NES with scrambled graphics

Discussion in 'Repair, Restoration, Conservation and Preservation' started by litephiter, Oct 16, 2016.

  1. litephiter

    litephiter Newly Registered

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    I picked up a faulty NES for a few bucks a couple months ago, and I've been having some trouble diagnosing what's wrong with it. I'm wondering if anyone here might be able to point me in the right direction.

    (Let me say up front that I don't have any electrical engineering experience. I've never had to troubleshoot a circuit before. So there may be some obvious way of testing all the different components for faults, but I wouldn't know of it.)

    When I opened the system for the first time, I noticed what looked like a botched repair job by a previous owner. Someone had tried to re-bend the 72-pin connector, but had destroyed it in the process. This same person (I assume it was the same person, at any rate) had attempted to disable the lockout chip—except rather than cut the pin near the PCB, he had ripped the pin straight out of the IC, possibly causing some damage in the process.

    I swapped out the damaged 72-pin for a brand-new replacement, and then when I fired up a game, I saw this:

    http://imgur.com/XUfmE5e

    Here's Bart vs. The World. Note that you can just barely make out Bart's sprite in the lower lefthand corner:

    http://imgur.com/TLtz5cu

    Sound and controls both seem to function well. The individual graphical tiles look fine, too—it's just that they're all mixed up.

    Using a plastic probe, I applied pressure to some of the components on the motherboard to see if there was an issue with electrical contact. The only thing that caused the screen to change at all was when I nudged the lockout chip. The graphics would jump around a bit, but they wouldn't unscramble.

    While searching these forums, I found a thread from 2012 that describes a similar problem. In that thread, it's suggested that the issue may be with the system's RAM, or with its PPU.

    I also noticed a small amount of white residue on one of the smaller capacitors, but there didn't appear to be any bulging, so it could be nothing.

    I figure the problem must be either with the lockout chip, the capacitor, the RAM, or the PPU, but I have no clue how to go about figuring out which one is the culprit.

    Does anyone have any guidance they could offer, or resources they could point me to? I'm happy to do a bit of reading, but I just don't know where to start.
     
  2. sayin999

    sayin999 Officer at Arms

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    Only person I know online that deals with a lot nes repair or mods is game tech. He strictly does rgb/hdmi mods now but go to his YouTube channel or email him on his site https://www.game-tech.us/ he maybe be able to give you some guidance.
     
  3. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    PPU or RAM most very likely. Since all the games seem to run fine I would suspect ppu first. It is actually fairly common.
    I have repaired many NES and encountered bad PPU several times. Since I have a lot of spare parts and motherboard I had the luxury of putting sockets in the motherboard and trying known good chips and that is how I found out 100% sure I had bad ppus.

    About the leg ripped out of the CIC chip. Actually very classic of the NES cic. As soon as you start to bend the leg a little it just extracts itself from the chip, always happen. It does not seem to cause any issue though, your CIC is deactivated and life goes on. No one will miss the lockout chip right?
     
  4. litephiter

    litephiter Newly Registered

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    Yeah, the PPU seemed a likely suspect to me. Do you know if there's any way to test a PPU? Like if I traced all the pins with a multimeter, would I notice breaks in continuity or anything like that? I don't want to go to the trouble of sourcing a replacement part unless I know what's up.
     
  5. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    No easy way that I know of. Not impossible that there could be a cold solder joint somewhere but not common on the NES, most likely bad PPU. You are better off selling the system for parts, the cpu should still be good and can be used to revive another unit.
     

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