Robotron CPU Issues

Pinball Wizard

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My Robotron bit the dust and I need some help. The CPU gets hot to the touch, and the monitor displays the attached image. I tried swapping it with a known good CPU chip, and still the same. What should I do?
 

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My Robotron bit the dust and I need some help. The CPU gets hot to the touch, and the monitor displays the attached image. I tried swapping it with a known good CPU chip, and still the same. What should I do?

With any game, the first step is to always check your Power Supply.

Williams games have a high failure rate on the power supply.
 
Power supply is good, I have the board set up with a switcher, along with a 4164 RAM upgrade, the 2 most common issues with Williams games.
 
Power supply is good, I have the board set up with a switcher, along with a 4164 RAM upgrade, the 2 most common issues with Williams games.

Ok, anything on the LED?

Could be bad RAM.

If you have a logic probe, check the following pins on the CPU:

Pin 37 to see if the Watchdog is barking. If the screen is static like the picture and the game doesn't reboot, then it's either not barking, or the the CPU is bad and it doesn't matter if it is.

Then check Pins 34 and 35 to make sure you're getting a clock. You probably are because you're getting something on the screen, but always good to check.

Then check pins 8 - 31 for activity (these are the Address and Data lines).

If 37 isn't pulsing, 34 and 35 are pulsing, and you see pulsing on 8 -31 (not all pins but some), then chances are the CPU is good.

You should also check the Ribbon cable between the ROM board and the CPU. YellowDog has posted a few links in the past on how to re-crimp/replace those connectors.

If that checks out, then you can swap some RAM around and see if that changes anything. If swapping RAM doesn't help, it could be an Address decoder or some other part of the Memory circuit that went bad.

If that's the case you'll need to break out the Schematics and start hunting it down.
 
I don't nomally see a crosshatch pattern unless the video output addressing is hosed. If the CPU is bad or the watchdog is not getting reset (resulting in pulsing on the RESET (pin 37) line) the video is normally either all white or a set of wide vertical bars, usually green or yellow.

Crimping the ROM cable might help. You can see if you get a different result by pressing on the top of the ROM cable connector or wiggling the ribbon cable after pressing the reset button on the MPU board.

ken
 
Quick question, would not having a sound board installed currently cause any issues? I have always had the sound board in but I just took it out to test in another machine and haven't had a chance to put it back in yet.
 
Quick question, would not having a sound board installed currently cause any issues? I have always had the sound board in but I just took it out to test in another machine and haven't had a chance to put it back in yet.

Nope. The only boards you need to test with are the CPU and the ROM board. You can leave the rest off.
 
I have activity on the address and data lines. The watchdog is pulsing away like crazy. I do have clock on pins 34-35. I put a known good ROM board in it and it didn't work so it doesn't seem like it is the ribbon cable, as I then put that ROM board back into another game and it works fine.
 
I have activity on the address and data lines. The watchdog is pulsing away like crazy. I do have clock on pins 34-35. I put a known good ROM board in it and it didn't work so it doesn't seem like it is the ribbon cable, as I then put that ROM board back into another game and it works fine.

It's could be a problem with the RAM or an Address Decoder.

Do you get anything on the LED at all?

Have you tried swapping RAM around? (Swap the first chip with the last chip in each column).

Chips you can start looking at:

1H, 3H, 4H, 2H
4E, 3D, 3E, 4F
1I, 2I, 3I, 4I
1J, 2J, 3J, 4J
 
You said that the watchdog is barking, did you see why? Check the inputs to the 74ls133 (at 5G) and see what is missing.
 
I don't nomally see a crosshatch pattern unless the video output addressing is hosed. If the CPU is bad or the watchdog is not getting reset (resulting in pulsing on the RESET (pin 37) line) the video is normally either all white or a set of wide vertical bars, usually green or yellow.

Crimping the ROM cable might help. You can see if you get a different result by pressing on the top of the ROM cable connector or wiggling the ribbon cable after pressing the reset button on the MPU board.

ken

Resurrection from the dead...

I never did get any further with the CPU board, too many projects and not enough time. Anyways, you mentioned the all white screen from a bad CPU or a watchdog not getting reset. I have another Robotron CPU that does that issue. What am I looking for to fix the issue. Swapped a good CPU in and had same issue.
 
You need to test the reset line one the CPU chip (pin 37). It should be high. If you press the reset switch, it should go low for a second then high again. If it is pulsing, then something is interfering with the CPU from resetting the watchdog.

If you are having the same issue with a known good MPU board, then it may be the ROM board. Pop the ROMs and verify them if you have a ROM burner. Of borrow a working ROM board from a different Williams game and try it. Also make sure the ROM board is getting power. It only needs +5V & ground.

Check the voltages at the corners of the RAM chips. I know you said that you are using 4164 chips so the voltages should be Pin1 = (indeterminate), Pin 8 = +5V, pin 9 = +5V, pin 16 = Ground.

ken
 
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