[sdiy] Kenton TB-303 sockets schematic?

Roman Sowa modular at go2.pl
Tue Apr 11 13:27:44 CEST 2023

No need to worry. Even if all cables fall apart, it's easy to recreate 
or make it from scratch again. If you will not feel like doing it with a 
little help of s-diy list, any synth tech will do that hapily.
Or you can send it to me, so I'll install MIDI inside.

The original TB circuitry will probably break faster than this mod. No 
need to unscrew that brittle plastic just for a photo.

What you're saying about 0V fed to CV jack affecting the sound confirms 
what I suspected that external CVs are just one resistor away from base 
of expo pair. I bet if you connect multimeter to filter CV input, you 
should see voltage changing when you move cutoff knob. Expo pairs inputs 
are not at 0V, so applying 0V to those inputs are actually pulling them 
And if that's so, it also means that with external voltage connected, TB 
will become a little flat on pitch tracking internal DAC compared to 
when nothing is connected there.
But without anything plugged, i's good as new!


W dniu 2023-04-11 o 13:00, rburnett at richieburnett.co.uk pisze:
> Thanks Roman.  You are right that there is a transistor of some sort in 
> there, and a bunch of resistors and that's it I think.
> I know that it should never break, but I worry that one of the wires 
> will fall off somewhere and I won't know where to reconnect it to.  I 
> guess the only solution is to open it up carefully and take some hi-res 
> digital photographs documenting where the wires go on the PCB now while 
> it is still working :-)
> The FILTER CV input definitely alters the sound if you insert a jack and 
> feed it from 0 volts (shorted jack plug).  The voltage input required on 
> this jack for "no change" seems to be about 3.1 volts.  As you hinted I 
> think it is fed into a summing node like where the VCF envelope sweep 
> and the accent envelope sweep get combined in the exponential converter 
> at the bottom of the ladder.
> -Richie,
> On 2023-04-11 11:51, Roman Sowa wrote:
>> Based on what I could see in Kenton's page photo this is very minimal
>> kit, with maybe 3 resistors and a transistor. CV for pitch and filter
>> are probably just resistors going to summing nodes of VCO and VCF
>> respectively, probably directly to the base of 2SC1583, so without any
>> voltage at their sockets, there's no difference from un-modded TB303.
>> The other 3 are just on/off switching signals.
>> All I'm saying is, the kit does not change anything when nothing is
>> connected and is so simple that has no chance of ever needing a
>> repair.
>> Roman
>> W dniu 2023-04-10 o 21:37, Richie Burnett pisze:
>>> The TB-303 that I got second-hand many years ago has a set of 3.5mm 
>>> jack sockets along the back of the enclosure marked SLIDE, ACCENT, 
>>> FILTER, GATE and CV.  I believe these are a mod for use with a Kenton 
>>> MIDI-to-CV interface?
>>> Does anyone know the details of how these sockets connect into the 
>>> rest of the TB-303 schematic?  Last time I had the 303 open to 
>>> replace the tact switches I noticed that there were various resistors 
>>> and transistors inside bits of heat-shrink tubing soldered to the 
>>> copper side of the PCB.  I'm trying to find a schematic of the 
>>> connections for a couple of reasons:
>>> 1. I want to know how to fix them if a wire should fall off at some 
>>> time in the future.
>>> 2. I'd like to make a judgement on whether these mods affect the 
>>> sound in any way compared to a standard production TB-303, even when 
>>> nothing is connected to the jack sockets.
>>> Before anyone says "open it up and trace out what wires go where", I 
>>> had thought of that!  I'm not being lazy, I'm just trying to minimise 
>>> the number of times I open up the unit as the old plastic enclosure 
>>> and PCBs are quite fragile.  There's also the risk of human error if 
>>> I trace out the connections vs working from a schematic if one exists 
>>> somewhere?
>>> Has anyone ever fitted one of these kits?  It looks like Kenton 
>>> Electronics still do them...
>>> https://kentonuk.com/product/roland-tb-303/
>>> ...but the PDF docs at the bottom of that page don't tell you how to 
>>> install the kit.
>>> -Richie,
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