Devil Fish mods to the TB-303

Simple TB-303 mods and maintenance

• 1996 September 15:  Original version.
• 2001 January: To cover replacing the pots.
• 2001 March 13: Reliability in general
• 2001 April 22: Fixed mention of resonance pot - should be linear.
• 2002 April 26 and 28: Mentioned the newly manufactured replacement pots, some other replacement parts and corrected my mention of the Decay pot to be log, rather than linear.
• 2002 May 2: More information about the shaft-length of the six pots, and the difficulty of fitting the knob on the new Resonance pot.
• 2002 May 26: Updated Rob Cyborgzero's email address.
• 2002 August 24: Updated Rob's web site, report on his translucent knobs and wrote some more about replacement pots.
• 2004 August 5: Updated contact details for http://www.technologytransplant.com .  Updated reliability information on dust-guard protected switches. Various other updates.
• 2005 August 25: Added note about restricted active rotational range of replacement Resonance pots.
• 2007 August 21: Linked to Mouser rather than Farnell for SKHCAA.
• 2007 November 20: Notes on beta of TIP30A.
• 2008 January 27: Pointer to new page on the 6 small pots.  pot-wear/
• 2010 January 3: Updated links to Technologytransplant.biz and deleted information which is either obsolete or better covered by the pot-wear/ page.
  

This is: http://www.firstpr.com.au/rwi/dfish/df-mods.html

Here are some modifications and maintenance tips.  Please also see the notes on spare parts in Q9 of the FAQ at the main Devil Fish page.

To the main Devil Fish page.
 

1 - Replacing the switches and installing a dust guard

The TB-303 has 24 "tact" (tactile) switches which are made by the Japanese company ALPS http://www.alps.com who make the pots and rotary switches in the TB-303 and the miniature pots in the Devil Fish. These are good switches, but they are not sealed against dust.

Household dust, especially flakes of skin, get into the switch and contaminate the contacts, causing them to become unreliable. This can make the TB-303 completely impossible to program.

These instructions tell you how to obtain new switches, and install them with a dust guard to prolong their life in the inevitable presence of dust. These instructions are directly relevant to the TR-606 as well. The TR-808 uses a slightly different version of the switch – the SKHCAB. It is possible to make a plastic dust-guard for the TR-808 - it is just a strip of plastic between the buttons and the switches.  This probably only partially protects the switches from dust, but it is worth doing.  The thin plastic I specify below adds slightly to the resting height of the buttons, but that is OK.

Even if only a few of your switches are bouncing, I recommend replacing the lot and installing a dust guard. I in 2004 only one person with a Devil Fish from 1993 or 1994 has contacted me about bouncing switches, and one person with a Devil Fish from 1996 has also had his switches replaced.  Both were heavily used machines.  So it seems that the dust guard greatly prolongs the life of the switches, but does not ensure that they last forever in well used machines.


Obtaining the SKHCAA switches

You may be able to buy the ALPS SKHCAA switches as spare parts from Roland or Roland service agents. It may be easier to purchase them from a general electronics distributor, or most likely one which specialises in distributing ALPS products. ALPS are a major (probably the major) Japanese manufacturer of switches and potentiometers. See the notes at the end of this section about ALPS branches in major countries.

You can obtain them from Farnell Electronics www.farnell.com. The SKHCAA is order code 733-271 and the SKHCAB (for the TR-808 and JP-8) is 733-283.

Mouser also has SKHCAB - for the TR-808: www.mouser.com/search/refine.aspx?Ntt=688-SKHCAB.

You will need 24 SKHCAA switches to replace those in the TB-303, but they are relatively cheap, especially when purchased by the hundred. These switches are not labeled SKHCAA, but have black buttons and bodies and a 130 gram operating force. A similar shaped switch (SKHCAC) has a dark grey button and a 260 gram force. You can use these if you like - they are harder to press, but not too hard.
 
 

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.

De-soldering the old switches is pretty straightforward. Take them off with their buttons still attached. 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.
 
 

Removing and cleaning the buttons

With a pair of pointy-nose pliers, it is relatively easy to prise off the buttons from the switches – once they have been de-soldered.

Wash the buttons and the knobs with a toothbrush (an electric toothbrush is better, as it is for teeth!), in water with detergent. Rinse thoroughly, shake out water from the holes and allow to dry in warm air.
 
 

Installing the new switches

This is easy – just solder them in. Don't put the buttons on yet!
 
 

Preparing the dust guard

The SKHCAA switches have a stem which the chrome-plated plastic buttons sit on. The dust guard is a piece of flexible plastic with 6 mm (1/4") holes for each stem, and four smaller holes for the guide pins on the Run-Stop and Tap switches.

I make the dust guard from plastic sheet cut from a 3M overhead transparency protector. These consist of two sheets of clear plastic sheet 200 mm x 300 mm, bonded together, with two white cardboard fold-out strips. They are readily available from office supply shops and are labeled "3M Flip Frame (tm) Transparency Protector".

Cut a section of plastic sheet film 285 mm x 50 mm. Now you need to punch 6 to 6.5 mm (1/4") holes in it - one for each switch stem and one for each LED. I have my own template for doing them en-masse, but you will need to lay the plastic over the switches and LEDs and mark with a pen where the hole centres should be. The stems of the newly installed switches should make excellent references for your pen marks.

I align the rear of the sheet about 8mm beyond the top row of LEDs. Then use a hole punch from the office supply shop to punch the holes.  (I use an acrylic template with 1mm holes to drill pilot holes in twenty or so sheets of plastic at a time.  Then I staple four or so together and use a specially modified and very sharp 1/4" hand-operated hole punch.  The punch has a filed down, pointed, piece of 1mm drill inserted in a hole I drilled in the centre of the punch.  This helps me locate it correctly.  The punch must be very sharp.  I find it easier to use it quickly with four to six layers or plastic - if I only punch one, the plastic is so thin and flexible that it does not cut cleanly.

There also need to be four small holes for the Run/Stop and Tap buttons.  I do these with a small leather hole punch.
 
 

Putting it all together

Now place the dust guard over the switches and LEDs, using the punch to correct any holes which are not quite where they belong.

Install all the buttons and carefully check that the dust guard plastic is not restricting their movement. By cutting the dust guard judiciously with a knife, any tensions can be alleviated.

Cut the dust guard to make way for the two screws which will hold the front panel PCB and battery guide to the top half of the case.

Check the alignment of the brown and orange wires on the left of the front panel. These should be pressed into the gap between C202 and S4. Test the operation of all switches and LEDs. Beware of broken wires or fractured solder joints in the cables leading to the front panel. Bend the white ribbon wires so they are not pressing against the top half of the case.  (I routinely resolder both ends of the white ribbon wires between the main and front panel PCBs - these joints often fracture if this is not done.)

LEDs can sometimes fail to light because they have been hit physically in the past, weakening their solder joints - so waggle them to test this and re-solder them as required.  Once I had a TB-303 with a LED which was internally corroded due to some liquid being poured on it in the past.  The LED worked fine, but it had a resistance in the reverse direction which upset the switch scanning, causing a curious effect: random activation of button functions within a set of switches when any one of the switches is pressed.

See the notes below on general maintenance before closing the machine up and giving it a final test.
 
 

Obtaining ALPS switches

The ALPS website should have more up-to-date distributor information: http://www.alps.com
 

2 General maintenance

Here are a few tips on keeping your TB-303 healthy, and in reassembling it.

2.1 Never spray anything in the machine

Do not under any circumstances spray anything, such as WD-40, contact cleaner etc. into any item of electronic apparatus.

If pure water gets into the machine, due to rain or some other source, just let it dry.  If there is lots of water, pull it apart and mop it up with tissues before letting it dry in warm air. The 24 tact switches will not like being immersed in anything – but they may survive water.  But how could they dry properly, since the actuator is closed against the retaining ring except when it is pressed?

If some other liquid - such as Bundy (Bundaberg Rum) and Coke - gets into a machine, open it up and wash it away with warm water, using a toothbrush and maybe just a little detergent. Then let it dry in warm air.   Best replace the tact switches!
 
 

2.2 Fixing little things - especially replacing the pots

There are a few things which often go wrong with the TB-303. This is not a comprehensive treatise on maintaining or adjusting the machine - just some tips which may be useful. 

Overheated power transistor

Sometimes, presumably due to application of more than 9 volts, the power transistor Q45 may be brown or defective. I replace them with a relatively common PNP transistor called a TIP30. (Farnell Electronic Components has them). Whatever you use, make sure it has the pin functions, left to right, of Base, Collector and Emitter.  The TIP30 has the same connections.  However, its beta (hFE) may not be as good as the original 2SB596 transistor.  The TIP30A spec sheet from Multicomp from Farnell quotes minimum beta of 40 for 0.2A at 4 volts VCE.  However the minimum is only 15 for 1A at 4 volts VCE.  The 2SB596 datasheet from: http://www.ortodoxism.ro/datasheets/WINGS/2SB596.pdf quotes a minimum of 40 for 1A with 5V VCE.

 

Pinched wires during re-assembly

It is very easy to pinch various wires when re-assembling the machine – especially black wire 28 near the output socket. Replace any wires which have been pinched and exercise care when re-assembling.
 
 

Knobs falling off

I have not got the perfect answer to this, but 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.
 
 

Batteries not connecting

Early TB-303s had a positive battery connector which was rather thin. Later ones were pressed differently and were a few mm more towards the positive end of the battery. Simply remove the old battery connector and form (IBM Selectric service instructions use the term "form" instead of "bend") the edges of the connector so the contact area sticks out a little more.

The negative spring connector can be a problem too – especially when it has been subject to corrosion from leaking batteries. Remove it from the machine, wash any corrosive materials and rust from it and file the end to expose fresh steel.

The wires to these connectors often fracture after several assembly-reassembly cycles, because the wire is impregnated with solder. Simply trim back the wires and re-solder them.

Technologytransplant.biz now sell a kit with replacement battery contacts (very nicely made, including the negative contact spring) and a new battery compartment cover.
 

Weirdo corrosion of RAM chip pins

On several occasions, I have encountered an extraordinary fault: severe corrosion of pin 18 (power) and pin 10 (/WE) of the 1K x 4 static RAM chips.  This is only a problem if there has been traces of corrosive battery material in the case for long periods of time.  The corrosive gunk does not need to be near the chips, but I assume that the gunk provides a corrosive atmosphere and that the chips must be battery-backed-up for long periods of time, since the corrosion only occurs on those pins which are at +5 volts from the battery supply. (Actually, it can be more like 5.5 volts!).  Only one type (manufacturer) of chips is affected.  I can't remember which, but it is not the NEC chips.  The pin is typically visibly "rusty", but in one case, it looked to be in quite good condition, despite being completely corroded inside the plating, and not making reliable contact.  Replace with hard-to-get low power chips, not with 2114s which are not suitable for battery backup.  (I have spare RAM chips if you want some - they are now hard to get.  They were last manufactured in the early 1980s.)

3 Increasing the range of the Cut Off and Env Mod pots

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.
 
 

3.1 Cut Off pot extended range

To increase the high range of the Filter Cut Off pot, turn TM3 (near the mode switch) clockwise.

To provide a lower limit to the range of frequencies, without altering the high limit, short out R47 - a 10K resistor just to the left of the Env Mod pot. Do this short with a small length of insulated wire on the rear side of the PCB.
 
 

3.2 Env Mod pot extended range

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.

To extend the range of the Env Mod pot to include zero envelope modification, then short out R61 (10K) which is located logically and physically to the anti-clockwise pin of the Env Mod pot.
 
 

4 Improving the bass response

The standard TB-303 has a weedy bass response. The culprits are C20 and C21 – both 0.01 uF. Take these out and replace them with 0.1 uF capacitors. I use monolithic ceramic caps for this because they are compact. It does not matter that mono caps have a wide capacitance tolerance and a lousy temperature co-efficient. This is a non-critical value and anything around 0.1 uF is fine.

This gives a worthwhile improvement in bass response. If you want to restore the original TB-303 sound, just do it externally with EQ.
 
 

Robin Whittle - First Principles and Real World Interfaces
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