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.
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.
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
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
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
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.
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
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 http://www.brunelhobbies.com.au
is very effective indeed. It is branded "Tetra ABS/PVC
Cement" and the label includes: "Suited for ABS/PVC/Styrene". The
manufacturer is http://www.simplyglues.com.au
- 2010 October 26: Initial version.
Robin Whittle 2010 –
First Principles and Real World Interfaces
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