How to fix PCBs - A Beginners Guide

[Womble]

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

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.

Chippery

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.

 

[Womble]

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. Have seen a NeoGeo board with classis signs of a missing ROM dataline, but it all ducked under the huge cart connector which had been soaked in something dark and sticky, coca cola probably. I never did find the break as I had no cct diagram to show which should go where. 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. Electrolytics age very poorly, they are also physically fragile, look for smashed bent or bulging electrolytics. Any capacitor with white gunk at its 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, 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 Bad News

To be honest repairing a dead board can take a very long time, even if you know what you are doing there are always boards that just sit on your "too hard" pile for months/years. That's why most folk on here that can fix boards, do not offer to do so for other members. These boards are now so cheap on ebay that you only need to spend a couple of hours on a board before the cost of your time exceeds a new board. 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 $20 postage to get 1 SRAM chip, of course in these cases you buy 15 or 20 of them, but the point is that postage costs 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.

All the above might sound very disheartening, but if you are willing to get in to electronics it is possible to chalk up some victories. If you are game to take the path I started out on some time ago I would offer the following checklist to start with.

Stuff you will need

Soldering Iron (as fine a tip as possible) $50
Solder (duh) $5
Solder sucker $12
ROM Puller $3
Multimeter $25 - if you are buying on make sure it has a beeper for continuity testing, the ultra ultra cheap ones don't. Checking tracks is hard enough without having to look up at the meter screen everytime to see if you have a connection, it if beeps its good and you can move on.
Logic Probe (sounds daunting and expensive but isn't - the best $25 you will ever spend really). This connects to the powersupply you are feeding the board with and the LEDs on it will tell you if the probed pin is high/low/or flipping between the two. A floating pin will show up as nothing, neither high nor low, on some chips this is normal, in other places its a fault. Get one with audio tones if you can, it means you can keep your eyes on where the probe is instead of the probe body. Also you can use it to hear audio in parts of the audio section which can be useful when trying to find out where the sound dies on a silent board.

Stuff that will make your life easier

Desoldering station, makes getting chips off boards a breeze, its good to be able to check a chip is ok once its off the board so not having to cut its legs off is a bonus, also means you are less likely to roast the chip you want to use as you take it off a scrap board.

Oscilloscope - logic probes are cheap and useful, but if a pin is active you have no idea what the signal looks like without a scope, duff RAM chips often remain active but the signals are utterly mashed, easy to spot with a scope, impossible with a logic probe. All a logic probe can tell you is if the chip is totally dead, most RAM chips struggle on so you are left with the blanket replace and retry option.

Fluke 9010 - Microprocessor troubleshooting device, allows you to "be" the CPU and see what it can see across the various buses. Combined with the address map from the MAME driver source it can save you a lot of time, you do need a high level of tech skills to drive it though, and you need the right Fluke pod for each CPU. Probably the last thing a repairer has on the list, expensive and quite rare these days.

Things to check on your dead board

1) 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 tho, as is the magnifying glass
2) Check the power supply, voltage and voltage drop across the board
3) Check the power supply again, some older boards require their 5Vs to be more like 5.2V.
4) Get the logic probe on the CPU - check the clock pin - it should be very very active. Without a clock signal the board will do absolutely nothing.
5) Probe the RESET pin on the CPU - google the pinouts for that CPU to determine what it should be, on the Z80 a RESET pin thats low means the CPU is in RESET mode - ie its 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".

6) With ROM puller, remove each rom chips and reseat it carefully, its easy to bend pins when putting it back in. Reseating can cure problems cause by oxidised legs. If you have an eeprom 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 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, I have found boards where someone was on the right track and gave up, or even soldered in a badly faulty chip to replace one they suspected originall.
7) Probe around the RAM chips, google the datasheet on those chips and check the Enable, OE and WR for activity. Check the address and datalines for signs of missing lines.
8) Same as 7 for the ROM chips.
9) Probe the address and datalines 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 surfare mount chips, once solder joins up the pins its impossible to to know if a couple of pins are joined because you joined them, or if they board is designed to join them up. Riser board pins and connectors are another matter, anything thats 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.

The above tests using a scope gives you far more info than simply a logic probe does, a scope will let you see what the signals look like, in many cases a logic probe shows a pin as active, but a scope will show if the activity is a mess of noise and corruption or if it is working correctly.

That pretty much my standard tests, usually you get some leads from doing the above.

 

[Broodwich]

great post, Arcade Repair Tips just ran a segment about this also

http://www.arcaderepairtips.com/201...rd/?utm_source=twitterfeed&utm_medium=twitter

vids not the greatest but there's good basic info there, yours is a lil more in-depth very nicely done

 

[Maxstang]

Great post! I have a "too hard" pile too. Maybe I'll get motivated!

 

[dezbaz]

Cheers Womble

Great info there

 

[jonathan1138]

Womble, through the good help here on the boards, ive been getting better - but board troubleshooting is my weak spot. This write up is exactly what i need - look forward to reading it and the related links. Thanks.

 

[channelmanic]

CPU 101

The CPU is the heart of any system but, the CPU relies on external signals, external program chips, and places to store information to be able to do its tasks. If your game is totally dead, here's a bit of troubleshooting stuff to help you figure out what is going on:

(NOTE: a / is used to signify a signal that is active when low such as /OE or Output Enable... or /RESET which is a reset signal that resets the CPU when brought low for a small period of time.)

== Clock ==
First things first. The clock signal. Without this, the CPU will do nothing at all. Check the clock pin for a signal. Some CPUs such as the 6890 have a built in oscillator to which a crystal can be directly attached. Other CPUs such as the 6809E require external clock signals.

Use a logic probe to determine if the clock signal is working. OR use an oscilloscope to make sure it's a clean looking signal. A dual trace oscilloscope is crucial if you have 2 clock signals such as E and Q on the 6809 CPUs to make sure you have the proper phase between them. Without the proper phase between the 2 clock signals things just won't work right.

== /RESET ==
Next a reset signal is needed. This knocks the CPU over the head and gets it to pull data from the default location it starts from. Some CPUs start at memory location $0000 and others from different places. If /RESET does NOT start out low momentarily then go high you must troubleshoot this. Physically broken caps are a common cause of issues here.

== Watchdog ==
The Watchdog timer is NOT a CPU line, but simply a timer that is periodically refreshed by the system in the normal course of it running. If it's not refreshed then it times out and resets the CPU. If your /RESET line is continuously going high/low then you must find out why the CPU isn't running. It could be a bad CPU, RAM, ROM, latches, or buffers.

== Read/Write ==
Sometimes broken out into separate read and write pins depending on the CPU. If these are dead check reset and clock. If they are good and these are stuck replace the CPU. If the write never comes active then it's most likely stuck trying to read the ROMs. Check ROMs, address bus buffers, data bus latches, and address decoding if that's the case.

== NMI and IRQ ==
Interrupts are used to tap the CPU on the shoulder periodically so other tasks can be done such as refreshing dynamic memory, reading control inputs, doing video tasks such as vertical blanking, and more. If these are looking screwey then you must trace down each chip attached to these signals and verify that they are good.

== Address Bus and Decoding ==
Controls access to various system components by memory address. These must be decoded to work properly since the RAM chips and other chips have only as many address lines as needed to access their full capacity and not all 16 address lines for an 8 bit system.

As an example, if you look at a simple 8 bit CPU with 64K of addressable memory then it can be broken down into 8 simple 8k blocks with a 74LS138 chip. The 3 inputs on the 138 correspond to 8 different output and if you drive those 3 inputs with the upper 3 most address lines (A13, A14, and A15) then you can divide that address space up into those 8 8k blocks as such:

111 - e000-ffff (upper 8k)
110 - c000-dfff
101 - a000-bfff
100 - 8000-9fff
011 - 6000-7fff
010 - 4000-5fff
001 - 2000-3fff
000 - 0-1fff (bottom 8k)

Use the outputs of the LS138 to drive the /CE lines and tie the /OE lines low then the RAM or ROM will come active as needed.

== Data Bus ==
Carries data to/from the CPU and other components. MUST be fully functioning or the system will NEVER boot.

== Address Bus Buffers ==
Buffers are one way logic gates that you will find connected directly to the CPU's address lines on many systems. These are there to add "drive" capability to the Address Bus. There may be more chips on the bus than the CPU pins have the ability to send signals to (too many milliamps of current needed) so these give it extra oompf. Check the enable lines on these buffers. If they are good and the chip is active then the inputs should be the same as the outputs. If not, replace the chip. If it's still stuck on the outputs then something else tied to those pins is dragging the Address Bus down and you must find it. A 74LS244 is common for this although older systems will use 74LS367 chips or even older ones such as 8T2x chips.

== Data Bus Latches ==
These control the flow of data to/from the CPU and give the extra current drive to send the Data Bus signals to more chips than the CPU has the ability to. Check the enable and direction pins to make sure they are working correctly. If the enable isn't working then no data will flow. If the direction pin isn't switching then data is stuck and not flowing bi-directionally. The inputs should be the same as the outputs. If pins are stuck high or low, replace them. If the replacements don't fix the stuck pins then you'll have to check all the chips tied to that pin on the Data Bus to find the problem.

Enjoy!

 

[ifkz]

Great write up as usual Womble! Your repair logs are very motivational, and I think I've read all of them. I plan on creating a test bench so I can get back to looking at my faulty Raiden DX board. Piles of scrap boards are great: spare EPROMS to erase, volume pots, etc. You never know what will come in handy.

Great job on finding the Vindicators PCB fault. I bet that was the reason it was allowed to fall into such bad state of storage.

 

[GaryMcT]

Great thread!

 

[troxel]

Thanks Womble and Channelmanic. Great information. I love reading both of your guys' repairs.

 

[dezbaz]

I have a few vector PCBs needing repairs too

Are the above basic principles similar for vectors too?

(Some are dead, some have random vector issues)

Any vector advice?

 

[Talon2000]

As for Vectors alot of the same things apply. The major difference is in the DVG, or AVG sections. Digital Vector Generator, or Analog Vector Generator.

As already stated Power is the first thing. Get your schematics out, and start checking the power supply. Check both the DC power, and AC. Also check for AC ripple on your DC circuits. If you have to much AC ripple this will cause problems. That is why you see posts about people replacing the Big Blue and it fixing their problems. Big Blue filters out the AC ripple.

Connectors, These are the downfall of any electronics. They are a necessary evil, and will cause more problems than just about anything else. Corrosion, poor contact, cold/cracked solder joints on headers from removing and installing connectors over and over.

Aux boards on Atari Vector games have these nice wide connectors on them. If the game hasn't been repaired before, you can bet you have cold/cracked solder joints on these. Re-flow, Replace, re-build these guys everytime!

Sockets, I don't think anyone has touched this subject yet. Alot of games used single swipe sockets, meaning the socket only touches one side of the chip leg. These are often the ones you hear about people replacing. A dual swipe socket which costs less than .20 cents can go along way in getting a game up and running. Alot of times you will find that legs on the sockets are cracked, and will only make intermittent connection, or the contacts are corroded and don't make good contact, or introduce to much resistance.

BattleZone boards seem to be the worst as far as Vectors go. Which is why you see the multi game ad ons take care of a dead board. They use the CPU socket which tends to be a better socket, and it eliminates up to 8 possibly bad Rom sockets.

Sometimes just doing these few basic things will get you up and running. If not, you've got your work cut out for you.

Next is the clock circuit, without this, nothing will work.

Then for Chips, I like to check them in this order.
CPU
Roms
Rams

Womble, and Channelmanic have already covered this pretty good, so no need to cover them again.

Unless you have an IC tester, and a Programmer it's going to be difficult to test Roms, CPU's and Ram.

Next comes the fun part, are the Rom, Rams, and CPU talking?

Easy way to tell is with a Fluke 9010a, Signature analysis, and or the Logic Probe. Some games will thankfully have Self tests to help out, pointing out bad Ram, and even tell you which one. The self test code takes up a small portion of rom space, and if you have a Good CPU and Clock it will most generally run. You're only problem is if you don't have video out. The ram test will typically beep, but Rom failures are generally displayed.

A nice thing about games like Tempest, Battlezone, or any Atari Vector that uses an Aux board. It will run self test without the aux board. This way you can check video out, Ram, Rom test etc.

Hopefully at this point you have the MPU side of things working, and if there is nothing wrong with the Vector Generator, you should at least be able to play blind.

The video out on vectors typically requires an oscope. Just makes things easier when you can see the signal.

Tempest for Example will have 6.8vdc, +-15 Vdc and a Digital +5v there are test points on the board for those. Missing any one of these voltages and you won't get any video. So test those first.

The X and Y outputs swing from + to -. The general swing you will see is about 2-3v AC, so set your meter on AC.

From here, you need to dig out the schematics, and work your way backwards through the circuit to see where you are loosing your signal.

This is just a basic overview, Troubleshooting the AVG, and Mathbox circuits requires more time, and testing tools.

Often times, I see boards that have been stacked, or just laying around and they will often have cut traces from rubbing against other boards, or broken components, and chips.

The Aux boards, have ALU chips on them, these get pretty hot, and are prone to failure. Also there are 7-8 custom proms on Aux boards for Tempest, Battlezone and the like. Any 1 of these having a dead output will cause a game not to run. These chips when you can find them Run $10-$20 each depending. You will find that the proms will tend to have black legs from oxidization. I use either a pink eraser to clean them, and will even soak them in Tarn-x and run a toothbrush on them to clean the legs.

Again these are just basic troubleshooting guides, once you have a game that is running, you can get more in-depth on a particular section that is not working right. Ie highscore not saving, sounds missing, button inputs not working.

Hope this helps.

 

[Broodwich]

Is there a good site on identifying the parts that are installed on a PCB? i have a board with what looks like some burnt diodes or capacitors (tiny lil guys marked 10 16v). just wondering if i might possibly be able to repair it.

I mean i'm sure anyone looking into PCB repair knows what a standard cap/resistor/socket/ect. look like.

 

[dezbaz]

As for Vectors alot of the same things apply. The major difference is in the DVG, or AVG sections. Digital Vector Generator, or Analog Vector Generator.

As already stated Power is the first thing. Get your schematics out, and start checking the power supply. Check both the DC power, and AC. Also check for AC ripple on your DC circuits. If you have to much AC ripple this will cause problems. That is why you see posts about people replacing the Big Blue and it fixing their problems. Big Blue filters out the AC ripple.

Connectors, These are the downfall of any electronics. They are a necessary evil, and will cause more problems than just about anything else. Corrosion, poor contact, cold/cracked solder joints on headers from removing and installing connectors over and over.

Aux boards on Atari Vector games have these nice wide connectors on them. If the game hasn't been repaired before, you can bet you have cold/cracked solder joints on these. Re-flow, Replace, re-build these guys everytime!

Sockets, I don't think anyone has touched this subject yet. Alot of games used single swipe sockets, meaning the socket only touches one side of the chip leg. These are often the ones you hear about people replacing. A dual swipe socket which costs less than .20 cents can go along way in getting a game up and running. Alot of times you will find that legs on the sockets are cracked, and will only make intermittent connection, or the contacts are corroded and don't make good contact, or introduce to much resistance.

BattleZone boards seem to be the worst as far as Vectors go. Which is why you see the multi game ad ons take care of a dead board. They use the CPU socket which tends to be a better socket, and it eliminates up to 8 possibly bad Rom sockets.

Sometimes just doing these few basic things will get you up and running. If not, you've got your work cut out for you.

Next is the clock circuit, without this, nothing will work.

Then for Chips, I like to check them in this order.
CPU
Roms
Rams

Womble, and Channelmanic have already covered this pretty good, so no need to cover them again.

Unless you have an IC tester, and a Programmer it's going to be difficult to test Roms, CPU's and Ram.

Next comes the fun part, are the Rom, Rams, and CPU talking?

Easy way to tell is with a Fluke 9010a, Signature analysis, and or the Logic Probe. Some games will thankfully have Self tests to help out, pointing out bad Ram, and even tell you which one. The self test code takes up a small portion of rom space, and if you have a Good CPU and Clock it will most generally run. You're only problem is if you don't have video out. The ram test will typically beep, but Rom failures are generally displayed.

A nice thing about games like Tempest, Battlezone, or any Atari Vector that uses an Aux board. It will run self test without the aux board. This way you can check video out, Ram, Rom test etc.

Hopefully at this point you have the MPU side of things working, and if there is nothing wrong with the Vector Generator, you should at least be able to play blind.

The video out on vectors typically requires an oscope. Just makes things easier when you can see the signal.

Tempest for Example will have 6.8vdc, +-15 Vdc and a Digital +5v there are test points on the board for those. Missing any one of these voltages and you won't get any video. So test those first.

The X and Y outputs swing from + to -. The general swing you will see is about 2-3v AC, so set your meter on AC.

From here, you need to dig out the schematics, and work your way backwards through the circuit to see where you are loosing your signal.

This is just a basic overview, Troubleshooting the AVG, and Mathbox circuits requires more time, and testing tools.

Often times, I see boards that have been stacked, or just laying around and they will often have cut traces from rubbing against other boards, or broken comments, and chips.

The Aux boards, have ALU chips on them, these get pretty hot, and are prone to failure. Also there are 7-8 custom proms on Aux boards for Tempest, Battlezone and the like. Any 1 of these having a dead output will cause a game not to run. These chips when you can find them Run $10-$20 each depending.

Again these are just basic troubleshooting guides, once you have a game that is running, you can get more in-depth on a particular section that is not working right. Ie highscore not saving, sounds missing, button inputs not working.

Hope this helps.

Thanks, this is great, very useful for the xy owners

 

[Womble]

Is there a good site on identifying the parts that are installed on a PCB? i have a board with what looks like some burnt diodes or capacitors (tiny lil guys marked 10 16v). just wondering if i might possibly be able to repair it.

I mean i'm sure anyone looking into PCB repair knows what a standard cap/resistor/socket/ect. look like.

Just google those component types and select images, it will throw up pretty much all the info you need. Your "10 16v" components are probably electrolytic capacitors, 10uF 16v, very easy to find and replace.

 

[Broodwich]

Just google those component types and select images, it will throw up pretty much all the info you need. Your "10 16v" components are probably electrolytic capacitors, 10uF 16v, very easy to find and replace.

yup yup that's what i did, could only find caps and diodes that looked like it, thanks for the clarification.

i attached a picture, C30,C31, and C38 are the ones in question. not sure if C26 or C33 will be salvage-able either and will prolly be next to impossible to replace as they are extremely tiny.

 

[SuperGunGuru]

What do you guys think about having a list of some of the most common TTL chips that you may want to have on hand for doing board repair. Is this something that's feasible or are there just too many different types you could see on arcade hardware. Having a TTL data book I know there are hundreds of TTL chips, but maybe there are some that are commonly seen on many different boards?

 

[DarrenF]

yup yup that's what i did, could only find caps and diodes that looked like it, thanks for the clarification.

i attached a picture, C30,C31, and C38 are the ones in question. not sure if C26 or C33 will be salvage-able either and will prolly be next to impossible to replace as they are extremely tiny.

Whoa, you're into surface-mount there. Labeling & ID on those tiny things CAN get really strange. I'd guess that the majority of what's been discussed in this thread refers to thru-hole. Most 70s and 80s vintage PCBs will have little or no SMT stuff.

Edit: The one or two times I've had to delve into SMT crap, I found this page useful: http://www.marsport.org.uk/smd/mainframe.htm

 

[DarrenF]

What do you guys think about having a list of some of the most common TTL chips that you may want to have on hand for doing board repair. Is this something that's feasible or are there just too many different types you could see on arcade hardware. Having a TTL data book I know there are hundreds of TTL chips, but maybe there are some that are commonly seen on many different boards?

Depends somewhat on the era and manufacturers you deal with mostly. My stuff is mostly late 70s to mid-80s; Atari skewed...YMMV. I keep the "LS" variety of TTL ICs on-hand; they're the most versitile. Only from time to time will you encounter a case where you NEED to match the original type (F, L, etc.).

I'd say (74LS): 00, 02, 04, 08, 16, 20, 32, 74, 75, 86, 139, 157, 161, 174, 175, 191, 245, 273, 367.

And if you want to get sockets too, the vast majority of TTL chips are 14 or 16 pin, with a smattering or 20 and 24 pin thrown in.

 

[Arcadenut]

If you're working on Defender boards, 7474's seem to be bad A LOT and using 74LS74 in some cases won't work (you get artifacting on the screen).

And if you want to get sockets too, the vast majority of TTL chips are 14 or 16 pin, with a smattering or 20 and 24 pin thrown in.

I have a lot of the 14, 16 and 24. I need to find some 20's that are narrow (for the 5101 CMOS). I usually wind up cutting a socket down to use in the 20's.