Since MOS and CMOS ICs are very susceptible to damage via static electricity, you need to take special care with them. As we believe that most makers are already familiar with techniques of working with these ICs (CD4017, CD4040, 2716, 2732, 6116 and Z80A), we’ll mention just a couple of basic pieces of advice:
Use a grounded soldering iron. If you don’t have one, convert an ungrounded soldering iron by wrapping a grounded copper wire around the cold end of the metal, that which is nearest the handle.
If the room in which you are working has a synthetic carpet, the static potential of your body can reach up to 300 volts! That doesn’t pose a threat to us, since that electric charge dissipates very quickly when we touch a grounded object, but if that discharge goes through a pin of a MOS or CMOS IC, it will be rendered useless. This why such ICs are kept in anti-static tubes, have their pins tucked into special conductive sponge or simply wrapped in conductive tape.
Once soldered in, the IC isn’t in much danger, so after we are done we can do away with all these protective measures.
The computer housing — a thread makes a suit.
The mechanical design of the housing we leave up to you, but we will make one suggestion: There’s plenty of copper left on the sides of the PCB, so you can use the same material for the box and simply solder the sides to the PCB. This way, the PCB with components becomes a mechanical base for the whole box, for which purpose FR4 satisfies all mechanical needs.
1. We need to carefully plan the dimensions of each part of the box on paper, knowing which side goes over which joints. You can use the popular OLFA scalpel to cut out the material by scoring the surface on both sides of the panel. It’s then easy to just break the panel if the marks are deep enough. After cutting, use a fine file to smooth the edges. Edges that will be soldered should be filed straight, and exposed ones should be soft.
2. First we clean the soldering surfaces with an eraser gum or fine sand paper. Then we let the 24 or 30 W soldering iron get really hot and put solder on all cleaned surfaces. This is much easier with flux.
3. Before soldering the whole side, we solder just a few points. That way we can make an inspection and perhaps a correction. Once fully soldered, the side of the box is practically impossible to desolder without damage.
4. When soldering the sides, one should remember that solder shrinks while cooling: if we want right angles, we orient to sides with a slight outward angle, as seen from soldering side, lower side on the picture. After soldering, the solder will pull the sides towards one another.
5. After thorough inspection of position and angle of the surfaces, we solder the complete joint. It might be necessary to wait for the tip of the soldering iron to get hot again after every few centimeters. You might be able to solve this problem by using a stronger soldering iron, but that can be dangerous: overheated copper can separate from FR4.
6. We can solder a couple of 10mm high sides to the top cover, which will be adjusted to make a tight fit with the sides of the housing. That will hold the top in place.
7. To make the top sturdy, we solder one narrow strip of FR4 along the middle. The only thing that’s left is the bottom, which we can make from any non-conductive material. We find that 4mm thick Plexiglas is the most suitable, attached to the main board with four M3 screws and spacers for separation.
8. There’s a well known procedure to paint the housing and markings which has all the qualities of screen printing process, looks good, is mechanically resistant, and can be easily done by an amateur. We will need two spray paints (one white and one blue, number 469), a bottle of gasoline for cleaning, letraset-letters and, optionally, lines.
9. With very fine-grit sandpaper, sand the whole surface to be painted. It must not be glossy in any place, or paint will fall off rather quickly. Clean it thoroughly and then degrease with gasoline.
10. Make an even coating with the white spray paint. This layer should be left to dry for at least 3 hours, but not in a cold or humid environment.
11. Use the letraset letters to print text on the now dry surface. If we pull lines by the edges of the box and keyboard opening, we’ll get much prettier design. Using a clean and dry finger, press each letter to make sure that it’s properly glued.
12. Carefully spray paint another layer, now with the darker color. This layer should be as even and as thin as possible, just thick enough not to see the color underneath.
13. After about an hour of drying, but not much longer, use your finger nail to remove all the lettering and lines. The cover might look a bit imprecise after this phase. Don’t worry about that for now.
14. Use a clean cloth or paper tissue dabbed in gasoline to rub the surface, and you’ll be surprised by nice looking lines and letters.
Power Supply Transformer and Regulator
Transformer electrical schematic.
Transformer mounting.
Transformer PCB
We need to say up front that the stabilized 12V supply is only used for RF modulator; you can leave it out if you are not using one, or if yours requires 5V. You save on components D3, D4, D5, C4, C5 and R1 this way. Capacitor C6 on the primary side of transformer is used to eliminated unwanted interference coming from the mains. The transformer is full-wave and you get 11V of direct current and filtered voltage on capacitor C1. The 7805 voltage regulator can supply about one amp at 5V. It’s a good idea to use a transformer with that much current, no matter that the computer will only use about 400 mA. The rest of the available current can later be used to power future expansions.
Capacitors C2 and C3 protect the 7805 from oscillating.
Because 7805 dissipates a lot of heat during operation, we need to mount it on a heat sink. If we don’t have a ready-made one, we can improvise it from three chunks of aluminum with dimensions of 35×80, 35×110 and 35×140, of which each is bent in two places to form a letter U. The opening on the metal tab of the voltage regulator is for an M3 screw to tighten it to the heat sink. It is advisable to put some silicon paste to the contact surface of heat sink and regulator, to ensure good thermal conductivity. You can choose your own box in which to mount this transformer. It should have cooling vents and if the case is conductive, you will need a three-prong cable to the socket. Use green-yellow cable wire to connect ground on the socket plug to the ground of the box and transformer.
Simple Procedure, Fantastic Effects
To be able to turn a regular black-and-white TV into a computer screen, we must respect one crucial requirement: video input can be added only to TVs that have an AC/DC transformer. TVs with a hot chassis are very dangerous for modifications because they are not galvanically isolated from the computer and therefore can endanger the life of the one using it.
How do we test if our TV has a hot chassis? If you don’t have enough experience and knowledge, skip this and let a professional deal with it. If you are sure about your knowledge, open up the TV and plug it in35 without ever touching its metal parts. Measure the potential between TV ground and socket ground. Unplug the TV, turn the plug 180 degrees, plug it back in and repeat the measurement. If at any point you read any voltage during measurement, unplug the TV, close it and give up on further modifications. The solution to your problem is RF modulator.
If in both cases there was no voltage, you can continue checking. Resistance between either poles of the TV plug and TVs ground must be infinite. (Measure this, of course, while the TV is unplugged.) If this checks out too, you have green light to continue with modifications.
First, get the schematic diagram of your TV — without it every effort is pointless. Find the entry point into the first stage of video amplifier. There you will find a marking for “white level” voltage and sync is two volts below it. Transistor TVs usually have white level at + 3V, and sync at + 1V. Leave the voltage from the splitter connected to the transistor base, cut the trace that leads the signal from video-detector and connect it as shown on the picture. You need to add one bipolar electrolytic capacitor of about 5
0 μF or, because bipolar electrolytic capacitors are hard to get, you can use two regular electrolytic of about 100 μF tied in parallel. (Pluses towards each other, minus to the video signal socket and a switch that chooses the TV function.)
On the back-pane of the TV, drill a hole for a switch and video signal socket. Use cables as short as possible, shielded or at least twisted around each other. Same goes for the cable that connects the computer to the screen. With that, we are done with modifications. Close the TV and connect it to the computer. When you turn them on, you’ll probably need to adjust horizontal and vertical synchronization, as well as image contrast until you no longer see letter ghosting.
TV splitter
Don’t panic, everything is going to be fine.
First, plug in only the transformer. Measure the voltages: stabilized 5V voltage must vary no more than ± 0,25V. For the 12V supply required by some RF modulators, variations can be up to ± 1V. After you’ve made sure that voltages are within safe margins, connect the transformer and computer grounds by a wire, set the amp meter to highest setting and touch the + 5V transformer output with a plus side, and minus side to + 5V of the computer. The meter should show a current between 300 and 500 mA. If the reading is within the margins, remove the meter from + 5V and do the same measurement with + 12V. Depending on the model RF modulator, as it’s the only component run by this current, the reading should be a couple milliamps. To be able to register it, we must lower the range on the meter.
If everything is all right, we can remove the amp meter and connect the display, then connect the transformer to the computer and turn it on. If we are using RF signal and TV receiver, we need to go through all three bands to find the best reception. The computer will display it’s first word ever: “READY.”
It’s important that it starts working, eventually.
If the computer doesn’t start up at first, do not panic: some difficulties are inherent in amateur work. If the picture is there, but is unstable, try to adjust vertical and horizontal sync on the TV or display. (These knobs are usually on the back side of the TV, but you might need a screwdriver.) If you can’t see anything on the screen, increase the brightness.
Perhaps instead of one, you see nine smaller images (three by three) with black edges without text. This is simple to fix: the crystal, instead of 6.144MHz, is oscillating at three times that frequency! To fix this, solder a C5 capacitor with a value between 10 and 30 pF. As with any other modifications, first unplug the computer.
If the computer is completely silent, carefully touch each component, especially the ICs. The voltage regulator’s heat sink should be warm just after a few minutes, same goes for transformer diodes and transformer. Only the CPU and EEPROMs out of all ICs can be hot, and even those not so much that we can’t hold a finger to them. If something is overheated, at least we know where to start looking for a short circuit.
Hidden and Intermittent Faults
It’s entirely possible that the fault is so well hidden that it hasn’t manifested yet. In that case, there might be a short circuit on the PCB printing. Turn off the transformer, take the multi-meter and test all adjacent traces on 1 Ohm range. While doing that, check again if all IC pins are soldered correctly, and then turn over the board and check the layout of the components.
Another possibility is that the computer is working, but with minor deficiencies: for example, when you press one key, two characters show up instead of one. In that case there’s most certainly a short circuit between traces from ICs 741LS251 and 74LS156 to the keyboard. If you examine the situation and conclude which keys show up in pairs, you can deduce which traces are short circuited by looking at the keyboard matrix scheme.
It is also possible that the lines of text on the screen bend horizontally, especially in last rows. This is due to a poor image sync signal, and some experimenting with resistors R9 an R10 is required. (R9 must not be lower than 40 Ohm, otherwise the IC 741S38 is in danger.)
Advanced Fault Debugging Tool
For especially hard core faults we need to make a helper tool. It’s called a logic probe, and it can be useful in many other situations. We need 74LS04 and 74US90 ICs, six LEDs, one capacitor and a few resistors. Using this probe we can determine if the logic level on a trace is high (first LED is on), low (second LED) or there’s a sequence of impulses. For pulses, the remaining four LEDs will blink, usually so fast that it appears as though they are constantly on. Constant input without pulses can never turn on all four LEDs.
It’s best if the ground and plus of the probe are two differently colored wires about 50cm in length that end in alligator clips. Connect those to the device that we are examining to get 5V, minding the polarity, as an error can damage the probe. Then we can read the logic states on crucial circuit points by touching them with pointed spike of the probe.
First we’ll make sure that the oscillator is working. Pin 10 of 74LS32 IC has to show the changing signal, which means that all LEDs should be on. Next we follow the divider chain: pin 2 of 74LS93, pin 14 of CD4040, pin 2 of CD4017. Each of these should show the same state on the probe, except the last one, where the frequency is low enough that we can see some LEDs flicker. If we find a static state at any of these, we’ve found the fault.
Carefully examine the surrounding printed traces: if there are no errors, we have to substitute the IC. Pin 26 of the Z80 microprocessor must test low for about half a second after turning the unit on, and after that has to be constantly high. If this is not the case, check the transistor that is connected to this pin as well as the electrolytic capacitor that is connecting R5 to + 5V.
Logic probe PCB layout.
Logic probe mounting layout.
Logic probe schematic.
Others may know more.
If after all this trouble you haven’t found a fault, you’ll have to seek help from somebody more experienced. We think that path is easier that for you to become an expert in electronics yourself.
There is one problem which can be fixed in software. If the image on your screen is shifted too much to the left, each time you turn on the computer you can type BYTE 11176, 12 and press RET, or in more extreme cases, BYTE 11176,13. Similarly, if the image is too far to the right, you can type BYTE 11176,10 (or BYTE 11176,9) and press RET each time you turn on the computer.
Acquiring parts for the Galaksija computer.
Building a computer yourself, even in places where you can buy microprocessors in bulk, is not an easy matter. Some key parts of the computer, such as ROM, cannot be freely bought in any parts of the world, and others, such as the keyboard, can be found neither easily nor cheaply. In our country, where it’s hard to find even the most common resistor, getting into this adventure might seem insane. But, it’s possible to overcome these obstacles. How?
Thanks to the understanding and love for computers by a handful of local manufacturers, Galaksija has managed to source for its readers all the core components without which building this computer yourself would have been suicidal—ROM, keyboard and printed circuit board—and at affordable prices! (The PCB will cost 40 percent less than “Elektronika Inženjering,” even though they are paying taxes for them!)
Besides that, we’ve managed to make a deal for procuring the semiconductor components from abroad. Only the housing and cassette we are unsure about at this time.
The ever-shifting dinar exchange rate increased the prices on everything, which affects the Galaksija computer too. Final prices will depend on the way ICs are sourced from abroad. In the worst case, if customs decide you have to pay import fees, those shouldn’t be bigger than 15.500 dinars,36 but it can’t be less than 11,000 dinars.
Mechanical Components
Mechanical components of the Galaksija computer—PCB, connector board, keyboard mask, keys with caps—are being made available by Institut za Vakuumsku Tehniku from Ljubljana (keys) and MIPRO, Elektronika from Buj. Keys which will be built into Galaksija really satisfy all professional standards; the same ones are
built into terminals of several domestic computer systems.
The FR4 printed circuit boards also have a professional look and quality. Traces are first protected galvanically, and then covered with a green solder mask to which all professional boards owe their charm. The upper side of the board has a component silk screen, which simplifies assembly a lot. The possibility for an error when placing the components or making a solder bridge is minimized.
The price of the full set is 4300 dinars which covers just the manufacturing and mailing expenses, as well as taxes which are responsible for almost a third of the price! (The price doesn’t include the connector board, which oughtn’t be more than 300 dinars.)
This kind of accessible pricing represents the support of the MIPRO and Elektronika companies from Buj and their owners Zvonko Juras and Blažo Krakić to the whole Galaksija project in spreading the ideas about home computers. These low prices come with a few limitations, unfortunately, but those shouldn’t worry those who make the decision to build the Galaksija computer early enough.
The prices are valid only till January 31st for orders received through Galaksija’s office. MIPRO and Elektronika will still accept orders after that, but at economically viable, and therefore higher, prices. This also means that parts can only be ordered in package.37 The first hundred orders get a special discounted price of 3660! Which first hundred? Well, the ones that first send in the orders, on or after the fifth of January! 38 Delivers begin on 15 January, and orders should be sent to Galaksija, 11000 Beograd, Bulevar vojvode Mišića 17.
PoC or GTFO, Volume 2 Page 6