User manual

35
they reduce the voltage drop between the four C-cell batteries and the CPU, enabling the
machine to run from a lower total 4 x C-cell voltage. The standard TB-303 draws about
85 mA, or more depending on the number of LEDs illuminated, whether the headphone
amplifier is on, and what signal it is driving into the headphone load. The Devil Fish
draws more current, depending on the three new LEDs it may be turning on – Gate and
either the Normal or Accent Decay LED. It may draw more current due to the headphone
amplifier being on (if a lead is plugged into the old headphone socket) and the MIDI In
system draws a little extra current too. The new Blue and other colour LEDs do not draw
more current. The new Red LEDs draw the same current as the original LEDs and the
other colours, especially the Blue, draw less current because the junction voltage of the
LEDs gets higher with shorter-wavelength light. The 32 Bank Memory system does not
draw any extra current.
Peak currents for the Devil Fish may be 150mA or so. This reduces the battery voltage
more than the usual TB-303 currents, and induces greater than usual voltage drops in
some of the internal power supply circuitry. The three 4.0C modifications reduce these
voltage drops between the battery (or the external power supply, after the internal 6 volt
regulator) and various of the machine. The first two reduce voltage drops to all sections
of the machine’s power supply – for the CPU and the switching supply which provides
+15 volts. The third one only reduces voltage drop to the CPU. The aim of all these is to
enable the CPU to operate with battery voltages which are lower than those which would
otherwise be necessary.
1. A 1k resistor is wired across R169 (1.5k). This more than doubles the base
current to Q44, which – in one machine at least – reduced the voltage drop
between Q44’s emitter and collector from 112mV to 76mV. (Actual voltages
would depend on the particular Q44, on the temperature and on the current being
drawn.) This adds about 1mA to the total current drawn, but it saves around
36mV (or more at higher load currents) and so enables the machine to work from
lower voltage batteries.
In V4.B, R169 is removed, and instead of adding a 1k, I use a 560 ohm resistor
driven from a resistor-equipped transistor (UNR4212, NPN with two 22k
resistors) which itself is driven from a 4.7 volt zener diode from the input voltage.
This shuts off Q44 if the input voltage drops below ~5 volts, to stop the C-cell
batteries being discharged excessively if the machine is run from them for an
excessive time.
2. Short out R168 (2.7 ohms). This is a special resistor, intended as a fuse. It would
only operate if there was a drastic problem with the internal circuitry, such as a
dead-short failure of Q44, or perhaps a dead-short failure after R168 – but I doubt
if a normally driven Q44 could pass enough current to blow this fuse. In the 250
or 300 or so TB-303s and TR-606s I have worked on, I have never seen such a
fault and I have never seen one of these “fuses” blow. I have on two or more
occasions found machines where this device has a higher than normal resistance –
and so needed to be replaced. Shorting out a protection fuse is not generally a
good idea, but since this is battery operated equipment, and since the fuse
sometimes causes trouble (by developing a higher resistance) and has never been
observed to prevent trouble, I think we are better off without it. The benefit is
that we no longer have a 368mV (in one machine) voltage drop. This change
significantly lowers the voltage the C-cells can go to before the machine will no
longer operate.