Game Board Repair Guide
Most of us pick up a certain basic degree of electronics knowledge during our foray into the arcade collecting hobby, but wouldn't have a bloody clue where to start on fixing game-boards. Instead, we often find ourselves gazing in amazement at repair logs from some of our more experienced peers which are often filled with techno-babble and moon-speak which is beyond the likes of us mere mortals. The worst part is, a fix can often take a few hours (if not days!), which rightly is an expensive amount of time from a trained technician, so often sending boards for repair is only viable when the board is of a high enough value to warrant repair, and even then there are no guarantees.
Alas, even if you know what you are doing there are always frustrating boards that just sit on your "too hard" pile for months/years. Combine that with the fact that many of the chips on these old boards have been out of production for 20 years and are therefore hard to find. These chips can often be found online but they are almost always overseas so you end up spending a lot on postage to get 1 SRAM chip or whatever; of course in these cases you buy 15 or 20 of the chip at once, but the point is that postage costs soon add up. If you can’t get all your spares from the same place then the multiple postage costs soon start to approach a large % of a known working board.
Unfortunately, all of this leaves a huge number of dead games to rot and potentially never be fixed.
It needn't be this way however; if you are willing to get in to electronics it is possible to chalk up some victories, so don’t be disheartened! Whilst no-one is going to pretend that repairing circuit boards is cheap, quick or easy, it certainly helps to dispel some of the voodoo surrounding the subject and break it up into clear chunks of plain language that the beginner can understand and identify with.
In this document, we will aim to archive and collate technical info from various sources to create as clear a "guide" as possible to those starting out in game-board repair, with a view to encouraging and expanding people’s interest in this intriguing side of the hobby.
As the aim is to provide the reader with a general working knowledge of how boards operate and how various tools can be used to identify issues, we will try and steer clear of being too specific with examples in the main document, but where relevant we will include "case studies" of specific board repairs where key techniques have featured.
Remember that this is Wiki so you can add to it whenever you please - if you have anything you wish to add that you think will assist others, please feel free to do so.
The information here is intended to be used alongside the Basic Diagnostics.
Tools Of the Trade
To perform even the most basic repairs, you need a Multimeter, Logic Probe, EEPROM Programmer and preferably a ROM Puller. More advanced equipment is not essential, but is highly valuable in repair work. Please see the Repair Equipment page for a full listing.
The Fix - Where to Begin?
Just looking at a board, it can be daunting to know what to do first to try and identify an issue, but if you think of it like this there can only be three types of fault with a board:
When the correct power is applied...
- if all the chips are working correctly
- and are all connected up correctly (including passive components on the board)
- and all the ROMS/PROMS contain the right code...
...the game will work perfectly.
So therefore, your issue must be one (or a number of) the issues above.
Now, let's talk about each aspect of the board in a little more detail...
Power is the very first thing to check. If a board isn't getting the correct power, then it's never going to work! Check the voltage level is correct, and that it is getting to the chips on the board using your multimeter. Check the voltage at the chips as far away from the power input as possible. Voltage drop across a board is normal, you may 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 alternatively 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.
Problems with faulty chips range from easy to find to virtually impossible. At the very least you will need a logic probe (without one it’s 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 it’s 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 such as a logic comparator and/or an 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 though, 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 it’s soldered in and you can’t 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. Due to the nature of these chips in that they are usually surface mount and usually have upwards of 50 microscopic legs; there is often absolutely no way to test they are working correctly, and 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 will 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 heat-sink. A quick and dirty test to check the 12V feed is getting through is to run your finger across the pins of the amp chip and/or across the pins of the volume adjust pot. You should hear a crackle. If not then push firmly on the body of the chip (if it’s 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 cluster of capacitors, if any of these are damaged you may not get a positive 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 dip in the power supply in their area. These caps are often damaged, chipped or deliberately cut in half (to change their capacitance). It is possible that you have damaged one that is shorted out, but not likely, so generally you should ignore these caps.
Electrolytic capacitors on the other hand, are another issue. Electros 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 the electrolytic caps are the type where the two legs are at one end of the component, it is easy for the cap to be damaged if it has been wrenched to one side. This kind of damage will often tear one leg out, but when the cap is straightened again the torn leg goes back inside! The capacitor is wrecked but it might not be easy to spot with wiggling.
One area where electrolytic caps are critical is in the amp section of a board. If these are damaged or faulty then the amp may not work, or the sound may be quiet, missing or raspy.
Electrolytic caps can die of old age, some do, and 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); instead it is the resistance of the cap that goes up. This resistance is known as the Equivalent Series Resistance (ESR).
A healthy electrolytic will have a certain resistance, though just 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. Unfortunately this is still too low to measure with a multimeter but it’s 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 out bad electrolytic caps on a board is by 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.
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 heat-sink is not where electros have long lives.
Connection Problems - The PCB
Connection problems are common on old boards. While the board was safely bolted inside a cabinet the worst it had to face was dust and the odd spider. Once the game was superseded it was often slung in a box and not well cared for. The underside of a PCB is full of sharp pins which will scratch the arse out of any board pushed up against it. Scratches lead to cut tracks, cut tracks will often lead to floating pins, but if a pin has 2 input tracks you may not find a truly floating pin. Track damage can range from the bloody obvious, to the microscopic.
A circuit diagram is essential as you will often find things that just "look" odd, but are correct on that particular board. In the absence of a schematic you really are stuck unless you have a working board to compare connections to.
Age is another killer of tracks, tracks are usually shiny beneath the lacquer, but often you will see tracks that look dark, or just black. This is corrosion, often the track will still be conducting, and sometimes it won’t. Often these tracks duck under chips or other components on the board so you can’t actually see where they go or what condition they are in for most of their run. What makes things worse is that any liquids spilt on the board will get trapped under chips and remain there for ages due to capillary action. Sugary drinks are very corrosive, and arcade machines often had things spilt on them. If somehow this dripped onto the board it will get under the components and slowly eat away at the copper.
Tracks often duck through the board itself via through holes called, unsurprisingly, "vias". Tracing a board out by hand in the absence of a schematic is probably something that only the really bored, or borderline insane will attempt.
Other components on the board also need attention, basically its resistors, diodes and capacitors. Electrolytic caps age very poorly; they are also physically fragile, look for smashed bent or bulging Electrolytic caps. Any capacitor with white gunk at the base needs checking, this could be leaked electrolyte, or it might just be gunk used at the factory to hold the cap down a bit better. Ceramic capacitors will be all over the board, they are pale orange disks, and they are decoupling caps and often look very battered. Unless one has gone short circuit it’s not likely to cause any massive problems no matter how chipped they look. Only look at these closely if your board sometimes crashes or resets randomly.
The Next Steps - Things to Check on Your Dead Board
Armed with all your new knowledge, you are now ready to venture forth into attempting your first diagnosis. Here we go!
- Take the board outside in the daylight with a magnifying glass and go over the whole board both sides, sometimes the problem is obvious. Natural light is essential for this, as is the magnifying glass.
- Check the power supply, voltage and voltage drop across the board using your multimeter.
- Check the power supply again, some older boards require their 5Vs to be more like 5.2V.
- Check whether your board requires -5v or not - some old boards did.
- Get the logic probe on the CPU, check the clock pin - it should be very active. Without a clock signal the board will do absolutely nothing.
- Probe the RESET pin on the CPU - Google the pin-out for that CPU to determine what it should be as it differs and is not always what you would expect. For example, on the Z80 a RESET pin that’s low means the CPU is in RESET mode - i.e. it’s parked. A working Z80 should have this pin high! A pin with a line above its name in the datasheet means "active when not high".
- With ROM puller or screwdriver (see tools section above for pros/cons of each method), remove each ROM chip and reseat it carefully. It is easy to bend pins when putting it back in, so be careful. Reseating can cure problems cause by oxidised legs. If you have an EPROM reader, then dump the chips contents to your PC while you have them out of their sockets and ID them with the software that comes with Mame. If you find a ROM with legs bent under this will definitely be a fault, its not likely to the only fault, its more a sign that someone has been there before you, also its worth not ruling out anything that looks like its been worked on before, You could find a boards where someone was on the right track and gave up, or even soldered in a badly faulty chip to replace one they suspected originally.
- Probe around the RAM chips, Google the datasheet on those chips and check the Enable, OE and WR for activity. Check the address and data-lines for signs of missing lines.
- Same as 5 for the ROM chips.
- Probe the address and data-lines on the CPU, look for missing (floating lines).
It is best not to go round touching up solder joints on chips, it is not likely to be the cause of the problem and you can do a lot more damage that way, especially with surface mount chips. Once solder joins up the pins it’s impossible to know if a couple of pins are joined because you joined them, or if the board is designed to join them up. Riser board pins and connectors are another matter, anything that’s had physical flexing in its lifetime could well have developed bad joints, but again a blanket approach can leave you with more questions later about the boards original state.
As you attempt to repair your board, it is valuable to make notes as you go, and, if possible, take relevant photographs.
These notes and photos can be combined to form a "Repair Log" which makes a very useful reference for others who may attempt to repair the same board in the future.
Be sure and visit the repair log category where you can browse our selection of archived repairs - feel free to add your own to the list and help grow the resource!
Who repairs boards?
- Forum Member VectorGlow (Wales)
- Forum Member channelmaniac
Rest of the World
- Forum Member Womble (Australia)
Huge thanks to Arcade Otaku tech-heads Womble, trmatthe and Aryssor for kindly agreeing to let us collate info from their forum posts whilst creating the initial version of this document.
Hopefully this guide will continue to expand and grow in value as more technologically minded people add their thoughts!