TB-303 potentiometers -
to the Devil Fish
This page contains a few notes and photos concerning the
six small pots of the TB-303. This concerns the original
ALPS pots and some replacement pots from Technology Transplant from the
mid-2000s until some time in 2009.
The material below does not apply to the
bluish-silvery replacement pots from Technology Transplant from late
Please see ../303-mods/ for
information on replacing the 6 small pots and other matters.
Updated 2008-08-17: bad wiper heights in the new
pots and a better description of my test circuit.
The original ALPS pots are no longer
the ALPS pots have a rotor with a rim around the edge. This is
either to help keep out dust or for some other purpose, such as taking
the strain of excessive downwards force on the shaft, which would
otherwise be taken by the centre of the phenolic paper-board which
holds the resistive and conductive tracks.
outer rim extends leftward in a slightly curved shape here, which is
also a spring, as evidenced by the gap between this section of the rim
and the rest of the rotor. This is from a resonance pot - a dual
50k log, so there are two sets of wipers.
Here is a general
view of the same rotor, with the two sets of wipers.
is the phenolic paper-board part of an ALPS resonance pot, with
some wear from this outer rim. This only happens when the shaft
is both turned and pressed downwards.
all the ALPS pots have this white epoxy where the metal lugs are
riveted to the metal tracks. Those without it may be less
reliable, with open circuits developing between the lug and the track.
from the outside we have:
(metal base, with carbon on top for good connection to the wiper, I
guess) track for the outer pot. This connects to pin 1, and is
solely to connect to the outer part of the outer wiper (3 metal brushes
in the previous image).
- Resistive track of the outer pot,
pins 2 and 6.
- Resistive track of the inner pot, pins 3 and
circle - conductive track to connect to the inner wiper, via two
separate metal brushes. Note this is largely, but not entirely,
coated in carbon.
are some extreme cases of pot wear. I don't know exactly how this
occurred, but there must have been a lot of turning and pushing to cut
through the tracks like this. This must have continued well after
the pot failed electrically. Perhaps someone was hoping to make
them come good!
"Scratchy" pots in a device which has not been
used for years can be caused by a thin film of grease on the conductive
and/or resistive tracks. I find this can often be made to go away
by turning the pot a few dozen or maybe a hundred times.
However, at least with these ALPS pots, please don't push on the shaft
when you do this!
the way, there is very little trouble with the ALPS volume and tempo
pots. Replacements are available from Technologytransplant.com
Moving the knobs up a little
affixing the knobs, I insert a 6mm disc of leather inside the knob.
This means the knob sits about 1.2mm to 1.5mm higher than normal.
This makes the knob easier to grasp and reduces the likelihood of
unwanted downward pressure and wear of the case around the knob.
also use a little Blu-Tack
(poster putty) in the knob to stop them falling off.
Do not glue
knobs onto the shafts!
Don't Don't Don't . . .
spray anything in or on pots and switches unless you really know
what you are doing. My life has been complete without having to
do this (apart from a little alcohol in one or two isolated cases).
It seems to be common to spray oily things into pots and
switches - just because stores sell spray cans full of oily stuff with
claims that this stuff will fix electrical problems. This is very
a bad idea.
these pots, we would be in trouble. Despite the difficulties
noted here, I am very grateful that these pots have been manufactured
and made available.
They are available only in sets of
six. Likewise, pots for the TR-606 are also available, though I
haven't used any of them yet.
This information may not be
relevant to the pots currently on sale. Below I refer
to pots I purchased in 2006 and in early 2008. There were
versions of the pots which I don't refer to here.
the pots, check in "Items for sale" at the (was Chipforbrains)
from the Resonance pot, these do not have a rim around the edge of the
rotor. So, apart from the Resonance pot, they should not suffer
from the problem of cut tracks due to downwards pressure.
experience the pots have been good, except for the problems described
below for the Resonance pots. I think the friction grease on
these pots can be rather stiff. This is grease between the shaft
and the metal case, not near the conductive tracks. I can
dismantle the pots and replace the grease with low-friction silicone
grease. This gives very little resistance to moving, which is
unusual in pots, but I think is good for easy turning for musical
One thing to watch is soldering the
two metal mounting tags of each pot. Some earlier versions of
these replacement pots had cadmium plated (I think) tags, which were
very hard to solder to. The current production is plated with
something else and is easier to solder. It might be best to file
away the plating on both sides and the edges and to use a high-powered
soldering iron. These tags are more than
mechanical. In the TB-303 they carry Ground to various parts of the
circuit. The machine will not work unless all the pots are
soldered in to complete these ground circuits.
It can be
difficult to solder the pots in exactly the right orientation so that
the knobs do not brush against the case. It may be necessary to
melt the joints and press against the pot shafts to slightly change
their orientation once you have found a scraping knob problem.
The latter is only apparent after the back has been screwed on
and the knobs put on.
Shaft splines too
first problem is that the shafts of these pots are too large to fit
comfortably into TB-303 knobs. There is no prospect of filing the
shaft down - the splines are numerous and the material is hard nylon,
or something similar. Filing would just make a mess.
seem to be two flavours of knob - some with a larger hole. These
can be found in the one machine, as if some parts from the mold are to
one pattern and the rest to another.
Sometimes one of
these larger hole knobs can be pressed onto the Resonance pot shaft,
but it can be very hard to get off.
is not such a problem with a TB-303, since the knob can be forced off
with pliers, levers etc. from below, pressing against the metal body of
the pot, once the case has been removed.
For a Devil Fish, this
is not possible. The knob has to be removed before the case can
be removed. It can be tricky to hold the knob with enough force,
for instance when using adhesive tape to grip the knob. So
forcing a knob onto a larger shaft would lead to serious trouble the
next time the machine needs to be dismantled.
My usual solution
to this problem is to drill the knob almost down to the bottom of
the spline with a nice sharp hand-held (in a chuck) 15/64" drill bit.
Then the knob will fit OK, with enough spline left to lock it
against the shaft and stop rotation. The splines in the knobs
seem to be narrower at the bottom of the knob than the top, inside the
second problem is that the pots are not linear. They have a
"logarithmic" taper at both ends of the rotation. This can be
seen by the thicker layers of resistive material printed at both ends
of the resistive track:
looks like layer 1 is printed first, to about 10 and 2 o'clock.
Then the main resistive track is printed, followed by a third low
resistance track to about 9 and 3 o'clock.
The result is that
most of the range is between 10 and 2 o'clock - rather than 8 to 4
o'clock in the ALPS pots. This doesn't change the sound of the
Devil Fish or TB-303. It only affects what position the knob must
be in to achieve a particular sound.
1 Outer wiper
2 Outer ACW
3 Inner ACW
5 Outer CW
6 Outer CW
Some replacement resonance pots have failed due to
problem may only affect one batch of pots, or a few individual pots, so
it doesn't necessarily mean that other Resonance pots are bad.
think the problem is primarily or solely that the wiper for the outer
section of the pot is not shaped correctly, and so does not make proper
contact. A secondary problem may be a kind of coating over the
resistive tracks, but this may not in fact be a problem at all.
I have a test
jig to test the connectivity of the wipers. The ACW and CW
ends of both pots go to ground. Both wipers go to +12V via 33k
resistors. A 100k from each wiper takes the voltage from each
wiper and mixes it to a common point, where a 0.22uF cap takes it to
the CRO, the audio amp, and a 1M to ground.
Turning the pot
produces audible noise as the wiper fingers find their way across bumps
in the tracks, with different voltages.
If there is a problem
with wipers not touching the track - the problem I am trying to fix -
then there is a much harsher noise and sharp transient voltages can
clearly be seen on the CRO.
Note how the outer
and inner conductive tracks are plain metallic tracks. In the
original ALPS pots (the first photo above) they are metallic tracks for
low resistance, topped with a carbon track, except for one sector of
the central track. I assume ALPS know that this is a better
surface for the wipers. ALPS know a lot about pots. The little
RK09K113 pots used on the Devil Fish boards have never (to my knowledge) failed,
despite the hard work they have done, in some machines since 1993.
wonder whether these metallic tracks will wear away faster than the
I wonder whether they will corrode over time.
If they do, we are in trouble . . .
At first I
assumed the problem described below was due to some kind of residue
remaining on the surface of the conductive tracks after manufacture.
Therefore, if it is fixed, the pot should be fine for a long
time. If it is a corrosion problem, then the trouble might
was able to get rid of the grey-brown blotchy contamination on the
track by wiping with ethyl alcohol (methylated spirits). It came
off quickly, leaving a clean metallic track. Then the pot worked
perfectly . . . but the real problem is probably due to wiper fingers,
as described below.
There is a
bluish-purplish tinge on the lower right - that is oxidation from some
oven step in the manufacturing process. That was not affected by
the alcohol and does not seem to be a cause of the trouble.
This contamination is not the only
problem causing scratchiness. Perhaps it is not a problem at all.
There was a bad case of
scratchiness with a pot I had dismantled and cleaned.
pot has 4 inner wipers for the conductive element and 2 for the outer
There seems to be a problem
with these pots
that the four inner wipers are not as extended towards the conductive
track as the two outer wipers. Since they share a common base,
and since this base may not be pressed hard against the rotor when we
look at the height of these wipers, there can be a serious problem
which definitely causes scratchiness:
The outer 2 wipers, being more
push the whole base away from the tracks, taking the base of the 4
inner wipers away, and so reducing the pressure these 4 inner wipers
place on the resistive track.
The fix is to bend the 4 inner
wipers away from the rotor until they are about as extended as the 2
are the wipers in their original state.
Note the set of 4 which is slightly lower. (This would be
"higher" in the final orientation of the pot in the TB-303, since in
these photos the shaft is downwards while in the TB-303, the shaft is
Below, after I
bent them all upwards a little, especially the 4 wipers of the outer
This appears to solve the problem.