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Refurbishing the TB-303 plastic case

2010-10-26  Robin Whittle


This is a description of some labour-intensive techniques for repairing TB-303 ABS plastic cases.

ABS is a combination of three plastics all polymerized together:  Acrylonitrile, Butadiene and Styrene.  Polystyrene is used for model aircraft and the like.  ABS is tougher than this.

Paint and markings worn around the 6 small knobs

Frequently, due to over-enthusiastic turning, while pressing downwards on the knobs, the paint and markings are worn way around some of the pots.  This is most common around the Cutoff and Resonance pots.

At present I don't have a fix for the wearing away of the ABS plastic of the case, but I do have a technique for lightly respraying the worn area and then by hand (under a stereo microscope) writing in the missing or worn markings.  This is not perfect, but it is a significant improvement over the original state.

Repairs to worn paint and markings on the case of the TB-303 Devil Fish

For those who want to try this . . . 

I use a small compressor driven spray gun and metallic acrylic automotive lacquer.  The first trick is to dilute the base lacquer only with acetone.  Using the ordinary thinners will cause it to dry slowly and perhaps not bite into the ABS as it should.  The acetone dries rapidly as the drops leave the gun, so I found ratios of 1:4 or so lacquer to acetone worked OK.   Multiple fast, light, passes spray small droplets and result in a matte finish which dries almost instantly, since the acetone goes directly into the ABS. 

The second trick is to cut a mask from paper, waxed paper, plastic sheet or whatever and mount it about 1mm or so above the surface.  This means there is no hard edge to the paint, and by moving the gun around and spraying very light amounts of paint, it is possible to get a feathered edge to the paint.  The photo above doesn't show where I sometimes didn't quite cover the worn area.  Fortunately, the cream-coloured ABS does not contrast strongly with the silver paint. 

The new silver paint invariably looks different to the original.  However, both are truly "biting in" to the ABS.  This is not a thick coat of paint simply sitting on the surface, ready to peel off.

I used a fine black solvent based pen under a microscope, with a little printed guide popped into the hole to indicate where the marks should be.

Its not perfect, and the new paint and markings will wear away if there is repeated finger pressure against it.

There should be less finger wear with the Devil Fish, since I mount the knobs about 1.6mm higher than in the TB-303.  This makes them much easier to grip.  Also, with the replacement 6 small pots, I alter the lubrication of these pots to make them easier to turn.   So there should be no need to press down on the knobs.

Other repairs

I can usually use epoxy or perhaps ABS glue to repair splits in the corners of the upper part of the case.

Sometimes, the holes in the internal supports are stripped by excessive screw tightening, and so it is difficult or impossible to make the screws which hole the case together hold in these supports.  I may fix this with a wider screw.  It is surprisingly difficult to fill the hole in a way which enables the existing screws to bite properly.  I have some special ABS glue, which is very effective at dissolving the ABS.  In principle, this can be combined with shavings of a scrap case to make a paste which will fill the holes, so they can be drilled again with a suitably small hole.  In practice, the glue does two unfriendly things.  Firstly, it soaks into the whole support pillar and makes it softer.  Secondly, it takes many days or perhaps weeks to evaporate sufficiently to stop the new plastic being gooey.  Screwing or drilling into new plastic which is even slightly soft makes a mess. 

Sometimes, due to oil being sprayed inside the machine, the plastic of the support pillars becomes fragile and crumbles away.  I may be able to fix this by using ABS glue to bond on some pillars cut from a scrap case.

My experience is that this glue needs many days to dry before the join can be used.  Although it is highly volatile, it seems to soak into the ABS and make it rubbery or gooey.

BTW, acetone is not an effective ABS glue.  Tricloro-ethylene (liquid paper solvent) is better, but I think not at all adequate.  The ABS glue I obtained from  is  very effective indeed.  It is branded "Tetra ABS/PVC Cement" and the label includes: "Suited for ABS/PVC/Styrene".  The manufacturer is .

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© Robin Whittle 2010 First Principles and Real World Interfaces

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