User manual

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capacitors in parallel with 33k resistors, to ground, for both the Clock and Run/Stop pins
of the Sync socket. The reason for this is explained is the following paragraphs:
There can be coupling between the two MIDI signal wires (for pins 4 and 5 of the 5 pin DIN socket
– the intermediate pins) and the two outside pins (1 and 3) which are for DIN Sync, if the cable has
all five pins connected. (A pure MIDI cable, not suitable for Sync, would have pins 4 and 5 as
signal wires in a shield which connects the centre pin: pin 2. Such cables will not cause the trouble
described here. ) The cause is capacitive coupling from the wires for pins 4 and 5 – and perhaps
from the ground wire, which may be a shield for the other wires – and the wires which connect to the
outside pins 1 and 3, which are for Sync: Run/Stop and Clock respectively. The source signal for
this coupling is most likely to be the AC signal of the MIDI Data waveform itself. The capacitively
coupled spikes (which this modification prevents) should not have caused any serious problems, but
they had been observed – with a long and so high-crosstalk cable – to cause a slight illumination of
the Run/Stop LED when it should be Off.
Another potential cause of capacitive coupling trouble in a 5 wire MIDI lead would occur if the
master device was not grounded, and was powered from a mains adaptor which was strongly coupled
to the mains voltage. (Many switch-mode adaptors have large value capacitors connecting their
output to both the active and neutral pins of the mains. This is done to reduce RF emission and so
meet the requirements of various regulatory authorities, but it means that if there is no grounding of
the device itself, it will typically have a ground voltage of about half the mains voltage. In 230 volt
counties such as Australia or the UK, this means over 100 volts 50 Hz AC.) However, this would
only occur if the centre pin (2) of the cable did not connect the ground of the master device to the
ground of the slave. Normally, with MIDI In sockets, there is no connection to pin 2, and this would
leave the master with a high AC voltage on its ground with respect to the slave device. However, the
Devil Fish retains the TB-303’s 22 ohm resistor which connects pin 2 to local ground. So the just-
mentioned 100VAC or similar difference between grounds of master and slave would only occur
with the Devil Fish if the middle pin of the cable did not connect on the Devil Fish side, which it
does, via the 22 ohm resistor. The same problems could occur if the master device was grounded
and the Devil Fish was not, with the Devil Fish being powered by an adaptor which had a high
capacitive coupling to the mains. Likewise, if neither device was grounded, but one or both were
powered in a way which involved significant capacitive coupling from the mains.
In late 2013 we began using a new set of replacements for the six small pots. These are
the same pots as are used in the Cyclone Analogic Bass Bot. The pots we used before
this had shafts which were not quite tall enough, so we mounted them no some laser-cut
phenolic board spacers. The new pots have a suitable length shaft and don’t need any
special mounting. We continue to mount the knobs a little higher on the shaft than usual,
since this makes them easier to grip.
As before, we dismantle the pots, remove most of the thick friction grease (this is where
the shaft and rotor meet the body, it is not in the resistive track and wiper area, though it
can migrate there in years to come), and add a little light silicone grease, to make them
easier to turn. We also modify them to support the base of the shaft a small fraction of a
millimetre higher than how they are normally supported. This is not so much to increase
the height at the top of the shaft, but to reduce side-to-side slop in the shaft and to make it
more difficult (and perhaps impossible) to cause the rotor of the pot to cut into the
conductive and resistive tracks if the knob is pressed down on while being turned.
Version 4.2B
[February 2015. Starting with SN 289.] We changed the arrangements for battery
backup supply for the memory. This includes an undervoltage protection system so the
power supply is only enabled if the battery voltage (or the adaptor voltage, after passing
through the internal ~ volt regulator) drops below about 5 volts. Please see the separate
manual DF-Memory-Backup.pdf