Building your P3 Sequencer

N.B. These pages refer to the previous revision P3 boards, for reference only.

The P3 sequencer is constructed from 7 separate printed circuit boards.
These are:

	DIN output board
	Main board	(schematic)
	Function switch / LED board	(schematic)
	Step Switch / LED board	(schematic)
	Keypad board	(schematic)
	Upper pot board	(schematic)
	Lower pot board

There is also a 2 line by 16 character backlit LCD display (which is bought separately as a ready-built module).
The power socket and switch, and two extra pots for DATA and TEMPO functions, are mounted directly to the case.

The assembly of the PCBs is easy enough as long as you have some experience of soldering through-hole electronic circuitry. I would not recommend building a P3 as your first ever project.

As well as the PCB/CPU kit, you will need to buy all the parts on the bill of materials.

There are some choices available in the selection of the step, keypad and function switches. The PCBs have been made to take any of three different types of switch in these positions.
This photo shows the choices:

switch choice

From left to right, the switches are Multimec 3FTL6 tact switches, generic 12mm tact switches with a 7.3mm square projection (available from a number of different makers), and a keyswitch only available from Maplin in the UK.
Part numbers for these switches can be found in the BoM.
My preference is to use the Maplin keyswitches for the step and keypad switches, and the Multimec tact switches for the function switch board.
My rack mounting Schaeffer panel layout is designed for this choice of switches, as you will see from the photos.
All of these switches need caps. The Multimec caps are available in a choice of colours. The Maplin caps are a two part type, with has a clear outer cover that can have a printed label inserted.

Building the Boards

The boards may be built in any order. I prefer to start with the most complex board, so the job gets easier as it goes on. But if you're wanting to start on the easy boards and build your confidence, the DIN I/O board is easiest to start with. This is the complete board:

It has nothing more on it than 3 PCB mounting 5 pin (180 degree) DIN sockets, and a 16 pin IDC header.
These are the MIDI IN, MIDI OUT and DIN SYNC sockets. Depending on how you are building your case, you may prefer to use chassis mounting sockets. In that case, I suggest soldering ribbon cable directly to the sockets once they are mounted on your case, with a 16 way IDC plug already attached. The length of cable required will depend on your case layout. Chassis mounting sockets were used on the unit pictured at the top of this page.

DIN I/O Connector pin-out for chassis mount sockets:

Gnd	1	2	DIN SYNC PIN 3
Gnd	3	4	DIN SYNC PIN 1
Gnd	5	6	n/c
Gnd	7	8	n/c
Gnd	9	10	MIDI OUT PIN 5
Gnd	11	12	MIDI OUT PIN 4
Gnd	13	14	MIDI IN PIN 4
Gnd	15	16	MIDI IN PIN 5

The Main Board

The main board has the bulk of the components on it, including the power regulator, CPU, main memory, and interfacing logic.
The bare board looks like this:

It's usually easiest to start with the smallest components and work up, so that the components are held in place against the up-turned board for soldering. I usually start with the resistors.
N.B. On the first run of boards, R26 is incorrectly labelled as '1k'. Oops !
The labels for R20 and R21 are rather crammed in too - R20 is the vertical resistor between U9 and C10. R21 is parallel to Q1.
After the resistors are installed, the board should look like this:

main board + Rs

Next, the capacitors are installed. The capacitor types used are shown here for the avoidance of doubt:

mainboard caps

From left to right, these are:

C1, C2 - 33p ceramic disc
C3, C7 - 10uF electrolytic, 16v or above
C4 - 150p ceramic disc
C5 - 10nF polyester, 5mm pitch
C6, C8 - C12, C14 - 100nF ceramic disc
C13 - 1000uF electrolytic, 16v or above

Remember to install the electrolytic capacitors the right way round. Unless you like smelly smoke.
After the caps are fitted, your board should look like this:

mainboard + Rs + Cs

Or at least it will look like that if you sneak in the 10k horizontal preset resistor used for the LCD contrast adjustment, VR1.
Next stage is to fit the diodes:

mainboard + Rs + Cs + Ds

I have also fitted a wire link here, in place of F1, and I have not fitted D1.
F1 is an electronic fuse, rated at 500mA. It will protect the board against damage if anything is short-circuited. I save myself the cost of one by always handling my boards carefully. Whether or not you fit a fuse here, I leave to your discretion. D1 is a zener diode, which is there to draw a large enough current to trigger the fuse if the supply voltage rises above 5.6v (due to an unlikely regulator failure). If the fuse isn't fitted, there's not much point fitting the diode either.

The next stage is to fit the ICs, or sockets for them.
I use turned pin sockets for ALL ICs. This makes life much easier if anything ever breaks. You may find it easier to fit sockets before the capacitors.
PAY CAREFUL ATTENTION TO THE ORIENTATION OF ALL ICs.
For ease of PCB routing, a number of them are 'upside down'.
Here is the board, now with the IC sockets, and also the two transistors, 18MHz crystal and lithium battery holder fitted:

mainboard stage 5

Next, I've fitted the 16 way IDC headers, the 3 and 4 way SIP headers, and the voltage regulator, with heatsink. I bend the pins on the regulator using pliers before fitting, to make the heatsink sit flush with the board:

mainboard stage 6

And that's the soldering done ! Now all we need to do is fit all the ICs into their sockets:

mainboard stage 6

Once the backup battery is fitted, the supply to the RAM chips will be on permanently, so it's best not to fit the battery until you have mounted the main board into the case.

Next Page... the Function Switch / LED board.

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