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Thread: For new people : Repairing PCBs yourself

  1. #21
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    Where abouts are you in Brisbane? I'd be happy to show you some of the basics time permitting.
    I never run my pcbs over 5 volts in facts I tend to go around the 4.8 mark, the IC's run a lot cooler which helps prolong their life.
    I usually gauge a pcbs heath on how low you can run it at

  2. #22
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    Yes you need to see how the power propogates through the board (for want of a better word). Standard TTL chippery needs between 4.8 and 5.2 volts to be happy. Any less and things go wonky, any more and things are stressed. The trouble with any circuit is that the voltage sags the further you get away from the point at which the power is applied, so chips a long way away from the edge connector will see a lower voltage than those close by. Thats why measuring the voltage at the edge connector is only half the issue, thats the point at which it is most likely to be perfect, the problem is the far end of the board, or on subsequent boards if there are ribbon connectors involved. Measuring it at the powersupply is utterly pointless except to confirm what the PSU is giving out, which doesnt actually mean the same thing as what the board is getting as there can be volt drop in the wiring itself.

    If faced with a cab I had never met before I would power it on without a board, use a multimeter to confirm that the PSU is giving out 5V and hasnt been cranked to something silly (or is just plain faulty), then I would plug in a board, adjust the PSU until I got 5V at the edge connector, then go and see what the voltage is at the far reaches of the board, and tweak the PSU up a bit if its getting low at the extremities.

    In some cases, usually bootlegs, its hard to get enough volts into the board at the edge connector to get a decent voltage at the far end without upping the input voltage to dangerous levels, in those cases you may need to run extra wiring, to inject power to multiple points, I have a bootleg board here that would never have run without extra wiring, at 5.2V input the far end is still only 4.5, to get 5V I would have to inject 6V at which point things near the edge connector would start to cook.
    Sic transit gloria Atari!

  3. #23
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    I don't even bother with bootlegs anymore (although there are a few exceptions) the majority of them are cheap rubbish populated with cheap parts.
    I admire your enthusiasm Womble, you remind me of myself a few years back where everything was worth saving. I remember the amount of chips I replace on my first Moon Patrol bootleg just to see it going again.
    Besides my dedicated stuff I don't do to many repairs anymore mainly because I'm sick of plugging games in only to find they have more faults. In fact I spent 99% of the time fixing pcbs, testing then sticking them in a box and rarely playing them.

  4. #24
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    SectionZ and I worked on a few boards on the weekend

    He has 2 burners, a Prom old school burner (hilo universal), and eprom newer one (willem). We we able to work out which of the 3 proms on a board was faulty by using MAME to compare with

    Eg:
    1. Unzip the mame rom for the game
    2. Change the name of prom 1 (We added "-x" to the end of the file name)
    3. Open a blank text file and save it in the same folder, and rename the blank text file with the proms original name.
    4. Rezip the file and place in mame / roms
    5. Don't "audit all games" in mame
    6. Run the game with the blank prom file

    Original fault in real PCB


    Simulated in mame Prom 1 using blank text file renamed


    Simulated in mame Prom 2 using blank text file renamed


    Simulated in mame Prom 3 using blank text file renamed


    Prom 2 was the only prom showing black background, so prom 2 was burnt on prom burner using the mame file corresponding to it.

    Fixed


    Finally, in MAME, delete the renamed text file
    Remove the "-x" from the end of the original file.
    rezip the correct rom file into mame/roms

    Mame file now back to normal
    Last edited by dezbaz; 11th January 2010 at 10:39 PM.

  5. #25
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    Quote Originally Posted by Arcade King View Post
    I don't even bother with bootlegs anymore (although there are a few exceptions) the majority of them are cheap rubbish populated with cheap parts.
    I admire your enthusiasm Womble, you remind me of myself a few years back where everything was worth saving. I remember the amount of chips I replace on my first Moon Patrol bootleg just to see it going again.
    Besides my dedicated stuff I don't do to many repairs anymore mainly because I'm sick of plugging games in only to find they have more faults. In fact I spent 99% of the time fixing pcbs, testing then sticking them in a box and rarely playing them.
    I think I am slowly coming round to your take on things AK, had a number of scruffy olf boots lately, the last being '99 The Last War, it was one of those boards that the more you fix the worse it gets. Found a number of blown TTLs but as these were replaced the board got further and further away from doing the right thing, ie when I got it it would throw up corrupt memory errors, but some of the faults were ROM addressing faults so that might have been a fluke. As its a bootleg I have no way to know if the MAME ROMs are actually right for this board, seeing as it came with Mitsubishi M5L EPROMs most of the original code was already lost. In the end it was pissing me off too much, had gone way past the point where it was fun work, so its now slung on the scrap pile. Same as the 1942 I fixed, it lived for a few weeks but is now a basket case again. Spent some time on it but I think I have lost a track somewhere, fairly tatty CPU board, shame as the video board is pretty mint. Ho Hum, bloody things.

    Hard to determine if I am fixing things coz I want them, or to keep them, or whether its for the fun of the fix, I certainly have less urge to fix crap games these days.
    Last edited by Womble; 12th January 2010 at 10:01 AM.
    Sic transit gloria Atari!

  6. #26
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    I read somewhere that Bootlegs are the ones to fix, as genuine boards were full of custom chips that can't be replaced easily?

  7. #27
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    It really depends on the bootleg, they range from well made to appaulingly badly made, a good bootleg can be as well made as an original, but bad ones are pretty bad.

    Positives
    No custom ICs

    Negatives
    Any doco will probably not be relevant
    Silkscreening missing or wrong
    ROM sets may not match MAME at all, once a game is up and running in MAME the urge to add every and all bootleg versions in is not there, so if you have bad ROMs and the MAME set ones dont work you are kinda on your own. Also have to know if the ROM is bad if the checksum is not a known one, ie is it bad coz its bad, or is it just not known coz its a bootleg version?
    Last edited by Womble; 12th January 2010 at 05:19 PM.
    Sic transit gloria Atari!

  8. #28
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    There are some really good bootlegs like the gallag board sets which plays identical to the original, Bombjack bootlegs are identical to the original, Defence command (Defender clone) I admire the bootleggers ingenuity on this pcb. They condensed the massive Williams 5 piece board set into a neat little 2 layer jobby. I worked out how to convert it to defender which is a bonus.

    I have an R-Type bootleg here that was given to be by Brad, the thing is massive. I actually got it working but it had graphics issue which for the time it would take me to track down the fault it just wast worth it.
    An original R-Type has a lot of custom chips which take up a small amount of room compared to the TTL equivalent the bootleggers had to use to reverse engineer them. This made the bootleg version of the game 4 times bigger and 4 times the amount of IC's to fail.
    I have a good collection of Irem M72 games and plenty of spares to keep them going, so I'm not worried about customs failing The same goes for any game you really want to keep going, spare parts are ESSENTIAL!
    Eventually all this hardware will fail so all we can do is enjoy it while they are still going.

  9. #29
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    Some Great reading

    Thanks !

    Trying to dabble

    Cheers
    Ando
    Pinball, Its an Addiction

  10. #30
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    Here's a good site for people getting started that explains some logic probe stuff and has good step by step info on troubleshooting/repairing a pacman board


    http://lawnmowerman.rotheblog.com/logicprobe.html

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