(Topic ID: 204080)

Williams System 4 MPU problem has me stumped

By msadan

6 years ago



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  • 10 posts
  • 4 Pinsiders participating
  • Latest reply 6 years ago by msadan
  • Topic is favorited by 2 Pinsiders

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    #1 6 years ago

    Once again I need help from the pros. I'm working on a Williams System 4 MPU. Not sure of its origin, I'm just working through a stack of bad boards. No obvious corrosion damage on this board. On this one I cleaned all of the header pins, de-soldered / re-soldered all of the headers, replaced the chip sockets at IC20, 17, 14, & 19, burned and installed new flipper & game (Gorgar) EPROMs, installed an NVRAM, removed the battery holder, and fired it up on the bench with a good driver board. The LEDs flashed so I stuck in in a good Gorgar. Here's what it does in Gorgar: Powers up normally and goes into attract mode (kind of, attract mode is missing a lot of the lights and I'm not sure that the lights that are working are flashing correctly). You can add credits and start a game, you get the "Gorgar speaks" callout, then the game goes back into attract mode. It gets stranger: If there's a ball in the ball trough it won't go into attract mode. A couple of the playfield lights strobe rapidly, only the player 1 and player 3 displays are lit, and they are both kind of scrolling zeros for lack of a better description. The switch for the center middle rollover causes the same symptoms. Here's a link to a video of what it's doing:

    One more symptom: Flexing the MPU board on the right side will cause the game to lock up or reset. I know, that one should be easy to track down, but so far I haven't been able to. It seems to be an issue with the upper right quadrant of the board but I haven't been able to narrow it down more. No obvious cracked or cold solder joints, at least none that I've found. Also all diagnostics run correctly. All of the controlled lamps work, all switches (including the two that cause the weird symptoms) register correctly. BTW those two switches are not in the same row or column. One last thing, the fuse for the logic voltage has blown twice while I've been working on this, once when I was flexing the MPU and once right after powering the game up after installing the MPU.

    Here's what I've tried so far: Swapped in known good EPROMs, NVRAM, PIA, and processor. New sockets for PIA and processor. New socket for IC13 (had been previously replaced, badly). IC5, 16, &26 are also socketed but I haven't replaced those sockets (yet). None of the sockets I replaced were RM, or seemed bad. The good news is that I haven't caused any new problems , bad news is it's doing exactly the same thing. Anyone have any ideas where to look next?

    #6 6 years ago

    Thanks for all the suggestions. I only had a few minutes to play around with it today so I didn't really accomplish anything. Tried it with a 5101 instead of NVRAM, no difference. The male header pins on the 40 pin connector weren't the greatest, plus I wanted to check under the connector for corrosion, so I replaced the header pins. No difference. I'll dig into it a bit more tomorrow.

    #10 6 years ago
    Quoted from barakandl:

    Make sure you are not missing the upper most address bit or a data bit at the interconnect. I think A13 or so being missing at interconnect (yanked high by driver board pullup) can cause an effect like this.

    which NVRAM module?
    system 3-4 games need a dual ce compatible NVRAM.

    Had a chance to go in with a logic probe today and when it's in attract mode the address and data lines look normal (looks the same as with the working System 6 board). When I drop a ball into the ball trough address lines A4 - A7 go a little crazy. Not sure what (if anything) this means...

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