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Behringer TD-3 Modifications Guide

(by Maffez and other slutz) – Version beta, December 2019

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Credits & Disclaimer

Many of the pieces of info and mods gathered here have been collected from pages on

classic TB303 alterations, dedicated XoXboX-ers, and contributors to the TD-3 modding

thread on gearslutz. Nordcore @ GS did an excellent job transferring the TB schematics to

TD part numbers for some sections of the TD-3 (especially the main envelope and the VCA

and its envelope). GS user El-Folie was the first to report and implement a modification of

the square wave in the VCO and also well documented some tests on the VCA envelope.

Although I transferred quite many classic mods from the 303 and the XoXboX, some I have

no interest in implementing. These I left untested, and they are marked brown in the

descriptions below. For one or two of them I was too daft to hear a significant different/find

the corresponding part numbers and indicated them as “*verification needed+”. Since my

main take on the TD is that it is a sweet little box that can be messed with, my focus is partly

less on making it sound closer to “the” TB303 sound (whatever this is in the end, given part

tolerances ect.), I also took the liberty to add a couple of my own mods that make the TD

sound less familiar, such as portamento, a filter mod and accent to pitch. And finally, and

most importantly, solder responsibly and at your own discretion.

Pages of interest:

http://dl.lojinx.com/analoghell/RolandTB303-ServiceNotes.pdf

http://www.timstinchcombe.co.uk/index.php?pge=diode2

http://www.firstpr.com.au/rwi/dfish/

https://www.ladyada.net/wiki/x0x/x0xd0x

https://www.subatomicglue.com/x0xl0g/mod%20guide/mod%20guide.html

http://23.235.199.139/~re303c5/forum/

https://www.gitarrebass.de/workshops/boss-ds-1-keeley-mod/

Pictures of the TD-3: https://imgur.com/a/MMNifIr

Demos of modifications: https://soundcloud.com/uibkmedan/sets/td-3-modifications

Td-3 Mod thread on gearslutz (with transferred schematics and frequent updates):

https://www.gearslutz.com/board/modular-mania-all-things-eurorack-and-modular-synths-

effects/1289500-behringer-td3-diy-mods-1.html

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Contents Technical information, Test Points on back of PCB, Part numbers Places for switches, sockets and potentiometers Notes on drilling and notes on SMD soldering VCO Envelope to pitch Accent to pitch Portamento VCF Pre-filter overdrive Extended filter mode Lower filter cutoff (when VR11 doesn’t do it for you) Filter tracking Higher/lower resonance Filter FM from VCA output Filter FM from VCO output Bypass ENV to cutoff VCA Bass boost C52 mod ENVELOPES

Different speeds for main envelope

Variable VCA decay

ACCENT

Change accent to VCF

Change accent to VCA

Change accent to VCA onset

Manual accent trigger

Variable accent decay

Accent sweep speed

FX

General info and tone mod

INS/OUTS

Gate input

1V/OCT input

Accent ON CV input (TB style)

Accent ON/OFF output (TB style)

Accent sweep output (Avalon style)

Filter CV input

Envelope output

VCO output

VCF output

VCA output

“Other” mods

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Technical Information

Part of the power supply in the TD is, like in the original, provided by an op amp (IC9). The

microprocessor handling the sequencer, midi, and pitch and gate, is an ARM 32P, type

GD32F350C8T6. Pitch CV, including slide, is generated by software in the TD-3 by a timer

output (10kHz PWM), with IC10 as filter and amp, not the CPU DAC (this was in via

Nordcore). Slide itself is scaled with tempo, which explains why it may sound “off”-ish to

some at higher tempos. A change of this behavior is a matter of firmware changes. The rest

of the VCO is realized according to Tb303 schematics. The VCF is a SMD recreation, yet with

very minor deviations (a couple of capacitor values). The VCA is not built around Coolaudio’s

new V662A but a comparable to the Open Music Labs clone of the Roland BA662:

http://www.openmusiclabs.com/projects/ba662-clone/index.html. Envelopes and accent

are faithful to the Tb303 schematics. The added distortion is equivalent to the Boss DS-1.

Test Points on back of PCB (brown needs further sleuthing)

TP01 GND

TP02 Main out (post-V14, pre “Mixer” network P5 303 service notes)

TP03 Power after diode 5

TP04 CV Output

TP05 Gate Output

TP06 Selected Wave Output

TP07 Accent on/off?

TP08 ENV Output

TP09 Filter Output

TP10 VCA Output

TP11 distortion fx something

TP12 distortion fx something

TP13 filter ladder first cap

TP14 VCO (saw out?)

TP15 15V (= top of R172 on front)

TP16 sequencer stuff? / from microprocessor

Frequently mentioned part numbers

Nordcore and I transferred some part numbers to schematics, but since those are

preliminary, I have not included them here. Best check the GS thread for updated one. A

work-in progress inventory of the Td-3 is also here: https://yourpart.eu/p/TD3_Inventory.

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Places for switches, sockets and potentiometers

Backside: There space for sub-miniature switches on the back just over the DC input socket,

USB and midi sockets. Over the decay potentiometer there are two e-caps, so unless you

relocate them, there is no space here. There is space over the pots of the distortion section.

Finally there is enough space on the lower part of the backside under the PCB.

Top: You have enough space for an insulated solder lug potentiometer or horizontal print

potentiometer in places where there are no capacitors underneath the chassis (see

backside). There is plenty of space in the area of the Behringer logo underneath the led.

The PCB allows for installing a second row of sockets over the existing one (you need to

scrape off a bit of the plastic spacer over the filter input socket if you want to have a socket

there too, but the rest is easy). Installing 3.5 mini jacks (Tayda) vertically is okay if you bend

the solder lugs. Common sub miniature switches also fit if you bend their legs.

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Front: Like on the RD8, there is quite some space between the PCB and the front of the

chassis, which means larger components can be placed here most easily.

Sides: Components may be best placed underneath the PBC. I implemented two trimmers

(glued to the underside of the PCB) for resonance and square wave pulse-width that can be

accessed with a small screwdriver from little holes on the side of the chassis. This way I

cannot accidentally change settings but don’t need to open the box if I want to adjust things.

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Notes on drilling

Before you drill always triple check your location! Will the component fit or are spacers,

capacitors or other things in the way?

Make a little dent in the center of your hole to be – drills can slip easily and scratch your

plastic surface while doing so. A little dent helps them staying in place.

Go from small to big: for plastic I personally like metal 2, wood 3 and 4, then metal/concrete

5 and finally metal 6 for jack sockets or metal 7.5 for big potentiometers. Use a reamer to

smooth the edges. If you don’t have one, a round file or even a pencil tip can be of use...

Drilling makes dirt and holes, so have something underneath your drilling area if you don’t

like holes in your kitchen table. Equally, drilling takes patience, and a wonderful insight into

being tranquil (and possibly on ketamine) can be had by watching Abel Ferrara’s 1979 film

Driller Killer, which must be watched *by all means* with Ferrara’s audio commentary on

(2005 DVD version).

Notes on SMD soldering

Components are small and can burn if heated for too long. Best watch some tutorials. For

adding caps or resistors to existing ones, I find it easiest to solder female Dupont wires to the

terminals of the existing components (small solder blob on the wire first and then fastened

to the component) and then insert capacitor/resistor legs to the female connector. This way

you can also change the values of your added components more easily. Unsoldering SMD

parts, on the other hand, works best for me if I heat the terminals alternatingly and when

warm enough lifting them up from one side first with my soldering iron. Maybe not super

pro but saves me a heat gun.

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SOUND MODIFICATIONS

VCO

Change pulse width of square wave: some TD users reported on the square wave of the TD-3

not sounding as “hollow” as the TB303 square. You can adjust this by exchanging R189

(22k) with a 25k resistor. Credit to GS user El-Folie for finding this. For replacement I’ll

settle on a resistor and trimmer in series, so I can fine-tune to taste.

Envelope to pitch: in the original, you connect as follows emitter of Q40 - diode - 100k

resistor - 100k potentiometer - junction of R88/R89 (from xoxbox wiki). Translated to the

TD-3 circuits, this would be TP8 to diode to 1M resistor to 200k potentiometer to Pin5

(non-inverting input) of IC10 (might need even higher values). You can also solder this to

the back of the PCB - between C47 and PIN5 of IC10 the trace goes to a solder point on

the back of the PCB which is located just under the “OSC” of “OSC SCALE”.

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Accent to pitch: why not go different? This may best be used with a “momentary ON” push

button. Wire either middle pin (for variable) or right pin (seen from component side of

PCB) of the accent potentiometer to a 1.5M resistor and a switch, then to PIN 5 of IC 10.

Alternatively, you can also go for the V/oct point shown in the I/O mods section later.

Portamento: wire a 0.1uf-1uf ceramic disc capacitor and an ON/OFF switch between PIN5

(non-inverting input) of IC10 (LM358 op amp) and ground. This, of course, is not the

most refined portamento, and you can improve this by adding variable time (a pot in

between) and/ or proper CV decoupling by using an op amp (PIN5 of IC10 is sensitive).

Yet, I personally find 1uf with a momentary ON cool enough for performance effects.

VCF

Pre-filter overdrive: in the TD-3 a 20k and an additional 200k resistor are used (R23 and R24)

rather than one single 220K resistor (OG R62). This seemingly odd choice, however,

makes it very easy to install overdrive by adding an ON-ON switch and a 100-250k log

potentiometer parallel to R24. This way R23 gives the circuit some protection while the

increase in pre-filter volume can be dialed in from louder to overdriven.

Extended Filter mode (maffez): wire TP6 to a switch, a 150k resistor and 100k-250k log

potentiometer in series, and then to the bottom of R123 for phasey, bandpassy effects.

You could use an ON-OFF-ON switch to combine this with the Pre-filter overdrive.

Lower filter cutoff (adapted from Xoxbox wiki): If adjusting VR11 just isn’t enough for you,

you can lower cutoff even more. Wire a 4.7K-15k resistor in parallel with R44 (OG R47,

10k). The range of cutoff on the TD-3 is wide enough to “catch up” at higher values, i.e.

cutoff has a lower end but pot goes still high enough. I find 15k just nice.

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Filter tracking: connect the tip pin of the TD-3 CV output socket to a 100k resistor and this to

a switch, which goes to the bottom of R47. There is a solder point of the back of the PCB

where you can do this too: it is located underneath “VCF OFFSET” just a tad left of “V”.

I’m not going to implement this personally, but included it for completion’s sake.

Higher resonance: Wire a switch and a resistor (between 2.2k to 7.5k) in parallel to R160 (OG

R97, 10k). I find 4k7 quite nice. Alternatively, you could replace R97 with a trimmer. For

lower resonance, you could swap R97 for a higher value or just turn the knob less.

Filter FM from VCA output: Route the output of the VCA (TP10) to a 0.1uf capacitor (for AC

coupling), this then to a switch and a to a 50k-100k log potentiometer in series; this goes

to a 100k resistor, depending on taste, which then goes that finally goes to the bottom of

R47. For the solder point on the back of the PCB see entry on filter tracking. Both

Devilfish and Subatomicglue describe them well. NB my values for pots and resistors are

slightly different than theirs.

Filter FM from VCO output: Wire TP6 to a 0.1uf capacitor (for AC coupling), this to a switch, a

100k resistor and a 50k-100k log potentiometer in series. The wiper of the latter goes to

the bottom of R47. For the solder point on the back of the PCB see entry on filter

tracking. NB my values for pots and resistors are slightly different than those of the

classic mods. This is mostly because I use an ON-OFF-ON Switch for selecting between

zero FFM, “from VCA”, and “from VCO”.

Bypass ENV to VCF: wire switch to both terminals of R58 (OG R61, 10k) [needs verification],

so you can bypass envelope modulation entirely. This is a classic Devilfish mod, yet I’m

not sure how practical it is in musical terms – IMO it makes maybe sense to implement a

momentary ON switch to deactivate ENV to VCF with a simple push for individual notes.

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VCA

Bass boost: the signal enters the VCA through C49 and C76 (OG C21 and C22, both 0.01uf),

with C49 carrying the signal from the filter and C76 being resonance level compensation

(both are small caps near the blue e-foil caps under the ENVELOPE pot). A classic mod is

replacing them with 0.1uf film box capacitors, although the effects of this as well as the

most sensible values for replacement capacitors have been debated heavily. Not so

super interesting to me personally, but included for historic reasons.

C52 mod (AKA magic cap, AKA fröhlichefolie, AKA angry a… mod): technically it softens the

attack of the VCA a little bit, which might also result in hearing effects to the extent of

“glue”. Replace C52 (10nf) with a 47nf cap or ad another cap that gets you around 47nf.

Nordcore was extremely helpful in explaining this chance discovery as he had invented a

mod to declick the XoXboX VCA (B might even have been inspired by his work).

ENVELOPES

The Td-3 has two envelopes (main ENV with variable decay and a fixed one for the VCA).

Different speeds for main envelope: exchange C89 with capacitors of different values to

taste. For folks who think the TD3 decay could be snappier, put in smaller cap values.

Using a switch, you can toggle between speeds. I find longer actually nice since it also

makes accent swing in – will add an ON-ON switch that adds a 3.3uf to the existing 1uf.

Variable VCA decay: VCA decay is fixed at ca. 3 seconds and set by R52 (OG R123, 1.5mega).

Higher values for R52 give you longer decay times, lower values shorter ones. You can

replace R52 with a 500k resistor and a 1M potentiometer. You could also wire a

potentiometer or resistor and switch across R52 to toggle between longer and shorter

notes. I personally use this with a 1M pot and a momentary ON button across R52.

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ACCENT

Accent boosts resonance and VCA level separately in the TD-3. On details, see Robin

Whittle’s vivid descriptions: http://www.firstpr.com.au/rwi/dfish/303-unique.html.

Change accent to VCF amount: For resonance, the accent signal passes through R39 (OG

R46, 47K). To increase the effect, you can wire another resistor in parallel. To reduce the

effect, replace R39 with a resistor of higher value or a 47k resistor and a trimmer.

Change accent to VCA amount: For the VCA, the accent signal passes through R26 (OG R119,

47K). To increase the effect of accent to VCA, you can wire another resistor in parallel to

R26. To reduce the effect, replace R26 with a resistor of higher value or a resistor of the

same value and a trimmer in series.

Change accent to VCA onset: after passing through R26, the accent signal for the VCA goes

through C26 (OG C36, 0.0033uf), which very mildly filters the sweep. By enlarging the

value of C26 you can soften the attack of the accent effect, which can make the onset of

accent blend a tad more in. With extremes values (1uf), you hear a very pronounced

fade-in of accent.

Manual Accent trigger: wire ground to one terminal of a momentary ON switch and the

other terminal of the switch to the base of T3 *or* the bottom of R128 (easier to find,

just right to the sequencer mode rotary switch). Note also that the point under R128

goes to the back of the PCB (nice!).

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Variable Accent decay (adapted from Xoxbox wiki): lift top of D15 (= anode of OG D28), and

wire a 1M log potentiometer between the lifted up top of D15 and the free solder pad

(where top of D15 used to be) on the PCB. For simple fine-tuning of accent decay, you

might want a trimmer instead of a potentiometer.

Accent sweep speed: this is different to the variable accent decay mod in that you add to or

exchange capacitors for the accent capacitor C64 (OG C13, 1uf) [verification needed].

Different capacitor values will not only affect the decay of accent but result in more

differences, see http://www.firstpr.com.au/rwi/dfish/Devil-Fish-Manual.pdf. I think it

might make sense to experiment with an alternative “slow” accent mod for the sake of

variety. You can do this by wiring a 1uf capacitor in parallel to C64 – positive leg is wired

to the backside of the PBC to the solder blob of the positive leg of existing C64 and the

negative leg (usually indicated by a white stripe of your cap) goes to the “in” terminal of

On/OFF switch, whose “out” terminal, in turn, is wired to the solder blob of the negative

leg of existing C64. This way, when your switch is ON, your additional cap discharges

together with the existing one, resulting in a longer charge phase and sweep action.

FX SECTION:

The inbuilt distortion of the TD-3 is based on the Boos DS-1. General information about this

classic foot pedal, including schematics, can be found here: https://www.electrosmash.com/

boss-ds1-analysis. The DS-1 has been subject to elaborate modifications, most notably the

so-called Keeley mod, some clear and graphic information about which can be found here:

https://www.instructables.com/id/How-to-convert-your-DS-1-to-Keeley-All-seeing-eye-/. My

interest in the distortion has been limited so far, yet a simple tonal mod made me reconsider

keeping it rather than repurposing that space.

FX insert, send, and more: this needs to be fleshed out a little. So far I found that when

distortion is switched ON, C91 carries the dry signal while C92 carries the distorted

signal. When it’s switched off, both are silent. There are many ways to play with this and

more info will follow. For instance, you could wire C91 to an output socket and thus get

an FX send. You could also think about more elaborate things, such as decoupling the

distortion, or removing C37 and C69 and using the headphone socket as an insert, or…

Distortion easy tonal variation: a simple variation to the tone of the distortion can be had by

playing with different capacitors in addition to C58 (DS-1 tone filter). I find a 220nf Tantal

capacitor across C58 makes for a sound I like better than “as is”.

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CV I/O

Gate Input (adapted from xoxbox wiki): Tip lug of input jack socket goes to 22k resistor. The

other end of 22k resistor goes to a diode, the cathode of which goes to base of T30. The

junction between T30 and R142 goes to a solder point on the back of the PCB, so you can

wire your connection to this point too.

1V/Oct CV input (adapted from doepfer): connect the bottom of R168 (OG junction of

r100/r118/r106) to a 27K resistor and a 4.7k cermet trimmer, which then goes to the tip

lug of your input jack socket. The trace to R168 also goes to the back of the PCB (you can

find the point leading to the solder side left of the label R174 on the front).

Accent ON CV input (hi=off, lo=on): tip lug of input jack socket goes to 100k resistor. The

other end of resistor goes to base of T3 *or* the bottom of R128 (which is easier to find,

see entry on manual accent trigger). The latter, also, goes to the back of the PCB. Mod

adapted from xoxbox wiki.

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Accent ON/OFF output (TB style): tap the junction of R144 and R128, which you connect to

the tip lug of your output jack socket (NB: 0V = accent high). Point goes to back of PCB.

Accent sweep output (Avalon style): the accent signal ON shorts the decay of the main

envelope to its minimum length and sends this signal through the accent volume pot.

You can tap this short sweep at the right lug of the accent pot (as seen from PCB top, of

course, you also can access the tips of the lugs from the back of the PCB); put at least a

10k resistor in between for protection or mirror the CV signal with a non-inverting op

amp. This short sweep is cool, which is why the Avalon has it too, for instance. I triggered

gate on my Model D with this, and it should work for triggering the SH101 sequencer.

Filter CV input: wire tip lug of CV input jack socket to 100k resistor, which goes to the

bottom of R47. For the solder point on the back of the PCB see entry on filter tracking.

Envelope output: tap TP8 on the back of PCB and connect to diode. Put protection in

between.

VCO output: tap TP6 on the back of PCB. Put some protection in between.

Filter output: tap TP9 on the back of PCB. Put some protection in between.

VCA output: tap TP10 on the back of PCB. Put some protection in between.

When installing jack sockets as shown below, note that you need to bend their lugs (slowly

and steadily). Also, you want to protect your PCB below against change shortages, even

though in this particular area there are no components. Some sticky tape will do.

The Switches you see installed over the distortion section need their lugs bent too because

otherwise they are too deep. Even potentiometers fit in this section (size of those installed

on the TD PCB)! The two bigger pots underneath the led on the front are for demo – I used

ones with plastic bottoms in the final version and sealed off the lug-wire connections with

heat shrink tube.

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OTHER

Crappy power supply mod (for the brave folks): wire Pin 2 of IC9 to a 1uf capacitor (can be

film box or e-foil), which then goes to ground. This slightly destabilizes the supply voltage

of 11.85V and results in an overall warbling, quite crappy sound. Honestly, though, think

twice before you go there: https://soundcloud.com/uibkmedan/agemod-extreme?in=

uibkmedan/sets/td-3-modifications. Better and safer version may come around later.

Antique chassis mod: This is a must have for ebay auctioneers! New synths are comparable

to new cars. They smell all shiny and fresh, but it’s only after a couple of crashes and

dents that you have built up a personal relationship with them. One way, of course, is to

use your unit for 20 years every weekend in the club, but who has time for this? So, a

good way of quick ageing your chassis is as follows. A) dents and scratches: remove all

internals (PCB) and put your chassis in a burlap bag; add grit AND pebbles and shake the

mix for an hour as forcefully as you can. It is crucial that the number of grit to pebbles is

2:1 for achieving the right ratio of scratches to dents. B) wear and abrasion are as

important as scratches since no good track has ever been made by reading the labels on

your synthesizer (it’s an instrument, not a book, duh!); years of wear and abrasion can

swiftly be had by some drops of nail polish remover on cotton gloves and kneading your

chassis for 10 minutes; remove any rest of chemicals with bacon rind/sesame oil (for that

shiny shine)! C) condensation and evaporation: clubs and techno studios are humid

places; people constantly move (and thus sweat), take substances (and sweat), and spill

drinks (liquids), introducing to your gear, cycle by cycle, a patina that may be the hardest

to recreate. Since second hand retailers tend to withhold information on such matters

there is hardly any reliable data as regards the exact composition of patina, but devout

enthusiasts have established basically two mayor strands of patina emulation. For

Detroit Techno emulation soak your *entire* unit in coca cola and then put it in a bag of

rice to dry (repeat at least 5 times); for Chicago Acid House use Pepsi (it is a common

misconception that 303 patina consists of LSD and beer – many clubs were liquor free

and DJs would seldom take their instruments to the restrooms). Other variants may

include Ribena (UK second summer of love) or Mezzomix (Dorian Gray, Frankfurt). In all

these cases you can optionally emulate a badly cleaned sale item by taking your thusly

prepped neo-vintage machine to your local shvitz and let nature do its thing.