados with a Bass Bot and interest in moding it. If you like this stuff, think about donating a little to a charity helping refugees. If you make money of the info given here, donate to a charity helping refugees.
I plan on expanding and refining the info given here a tad over time, do also check in with my TD3 and Geosync TB-EFA modding pages, especially if you are a beginner. Part numbers and layouts will be different, of course, yet this can help you gaining some further insights into 303 modding as the principles are the same. As usual, the xoxbox wiki very helpful in this context too!
Very generously, @Green Dino offered their services as a proofreader, which helps in counterbalancing some of my idiosyncrasies or nincompoop spellings! Thanks!
Some general info
The revision I have is R2V1.3, designed by Rezonance Labs in 2016. CPU is an STM82S207-R8T6, with a 14174 hex flip flop interfacing 1v/oct and slide/accent switching. PSU features a sumida coil. Slide, Accent (and I think Hammer and Mute also) are done via 4066 IC switches. VCA is a discrete BA662 replacement. Headphone amp is a LA4140 (with a dedicated +6V supply rail). There are three trimmers: cutoff, resonance, and +5.33v regulator.
The MK2 is a tad tight, but there is space on the side panels and in the following pictures you see areas marked under which the PCB is not too busy.
Some notable aspects before turning to mods: the VCO on the TT303 has an auto-tuning feature (seems TM4 and TM5 equivalents to the TB303 are IC11 and IC15, which are digital pots). Q3 (equivalent of TB-303 Q8) is an 2SA812-M7, i.e. high hfe transistor. Most notably, VCO and audio input are buffered per opamp (IC2) before entering the VCF.
Square PW adjustment: R19 (22k) replaced by trimmer/resistor. I settled on 25k as a replacement but you might find other values nice(r). NB hollower square = less volume on square.
1V/OCT CV Input: tested with erica black sequencer, works! Pitch CV that tracks 1V/Oct can be fed to the left hand terminal (facing away from IC7). No resistor is needed, but NB that wiring a CV input directly does not offer any protection against potential hazards, so using a simple op-amp buffer (gain x1) makes absolute sense.
SLIDE time: slide is analogue, with Q15 and IC5 regulating the on/off-switching. Since you can set the slide time on the TT303 per software as well as manually engage slide when playing a pattern, I am not very interested in pursuing a mod here at the moment.
Pre-filter overdrive: On the TT303 MK2, there is no exact equivalent to TB303-R62 (220k). After the wave select switch, signal passes R13 (22k) and R14 (100k), then goes via the VCF input socket (switched socket) through R22 (100k) and C9 (1uf) to the filter. Now, the classic TB303 mod of reducing one resistor doesn’t really work here. Eventually, I just went for bridging R13+R14 with a 1m pot, but this does not give you strong overdrive, just works conveniently for level compensation and some crunch.
For the audio demos below I went a different route and put a 1m pot +10k resistor in series between C7 and C9. Now, that’s some vrooommm…
One side effect of going the C7/C9-route is that you can also mix the internal VCO with external sound sources (normally, plugging a jack into the filter in socket silences the internal VCO). There’s always a bit of internal VCO signal, so this may need a tad optimization, but works well if you don’t want to use an external mixer.
FFM from VCA: anode C66 via 100nf capacitor and 1m pot + 20k resistor in series to base Q27. C66 picks up a bit of hum on headphones, not so much on line out. Might need to improve that. Also weird: as I know this mod from other machines, a 250k potentiometer would be sufficient to conmpletely cancel FFM when turned fully counterclockwise, yet here, even 500k is not enough – maybe tapping the wrong capacitor…?
FILTER CUTOFF CV INPUT: input jack via diode and 100k resistor to base of Q27. As always with 303-based circuits, this may be a tad unexpected since filter cutoff is not referenced against 0V, so there’s some offset. Yet, I find it useable enough and have it installed on mine.
FILTER CUTOFF KEYFOLLOW: this one is one of my current faves. Tap the internal pitch CV output at the anode of D3 (the terminal facing R31) and feed it through a 1m pot and a resistor in series. With a resistor value of 100k you get 1v/oct tracking of the filter cutoff, but I like an even steeper spread, so I use 33k. Feed you keytrack CV to the base of Q27.
Lower cutoff: use trimmer.
Increased/decreased resonance: use trimmer.
VCA CRUNCH: remove R129 (220k = TB303-R128). This raises the output volume of the VCA and on louder/accented notes, you get a wee bit of crunch. Without R129 the timbre is quite a bit brighter, right?
Bass Boost: augment C35 and C36 by 100nf each. Will probably not implement this, but here is how it sounds:
VCA CV INPUT: Input jack socket goes via 47k resistor to the VCA_CTRL test point just above the mode selector rotary switch. 100k for your resistor would conform to Eurorack “standard” and also work well, yet I find 47k gives me better results for audio rate AM.
AM from VCO: the MK2 has a VCA-CTRL test point, which is just under the accent control. Fed the VCO output from anode C7 via a 1m pot and 20k resistor in series to the VCA_CTRL test point– makes the Tt303 quite loud!
Variable VCA decay: R126 (1,5M) replaced by potentiometer/switch. When R126 is at zero, only accented notes will sound.
Change minimum envelope decay time: R84 (68k) bridged/replaced by resistor. IMO this makes sense in conunjunction with variable accent decay.
Increase ENV AMOUNT: the ENV amount you can dialling in on the TT303 is relatively moderate, so increasing the maximum amount might be what you’re after (I keep mine as is). For doing so, reduce the value of R75 (220k = TB303-R63). Here’s a comparison with an additional 330k resistor in series with R75.
BYPASS ENV & ACC on VCF: bridging R70 (2.2k) switches any modulation amount by envelope an accent to the VCF off.
ENV SLEW: this is one of the tinkerings I “developed” for the TD3 and I like it here too. Put a 2.2uf capacitor between ENV AMOUNT POT PIN2 and ground via switch. On lower ENV amount settings this makes your envelope go bwoww.
Variable ACCENT decay time: 500k potentiometer between D6 and D7. Not yet tested.
ACCENT slew: not to be confused with the legendary accent sweep time mod. Put a 1uf-4.7uf cap between the cathode of D16 and ground via a switch. When engaged, this draws out the effect of accent on the filter, with subsequent non-accented notes wobbling. Sounds like this (2.2uf cap in example):
Gate ON forever: tie D10 cathode to +12V (at PSU section) via switch.
VCA ON forever: tie D13 cathode +12V (at PSU section) via switch. Alternatively, take +6V from the PSU test point via a 10k resistor and connect it to the VCA ACC IN test point above C51.
ENV Retrigger (not yet tested): momentary +12V (less should work too?) to cathode D5. Used when gate on forever is engaged to retrigger the main (VCF) envelope.
Gate Input: new gate in socket via diode and 22k resistor to base Q33.
Note the +6V (headphone amp supply) and +12V test points available for tapping. R172 is the equivalent of TB303-R172, but tbh I’m no very much interested in tinkering with PSU starve mods here. TM3 is for adjusting the +5.33V rail.
and some shaky schvideo