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Thread: Pacman PCB Repair 20/05/2019

  1. #11
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    Do you have any info or links to that little tester your using?

  2. #12
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    Quote Originally Posted by Arcade King View Post
    Heat shrink on the caps to keep @channelmaniac from having a meltdown.
    scary-terry-gif-3.gif


  3. #13
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    Quote Originally Posted by GIBO View Post
    Do you have any info or links to that little tester your using?
    Just cheap shit from ebay mate
    https://www.ebay.com.au/sch/i.html?_...ester&_sacat=0

    It only does gates,counters etc I need to find one that does buffers,octal transceivers and what not. Having said that its a neat little thing to have.

  4. #14
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    I use this one...

    https://m.aliexpress.com/item/327168...g1559360666131


    I have the USB powered version which seems to no longer be available.
    @Arcade King I'm happy to test any chip numbers that you want to know if it works with them.
    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

  5. #15
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    Thanks mate all this discussion I ended up buying one of these to try.
    https://www.ebay.com.au/itm/Digital-...53.m2749.l2649

    Probably double the price but I noticed it had 74HC245 on the screen which is one of the types of IC lacking from the other one, I've also got an old school logic comparitor but hardly if ever use it.
    I'll be honest I still rely far more on my trusty Elenco LP560 logic probe and scope than one of those chip testers. Problem I have with the chip testers is there's no stress test so you can get chips that say their ok but on the board they start playing up in a few minutes. Still good to verify if an IC is completely dead though.
    I'll let you know what I think of this tester when I get it.

    Just a heads up though if anybody is looking for one of those Elenco LP560's I bought 2 from Amazon 2 weeks ago for $15 usd ended up costing $80AU for 2 with postage. Cost over $60 just for one on ebay. I've never found a better probe than these.

  6. #16
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    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

  7. #17
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    Quote 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.

  8. #18
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    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.
    IMAG0144.jpg IMAG0147.jpg
    IMAG0142.jpg
    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.
    Untitled-2.jpg IMAG0148.jpg
    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.
    IMAG0150.jpg IMAG0151.jpg
    Things are looking much better on those two pins now.

    Fault. Board ran for half the day but once again another problem popped up.
    IMAG0155.jpg
    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.

  9. #19
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    Here's my final log on this board (hopefully).
    IMAG0145.jpg
    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
    IMAG0159.jpg
    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.
    IMAG0160.jpg IMAG0153.jpg
    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.
    IMAG0164.jpg
    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
    Untitled-3.jpg
    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.
    IMAG0165.jpg
    If this was a anything other than a original board I wouldn't have wasted my time.
    Last edited by Arcade King; 12th June 2019 at 06:33 PM.

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