How to fix PCBs - A Beginners Guide
There have been a few posts scattered around from people asking how to fix their boards, and or fishing for people to fix it for them. I wrote this a couple of years back on another forum, it may shed some light on the topic. Have had to split over two posts due to the 10,000 chr limit here
I have had about 80% success with fixing boards and am totally self taught, all the info you need is online these days, when I started out I knew how to solder kits together but not much else.
The purpose of the original post was to dispel the idea that fixing these things is cheap/quick/easy, its not even fun sometimes, but if people fancy getting their soldering irons dirty its not a bad introduction I think.
Anyway - if you think of it like this there can only be 3 types of fault with a board.
When the correct power is applied.
1) if all the chips are working correctly
2) and are all connected up correctly (including passive components on the board)
3) and all the ROMS/PROMS contain the right code.
the game will work perfectly.
Power is the very 1st thing to check,check the voltage level is correct, and that it is getting to the chips on the board. Check the voltage at the chips as far away from the power input as possible, volt drop across a board is normal, you many have 5V at the input of the board, but if that’s dropped to 4.5V at the farthest reaches of the board then things will get screwy. Either up the voltage a tad, or run a power wire to the far side of the board, operators often did this back in the arcade heyday. TTL chips do not like voltages much below 4.8V - they do odd things below that. If no matter how high you turn up the 5V on the PSU you cannot get 5V at the board and the voltage at the PSU is cranked up (you need to measure with a multimeter) then it is almost certain that your power wires between the PSU and the PCB are too thin, get some more copper in the path, either with thicker wire, or multiple runs of whatever you are using. Turn the PSU back down and try again, the problem will probably be gone, this is usually only a problem with harnesses that convert from one pinout to another, especially if your other games work and the one with the harness won't.
Problems with faulty chips range from easy to find to virtually impossible. At the very least you will need a logic probe (without one its like trying to fix a car in the dark) and you will have to go round the board looking for pins that are floating, neither high nor low. This might be normal as some chips are dual, quad or octal chips - i.e. they have 2, 4 or 8 of the same logic gates on them, its not uncommon to find a quad chip where only 3 of the gates are used. The inputs to the 4th gate will be floating as they are not connected. To check this out you need to pull up the datasheets for that specific chip. If the inputs are active and the output is floating then you have found a dead chip. You can go one further here, if the inputs are active and the output should be doing something (based on the logic table in the datasheet) yet it never changes from low to high then you have a stuck pin, this gets harder to work out by eye the more inputs a logic function has. If you have say 6 inputs that determine what the output is doing, and all of them are active and flipping then its very had to tell if the output should be changing at any given point. The worst kind of fault is where the chip is still functioning but its thresholds are wrong, it changes as it should but is no longer latching cleanly or on time, finding these chips needs special hardware, logic comparators, oscilloscope.
RAM chips are a common failure, google the datasheet and check the address and data lines. If you find floating lines then you are on to something, follow those lines back and see where they should go. Something is dead somewhere, it might be RAM.
RAM needs controlling tho, you need to check the chip enable pin is actually enabling the RAM, and the WR and OE lines are doing something, if these are dead the chip will sit there doing nothing not the wrong thing.
ROMs, same deal as with RAM, the control lines need to be working, check the output pins for signs of life. The contents of the chip need to be checked too, you will need an eprom reader to do this. The game board could be in perfect working order but a single error in an elderly ROM will cause the board to crash straight away. You could spend hours chasing a hardware fault that doesn’t exist if the problem is due to duff software. PROMs are like EPROMS but they are write-once chips. Contents of these chips can be checked against the roms in the MAME set, there are apps to do this.
CPUs - virtually impossible to debug due to their complexity,
if they are socketed its easy to rule them out, stick the CPU in another board that uses the same chip and use that board to test the CPU. Or stick a known good CPU in your board. If its soldered in and you cant remove it then you will have to assume it works until you have evidence to the contrary. Pull up the datasheet and see what the address and data lines are doing, bear in mind that a stuck pin doesn't necessarily mean the chip is bad, a track on a board has at least 2 ends, if the chip on the other end is shorted then the chip at your end wont be able to drive that line, so you may have found a fault from the other end.
Custom chips - these are project KILLERS. To save money on the chip count the makers made their own custom chips that combined dozens of other chips into one monster chip. They are usually surface mount, usually have upwards of 50 microscopic legs and often there is absolutely no way to test they are working correctly, even if you could test them there is no information around these days about what they do. Swapping them is not an option as the only place you wlil find another chip of the same sort is on another board of the same game and without specialist equipment there is no way to remove or replace them. If you have a dead custom chip then the board is scrap, its cheaper to buy a working board than take the dead one any further.
Audio Amp chips - these often are dead on old boards, they are usually the only chip bolted to the board, or to a heatsink. A quick and dirty test is to check the 12V feed is getting through then run your finger across the pins of the amp chip and or across the pins of the volume adjust pot. You should here a crackle, if not then push firmly on the body of the chip (if its firmly bolted or soldered down) you should here an angry buzzing noise. If you do get a noise then the amp chip is fine, especially if you can change the volume of the buzz with the volume pot. Bear in mind that the amp chip depends on the circuitry around it, often a cluser of capacitors, if any of these are damaged you may not get a possive result, even if the amp chip is fine.
Discrete Components - Resistors, diodes, capacitors - mostly you can spot damage by eye, but there are a couple of caveats, often you will find an orangey beige disk near many of the chips, these are ceramic capacitors used to smooth out any slight dips in the power supply in their area, they are often damaged, chipped of cut in half (to change their capacitance). Its possible that you have damaged on that is shorted out, but not likely, I would ignore these. Electrolytic capacitors are another issue, they are the ones that look like cans. Again these are often used as smoothing capacitors, often you will find 1 or 2 big ones by the JAMMA connector. If these are damaged the board will be more susceptible to voltage ripple, but the board will probably still run, it will just crash more. If they are the type where the two legs are at one end its easy for the cap to be damaged if it has been wrenched to one side, this will tear one leg out, but when the cap is straightened the torn leg goes back inside, the cap is wrecked but it might not be easy to spot with wiggling. One area where electrolytics are critical is in the amp section, if these are damaged or faulty then the amp may not work, or the sound maybe quiet, missing or raspy. Electrolytics can die of old age, some do, some don't. The problem with electros is they are wet capacitors, the electrolyte inside is a damp paste which can dry out if the cap has lost its seal. When they dry out ironically the capacitance doesn't change (so a capacitance meter is useless), its the resistantce of the cap that goes up - the resistance is known as the Equivalent Series Resistance (ESR). A healthy electrolytic will have a certain resistance, a tiny amount, often in sub ohm range. When it goes bad the cap can end up with an ESR that is 10 to 100 times higher, still too low to measure with a multimeter but its the equivalent of dropping lots of small resistors into the circuit so it can bugger a lot of things up. The only way to rule this out is using an ESR meter, or by replacing all the electros on the board, which is tedious and can get expensive if you have to buy new ones all the time. Personally I have an ESR meter which not only lets me spot the bad ones, but allows me to use caps from scrap or smashed TV PCBs which I can test as being fine. An old electrolytic cap is not necessarily a bad one, but a lot are, especially ones that lived in hot parts of the board, up close to an amp chip heatsink is not where electros have long long lives.
Regular member of www.aussiearcade.com.au. swing by if you like reading PCB repair tales, there are hundreds there, including mine.
Last edited by Womble; 09-18-2011 at 10:02 PM .