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Devil Fish mods to the TB-303

Simple TB-303 mods and maintenance

Robin Whittle  2 January  2010
Update history: 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 switches and installing a dust guard

SKHCAAs are available at Mouser in the USA:  www.mouser.com/search/refine.aspx?Ntt=688-SKHCAA  They used to be available at Farnell:

2 General maintenance

2.1 Never spray anything in the machine

2.2 Fixing little things - especially replacing the pots

Please see the page pot-wear/  for photos and more discussion of the 6 small pots.
The Devil Fish requires more power than the TB-303, due to the MIDI In system, the other circuitry and the new LEDs.  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.

I picked those transistors which required less than1.5mA and put a 2.2k across R167, to boost the base current at lower voltages.  Perhaps some manufacturer's TIP30A and TIP30Cs (higher collector voltage) would have generally better betas.  

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

3.1 Cut Off pot extended range

3.2 Env Mod pot extended range


4 Improving the bass response



 

Robin Whittle - First Principles and Real World Interfaces
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