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Thread: Another dead SuperPET!

  1. #11

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    Still looking for help with this, but I've made some progress.

    A few 74LS244 chips have been replaced, as they had rusted through pins. Replaced two memory chips that weren't showing any signals and checked the traces in the memory area. One blown inductor replaced.

    I acquired a Tynemouth RAM/ROM board, but it didn't help. Still no display, no signal on J7 HSync and VSync. Then today, with the Tynemouth board set to replace all the RAM and ROM, I got a display! It was only once though. Power cycles didn't bring it back. Then I got magic smoke from CR2.

    So I guess I'm dead in the water until I replace the CR2 bridge rectifier, right? I've been looking around for parts to replace it. I know it's 50V/4A, but does anybody have more detailed specs or a specific part number from Mouser or Digikey or something that I can order?

    While I'm at it, any advice on what fault might be causing half the screen to be garbled like it is? Remember, that's with the Tynemouth board set to replace everything it possibly can.

    Thanks folks. Getting there! It was nice to see the screen is bright and clear, at least.

    Scott
    image1 (4).jpg

  2. #12
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    Quote Originally Posted by csc316 View Post

    So I guess I'm dead in the water until I replace the CR2 bridge rectifier, right? I've been looking around for parts to replace it. I know it's 50V/4A, but does anybody have more detailed specs or a specific part number from Mouser or Digikey or something that I can order?
    CR2 supplies the unregulated -9V for the -5V regulator. This is only for the -5V bias on the 16 dynamic RAM chips. That is not much of a load. Any full wave rectifier package should do. Match the proper pinouts. That chip should not have failed with such a light load. Something bad going on there. Possible short circuit problem.


    While I'm at it, any advice on what fault might be causing half the screen to be garbled like it is? Remember, that's with the Tynemouth board set to replace everything it possibly can.
    It appears the the upper 1K video RAM (UC6 and UC7) are not working or getting proper signals. They are 1K X4 static RAM chips 2114.

    It seems like you still have a lot of bad connections that do not allow the CPU to run. Check connections to data bus and address bus. That is a big job but a NOP Generator may help. do you know what that is?
    -Dave

  3. #13

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    Okay, thank you. I think I've got a few 2114 chips lying around. I'll try to swap them out.

    I do know what a NOP generator is, in theory. Never built/used one though. I do happen to have a pretty fast signal generator on my logic analyzer though. I could probably use that, right?

    Thanks so much!

  4. #14
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    No, a signal generator is not needed. It is a simple circuit made from two 40 pin sockets that force a constant NOP instruction to the CPU (6502). It lets the CPU increment the address lines forever so you can take your time and scope address paths. Someone here has a good writeup on how to make it.

    For some reason your posts still aren't posting quickly. A few more posts from you should fix that.

  5. #15
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    See this thread message 6 and 7 for info on NOP Generator. http://www.vcfed.org/forum/showthrea...screen-problem

  6. #16

    Default Making progress...I think...

    Okay, thanks everyone for the help.

    First, when I said signal generator on my LA, I meant pattern generator. I've used that pin the past to send different signals down each line of a bus to trace them. But I did look at the NOP generator links and built one of those too. Very handy hack! Thanks.

    Before I get to the NOP results, I did piggyback the 2114 video RAM chips. UC4 showed a slight change, so I replaced that one. Now I have both the garbage on the lower half of the screen AND every other character lit (see attached photo). But the characters are properly formed at least, so I'm guessing the character generator is okay?

    I also picked up a Tynemouth 6502 ROM/RAM board. It didn't get me back to consistent boot tone until I replaced the 555 timer. Looking at the schematics, I'm not seeing how that would help, but it was showing some wonky signals. So I went ahead and socketed it and replaced it. Now I have consistent boot to the tone if I use the RAM/ROM board to replace everything. Selectively turning off replacement components from the Tynemouth board, it looks like the lower half of RAM, E000 ROM and Kernal ROM are all failed. But I can deal with those later. For now, the Tynemouth board gets me to a very consistent boot tone.

    As for the NOP gadget, I didn't get very far. Testing the signals coming from the address lines on the 6502 itself with the NOP, the A14 pin shows 2.0V P-P and the other address lines are 3.8V. All are correct frequency. What might cause this low voltage on A14? I did check UD14, which I had previously replaced because one of the pins was corroded through. It shows the same thing (all address lines correct frequency but A14 half the voltage of the others). A quick check of some more remote chips shows some pretty wonky signals with the NOP gadget installed. But I want to deal with that A14 before I get any further out in the circuit from the CPU.

    Any tips on where to go next?


    IMG_2954.jpg

  7. #17
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    I take it the Sync (UB14-pin 7) is still low? If so, this indicates that the CPU goes wrong a short time after beeping and initializing the 6545 CRT Controller. With the Tynemouth board in place you are not dependent on good RAM and ROM in the main board, but there may be a fault on the address bus or data bus. Also some faults on the PIA and VIA chips can keep the system from booting.

    The video data is not random as it should be at power up. There are some problems there. The most significant bit of the video data (ESD7) seems to be connected to one of the screen address lines which may cause inverse video on some of the video columns. Also it looks like the even addresses of the lower 1K of the screen are always blank.

    I'd start by looking for shorts, opens or flakiness on the data bus.

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