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Pacman PCB Repair 20/05/2019

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Share any logs or helpful information you may have to repairs of arcade PCBs, Monitors etc

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  • #16
    I usually use my tester to confirm the chip I remove is faulty or if I have a random chip floating around that I can't remember if it was good or not.
    If in doubt, for the cost of chip it's better to just replace it.

    The Elenco's are great, I bought mine 4 years ago and never had an issue with it though the next one I buy will have audible tones so I dont have to shift my gaze from the chip to the LEDs.
    Namco Starblade - Taito Cocktail 14" x 3, 20" x 2 - NZ SI PtII Upright - Chase HQ DX, Taito Top Speed - Taito Grand Champion - Taito Safari - LAI SI Head to Head Cocktail - SI Cocktail Clone x 2, Pong Clone x 3

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    • #17
      Originally posted by Kaizen View Post
      I usually use my tester to confirm the chip I remove is faulty or if I have a random chip floating around that I can't remember if it was good or not.
      If in doubt, for the cost of chip it's better to just replace it.

      The Elenco's are great, I bought mine 4 years ago and never had an issue with it though the next one I buy will have audible tones so I dont have to shift my gaze from the chip to the LEDs.
      Don't wait. Get it NOW. You'll hear a distinct difference with racing gates, weak outputs, and more. It'll greatly expand your troubleshooting.

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      • #18
        Ongoing issues with this poor old neglected board.

        In my last log I wrote about a fault with the dip switch settings going nuts in test. Unfortunately this only got worse as the real culprit failed.

        Fault: Game just goes nuts. Will generally boot when cold but is very picky about the ram, CPU and even sync buss controller used. Watchdog isn't barking it all so the program is just crashing.
        Sometimes it boots coining itself, sometimes boots into the game however the right hand side of the screen starts filling with random letters and characters. When Pacman reaches the right hand side of screen he will continuously move down the edge of screen from top to bottom until it returns to attract mode. Ghosts will follow the same pattern if they hit the edge of the screen (see pics.) Further more to that the sound was funny.
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        Eventually it would just boot into Bad V Ram-0

        Fix:Annoying fault to say the least. After going over the data buss with a fine tooth comb I couldn't find any issue so I turned to the ram address buss in particular the chip select circuit for the main scratch pad rams.
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        Long story short I found pins 5 and 6 shorted on a 74ls02 @ 7L you can see the odd signal on the scope however removing the IC the short remained.
        Traced it back to a bad 74ls138 @ 7J pins 5 and 6 shorted. Looking at the schematics it now makes sense as to why so many things were effected and going haywire, it's amazing I could get the board booting at all. I was able to get the board booting by just touching my probe on pin 1 of the 74ls02 which gave me my first clue.
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        Things are looking much better on those two pins now.

        Fault. Board ran for half the day but once again another problem popped up.
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        Fix. 74ls273 @4D just decided to pack it in.

        After all that I'm still getting an intermittent graphics fault...that's tomorrows problem.

        I didn't think this board was going to be easy to fix and was expecting more problems to pop up and I'm sure this isn't the end of them but at least the program is running 100% now without any crashes. I have no doubt this pcb hasn't been fired up in over 20 years.

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        • #19
          Here's my final log on this board (hopefully).
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          Forgot to put this up last log.
          While probing around I found a bunch of floating pins on the 4 74ls161 counters. R59 was completely missing, it's supposed to be a 1k resistor so I put one in.

          As mentioned in the previous log I was getting intermittent graphics glitches both separate issues.

          Fault: Ghosts started flickering changing colour then finally horizontal coloured bars across the play/maze area of the screen
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          As you can see only the maze area is affected.
          Fix: I've read that a failed attack ram(s) 2A-2D can cause this issue however I couldn't find any obvious issue with the data or address lines.
          Looking at the schematics I suspected either 3A or 3D as the culprit. Since 3A 74ls158 looked the worse with rusty legs I just swapped it out and was right. Game now 100%...for now.
          I've also desoldered the last 3 attack rams HM2510-1 (3B-3D) and replaced them with MCM2125's I had floating around which run soooo much cooler.
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          Picture on the left shows how bad the rust on the TI 74ls158 is along with the crappy looking HM2510 rams. Picture on the right I meant to put up earlier which shows more crappy IC's and you'll notice legs have been snipped on the two IC's top right. Somebody had a good crack at repairing this board in the past and did the old snip the leg trick to find the culprit. For reliability sake I've replaced all IC's I could find that Mr Snippy has been at which was around 5 or 6.

          Last fault.
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          The picture says it all. It started off intermittent and eventually sprites were affected which weird little sparkly blocks at the top of their heads.
          Fix. Pretty easy fix, usually when I see faults like this I head straight for the 74LS86's
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          Culprit was the 74ls86 @ 2H the outputs were weak. I could easily replicate the fault by rubbing my finger along the side of the rom pins 8-6
          For future proofing sake I also replaced the 74ls86 @ 4F both were rusty TI (Texas Instruments) branded IC's. TI seem to be prone to rusty legs.
          A lot of the IC's I've replaced on this board were the SA brand or rusty TI's. I think I've replaced almost if not more than half the IC's on this board and pretty much encountered every fault possible repairing it. There's only 2 TI chips left 74ls08's I'll probably replace these however the board ran 8 hours straight without a missing a single beat so I'm pretty happy with the effort.
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          If this was a anything other than a original board I wouldn't have wasted my time.
          Last edited by Arcade King; 12 June 2019, 06:33 PM.

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