Oakley 3031 - Manual (Page 8)

The ceramic capacitors should be ‘low-K’ or C0G ceramic plates. The lead spacing is 0.2” or
5mm. Do not chose cheap and nasty ceramic types, usually ‘high-K’, obtainable from some
surplus places. These can lead to a noisy audio output.
The horizontal preset or trimmer resistors are just ordinary carbon types. No need to buy the
expensive cermet types for these positions. Carbon sealed units have more resistance to dust
than the open frame types. Piher make a suitable type to use here. Pin spacing is 0.2” at the
base, with the wiper 0.4” away from the base line.
The multiturn trimmers are the ones that have the adjustment on the top of the box. Spectrol
and Bourns make these. Some types are 22 turns, while others are 25 turns. Either will do.
They should have three pins that are in a line at 0.1” pitch. Don’t chose the 10-turn ones with
the adjustment on the end, they won’t fit on the PCB.
The BC549 transistors can be pretty much any NPN transistor that corresponds to the same
pin out. For example: BC550, BC548, BC547 etc. However, I recommend using BC549 or
BC550 only as these are low noise devices.
The transistors specified as BC559 can be any general purpose PNP types that have the same
pin out. For example the BC560 may be used instead.
Quite often you see an A, B or C suffix used, eg. BC549B. This letter depicts the gain or
grade of the transistor (actually hfe of the device). The 3031 is designed to work with any
grade device although I have used BC549B and BC559B throughout in my prototypes.
The SCR in the VCO is different. Here I have used BC212L and BC182L because when
testing the prototype they gave better pitch stability. I am unsure why this is, so if anyone has
any idea I would like to know. I suspect it has more to do with the layout of my original
breadboard. The three BCxxxL types needed in the SCR are provided in the semiconductor
kit.
The two J-FETs in the TB3030 are the 2SK30A and the J201. The original TB303 used two
variants of the 2SK30A, the O and the Y types. These two different types differ only by their
value of Idss. In earlier editions of the TB3030 User Guide I stated that the variants differed
by values of Vp, the pinch off voltage. This is true. But they are formally defined on the
datasheet as having different ranges of Idss. This is the current that runs through the device
when the gate and source are grounded and the drain is taken to 10V. The O types are defined
as having a Idss of 0.6mA to 1.4mA. The Y types are defined as having Idss of 1.2mA to
3.0mA. Look at the range of Idss for the 2SK30A-O. It is interesting to note this, because the
sound of the squarewave is dependant on the Idss of the O type device. So no wonder that
people have commented some TB303s sound slightly different to one another.
The 2SK30A-O is getting hard to find in large enough quantities, so I needed a substitute. The
3031 uses a J201 in place of the difficult to get 2SK30A-O. It has a different pin out, but its
electrical performance is very similar. What’s more there is less variation between Idss in
various specimens of the J201, so everyone’s 3031 should sound the same.
8