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Devil Fish mods to the TB-303

TB-303 maintenance and simple modifications

2013-11-24  Robin Whittle
Search engine bait:   TB303 pots, TB-303 pots, TB 303 pots, Bassline pots, TB303 switches, TB-303 switches, TB 303 switches, Bassline switches.


1 - Replacing the tact switches with Omron B3W-4050 sealed tact switches
2 - Six small pots from Tuning to Accent
3.1 - Never spray anything in the machine
3.2 - Overheated Q45 power transistor
3.3 - Pinched wires during re-assembly
3.4 - Knobs falling off
3.5 - Noisy volume pot
3.6 - Batteries not connecting
3.7 - Memory problems - corrosion of RAM chip pins
3.8 - Buzzing due to LED activity
4.1 - Increasing the range of the Cut Off and Env Mod pots
4.2 - Improving the bass response
Update history

1 - Replacing the tact switches with Omron B3W-4050 sealed tact switches

Please see ../tact-switches/ for a comparison between the original ALPS unsealed tact switches, the ALPS  sealed tact switches and the Omron sealed tact switches.

Modifying the Omron switches

It suffices to cut the Omron switch stems by hand, with a Dremel tool or similar, with a 0.6mm cutting disc.  (The old Dremel cutting discs are a dull red or brown colour.)  It would be best to allow for some rejects with this hand-held approach. 

I adapted my original jig to do the cutting.  I use a tiny metal circular saw, with appropriate safety guards.  If you already have the Dremel saw attachment, that might be OK to use.  Otherwise, I suggest using a cutting wheel.

It takes me longer to remove the swarf from the cuts than to actually make the cuts.  I do this with a scalpel under a stereo microscope.  It is not necessary to remove all the swarf, since as long as it remains attached inside the button, it can't do any harm.  Still, I try to remove the main pieces of swarf from the two cuts.  Most of the swarf is still attached because the plastic partially melts during cutting.

Here are some photos:


Hopefully you will be able to obtain Omron switches and cut them like this.  However, please see the Devil Fish page: ../#tact_switch_kits if you want to purchase modified Omron switches from us. 

Removing the old switches

When you dismantle the machine, you may want to keep it connected to the batteries (taped into its battery compartment) in order to retain the memory contents. Often the on-board capacitor will retain memory data for days, but it is probably best not to trust it when you are working on the machine.

I suggest removing the buttons before desoldering the switches.  This can be tricky - I use very long pointy-nose pliers to prise the buttons up evenly. 

De-soldering the old switches is pretty straightforward. I use a Weller vacuum de-soldering station. A second alternative is a standard soldering iron and a push-in, press-to-release solder sucker. The only problem with these is that the sucker may jump forward and damage tracks. A third alternative is to use solder-wick to soak up the excess solder.

Installing the new switches

This is easy – just solder them in.

2 - Replacing the 6 small pots from Tuning to Accent

Here is a photo of the rotor and phenolic board of the Resonance pot:

The central part of the shaft goes through the hole in the middle and presses against the metal backplate.  This takes all the downward force applied to the shaft.  The outer part of the rotor is not close to the phenolic board, so it can never press against it or cut the conductive tracks.

The black epoxy is to protect the riveted connections between the conductive tracks and the terminal pins from corrosion.

There are two remaining problems:
  1. The Resonance pot still has the log curve at each end, rather than being linear. As noted above, this nothing to worry about, since it just alters the knob position for a given sound, rather than alters the sounds which are available.

    The photo below shows how the three resistive layers are printed, for both sections of this dual pot.

  2. The shafts are about 1.6mm too short.  See the discussion below about what to do about this.  The knobs on a TB-303 are already marginally too low.  I have been boosting them up about 1.6mm for quite a few years.  That technique - a small circle of leather inside the knob - could be applied to these pots to restore the knobs to the original height, but it is not possible to extend this technique to more than about 1.6mm.
I understand that later in 2010, Technology Transplant will have another batch of pots which solve the shaft length problem by having an extended frame at the bottom.  This should work fine.

In the mean-time, from August 2010 onwards, I am using the above pots, with their shorter shafts, and boosting their mounting position with some 1.6mm thick phenolic paperboard spacers I had laser cut.

We are very fortunate that Technology Transplant has these pots manufactured, and sells them world-wide.  This is an obscure pot-shape, and ALPS haven't made this kind of pot since the early 1980s.

The values of the pots are:
The original parts are no-doubt unobtainable, but here are the Roland part numbers anyway:

To help with debugging, the following diagram shows the connections and function of the Resonance pot:

3 - General maintenance

3.1 - Never spray anything in the machine

3.2 - Overheated Q45 power transistor

I picked those transistors which required less than1.5mA and put a 2.2k across R167, to boost the base current at lower voltages. 

The Devil Fish requires more power than the TB-303, due to the MIDI In system, the other circuitry and the three LEDs in the Devil Fish panel, and for MIDI, the Blue LED too.  The 1.5k R167 would only have about 2.4 volts across it - 1.6mA (assuming there was no voltage drop in R168 and Q44) when the input voltage is 9 volts.  Actually, this is the voltage with 9.5 volts going into the machine and with R168 shorted and 1K across R169 (Devil Fish 4.0C and later).  So without these mods and with a genuinely 9 volt supply, the voltage across R167 would be more like 1.5 volts and the base current only about 1mA.  We need a transistor with a beta of 180 or more when running with a collector current of about 180mA .

I tested a batch of  MOSPEC TIP30As to see what base current I needed to get 200mA Ic with about a 4 volt VCE.  The best was 1.02mA and the worst was 3.58mA.  The 2SB596 needed 0.94mA, which is a beta of over 200.

3.3 - Pinched wires during re-assembly

The Devil Fish has many more wires.  If you dismantle one, please be very careful re-assembling it.

3.4 - Knobs falling off

A little piece of "Blu-Tack" (grey putty-like stuff for putting posters on walls and a vast array of other crucial tasks: ) is the best solution I have found.  Never glue the knobs in place.  They must be removed for installation and maintenance of the Devil Fish.

Keep an eye out for loose nuts on the tempo and volume pots.

I find that the shafts of the two rotary switches can vary somewhat. If you take off their knobs, it is best to mark them so they can be placed back on the same switch.  The same applies to the Tempo and Volume pots, which have very different shafts.

3.5 - Noisy volume pot

The original ALPS volume pots, with integral power switch, rarely fail.  However, over the years, they can become noisy when turned.

I fix this by desoldering the pot and prising the back off it, by bending back four lugs which wrap around the front face of the pot.  Then I clean the conductive tracks and the wipers - especially the two inner wipers to the rotor's slip-ring - with isopropyl alcohol and some thin cardboard.  I spin the rotor around clockwise.  Doing it anticlockwise could cause the wiper to jam against the metal strip in the middle of the zone it does not normally travel in.

In my experience, this always fixes the noise problem. 

Technology Transplant have replacement Volume pots, but I generally keep the ALPS pot, after cleaning it, in part because the ALPS pot shafts fit the knob perfectly, while I find the Technology Transplant shafts may be somewhat too big.

In my experience there is very little trouble with the Tempo pot, probably because most people sync their machines externally, rather than using the internal oscillator.  Tempo pots can be cleaned as described above.  Technology Transplant make replacement Tempo pots too.

I have received a report from a customer who fixed a noisy Resonance pot by placing a drop of DeoxIT Fader Cleaner inside the pot.  I am not sure whether he used the spray which is only 5% or the needle dispenser form, which is 100%.  I guess the latter.  Nor do I know how long the fix lasted for.  I have never used these materials but they seem to get a good rap in various discussion forums.  Other companies make liquids and sprays for rotary pots, carbon or conductive plastic and for linear faders, which have special lubrication requirements.  There are a large number of different DeoxIT products.  This is the page for DeoxIT Fader Cleaner:

3.6 - Batteries not connecting

3.7 - Memory problems - corrosion of RAM chip pins

3.8 - Buzzing due to LED activity

Here is something I discovered in January 2012 and have been doing to all the machines I work on since then.

Some, many or all TB-303s have a low-level buzz in the background, irrespective of the Volume pot setting.  While this is well below ordinary signal levels, it might be annoying.  The buzz from LED activity – especially with four LEDs on at once, such as when selecting patterns 1, 2, 3 and 4 – is apparent when running from batteries. 

A higher frequency buzz, from the Interrupt oscillator, may be audible when running from an external power adaptor.  The lower frequency buzz problem seems to be most apparent with the original LEDs or perhaps with any Red LEDs.  Blue LEDs have a higher threshold voltage and draw less current. 

The cause is ground loop problems within the machine.  The fix is to install a wire (such as a multi-strand piece of hookup wire) between the Output socket ground terminal and the ACW (ground) terminal of the Volume pot.

4 - Modifications

Here are a few mods which you can do with little trouble to a TB-303. There is no discount for subsequent Devil Fish mods to a machine with these mods – which represent a small proportion of the complete set of modifications.

These instructions are given on the understanding that they will be carried out by someone with some electronic expertise. Don't do them if you are unsure of yourself and don't email me for support if you get into trouble. If you get into trouble doing these simple things then you should have recognised that you lacked even the most basic expertise.

4.1 - Increasing the range of the Cut Off and Env Mod pots

To increase the high range of the Filter Cut Off pot, turn TM3 (near the mode switch) clockwise.
To triple the high range of the Env Mod function, solder a 100 K resistor to the back of the board in parallel with R63 - a 220K. R63 is driven by the wiper of the Env Mod pot and is located directly adjacent to the wiper pin of the pot. Put the resistor on the rear side of the board. I use a 1/8 watt miniature resistor, but you should be OK with a standard 1/4 W.

4.2 - Improving the bass response

Update history:

© Robin Whittle 1996 to 2013 – First Principles and Real World Interfaces
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