SC502, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC502, v1.0, Power supply, reset, and status card designed for Z50Bus.

Important note: How to modify box headers

Experienced builders can just go ahead and populate the board. There shouldn’t be any surprises to catch you out.


This guide assumes you are familiar with assembling circuit boards, soldering, and cleaning. If not, it is recommended you read some of the guides on the internet before continuing.

First check you have all the required components, as listed in the parts list.

Before assembling it is worth visually inspecting the circuit board for anything that looks out of place, such as mechanical damage or apparent manufacturing defects.

If you have a multimeter that measures resistance or has a continuity test function, check there is not a short on the power supply tracks. Connect the probes to each terminal of connector ST2. This should be an open circuit, not a short circuit.

The picture below shows what a completed SC502, power supply, reset, and status card should look like.

Switch Polarity

The power ON/OFF toggle switch can be set to either up for on or down for on. Solder a wire link at JP1 to select the desired polarity. An offcut resistor leg is a suitable piece of wire.

To select UP for ON, solder the link in the position illustrated below in green. To select DOWN for ON, solder the link in the position illustrated below in red.


Fit and solder the 1k resistors, R1 to R5 (shown below in red).

Fit and solder the 4k7 resistor, R6 (shown below in green).

Resistors can be fitted either way around, as they are not polarity dependent.

Diode 1N4001

Fit and solder the 1N4001 diode, D1.

Diodes must be fitted the correct way around. The light-coloured band at one end of the diode must be fitted into the circuit board at the end indicated by the silkscreen.

IC socket

Fit and solder the IC socket.

Be sure to fit it with the notch matching the legend on the circuit board, so you do not end up fitting the IC the wrong way around too.

Header pins

Fit and solder the header pins.

JP2 and JP3 together are a double row of right-angled pins.

P2 and P3 are also a double row of right-angled pins. These can either be fitted as two separate 2-pin long sections or as one 5-pin long section with the middle pair of pins pulled out (with pliers)

You may need to cut these sections of header pins from a longer strip, by cutting through the plastic with wire cutters.

Light Emitting Diodes (LEDs)

Fit and solder the LEDs, LED1 to LED5. The suggested colour scheme, a mix of 2 red and 3 green LEDs, is illustrated below.

If using angled LEDs, as illustrated to the right, the polarity is predefined and matches the PCB.

If using standard LEDs, as illustrated to the left, care must be taken to ensure they are fitted the correct way around.

It is important to fit the LEDs the correct way around. LEDs usually have a small flat side to indicate the cathode (the negative end). This should be positioned to match the flat side shown on the circuit board (illustrated to the right). Also, the cathode pin on the LED is usually shorter than the other pin (the Anode).


Fit and solder the toggle switch, SW1 (shown below in red).

Fit and solder the reset switches, SW2 and SW3 (shown below in green).


Fit and solder 100 nF ceramic capacitors, C3 and C4 (shown below in red).

Fit and solder 1 nF ceramic capacitors, C5 (shown below in green).

These capacitors can be fitted either way around, as they are not polarity dependent.

The exact value of the 100 nF capacitors is not critical. The use of very cheap capacitors within the range of about 30 to 200 nF is acceptable.

Capacitor (100 uF)

Fit and solder the 100 uF electrolytic capacitors, C1 and C2.

It is important to fit this capacitor the right way around. The negative terminal is indicated with a ‘minus’ sign, as illustrated to the right. The negative terminal also has a shorter lead.

The silkscreen has a plus sign for the positive terminal and a hashed area for the negative terminal

Bus connector

Fit and solder the bus connector, P1. This can either be a right-angled box header or right-angled header pins. The box header is recommended.

Take care to ensure the connector is fitted such that the card will stand vertically when fitted to a backplane. A good method is to just solder two pins, one at each end, and then check it looks correct. Then solder two more, one at each end but in the other row, and check again. At each stage, if necessary, adjust the position by heating the required solder joint and moving the connector slightly. Solder two pins in the middle of the connector and check again. Then solder all the others.

Power connectors

Fit and solder the 2.1 mm power jack socket, SK1 (shown below in red).

Fit and solder the two screw terminal blocks, ST1 and ST2 (shown below in green).

Ensure the wire entry holes face the edge of the PCB.

Voltage supervisor and reset

Fit and solder the DS1233 voltage supervisor and reset device, U2.

This device must be fitted the correct way around. Match the shape of the device to the legend on the circuit board.

It is necessary to spread the legs of the device to match the hole spacing on the circuit board. Do this gently so as to avoid straining the legs where they enter the plastic casing. DO not press the component hard into the board as this will also strain the legs.

Voltage regulator 7805

Prepare to fit the 7805 voltage regulator, U1.

Bend the legs such that the hole in the heatsink part of the regulator lines up with the hole in the printed circuit board.

If you intend to expand your system or run it from a power source near the maximum 15 volts, then it is recommended you fit a heatsink. Sandwich the heatsink between the PCB and the regulator.

Bolt the regulator in place with an M3.5 nut and bolt or screw. The bolt should be fitted from the back of the PCB (solder side) and the nut fitted on the front (component side).

Finally, solder the regulator’s pins.


A spacer may be fitted, in the position shown below, to help prevent cards from touching each other when fitted to a backplane. The spacer should be made of a non-conducting material, such as nylon. and be held in place with an M3 (3mm diameter) screw.

Header pins (P4)

Header pins, P4, are not included in the kit. The header is only required if you plan to mount the LEDs off the SC502 PCB, such as on the panel of a case.


Remove any solder ‘splats’ with a brush, such as an old toothbrush.

Visually inspect the soldering for dry joints and shorts.

Clean the flux off with suitable cleaning materials.

Visually inspect again. The importance of visual inspection can not be overstated.

Quick Tests

Repeat the check made earlier for a short. Connect the probes to each terminal of connector ST2. This should be an open circuit, not a short circuit. If you are using a digital meter set to measure resistance it will likely take a few seconds for the reading to stabilise as there are now capacitors on the power lines. A reading of more than 100 kΩ (100000 ohms) is acceptable. Repeat this test on ST1.

Integrated Circuit (U3)

Insert the integrated circuit (74HCT14) into the socket, taking care to insert it the right way around, as illustrated below. Be careful not to bend any legs over.

Functional testing

With the PCB on a non-conducting surface, connect a power supply to either SK1 or ST1. SK1 is a 2.1 mm barrel socket (centre positive) and ST1 is a screw terminal block. The power supply should provide between 8 and 15 volts D.C. and be able to source 300 mA or more. Note, an expanded system may require up to 1000 mA (1 ampere).

When the toggle switch is in the ON position the power LED (LED1) should light. The other LEDs may be on, off, or flickering, as the PCB is not connected to a working system.

If you have a voltmeter, check the voltage at ST2 is between 4.75 and 5.25 volts. If it is outside this range there is something wrong.

Now switch off and connect jumper shunts to JP2 and JP3. The shunts should be parallel to the circuit board, not perpendicular.

Turn on again and you should see the reset LED (LED2) light for about 0.5 seconds and then it should turn off. The LED lights for a short while following switch on as the voltage supervisor and reset device (U2) generates a nice clean reset signal. If the reset LED stays on it usually indicates the 5 volt supply is less than about 4.75 volts.

Test each reset switch causes the reset LED to light while the button is pressed and continues to light for about 0.5 seconds after the button is released, and then the LED should turn off.

You are now ready to use SC502 to power a Z50Bus system.

More information about SC502 can be found here.

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