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Thread: Superbrain QD repair

  1. #71

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    Oops..

    Obviously you will have swapped Z56 back to the "sane" one. Have you attempted without the boot disk? We're looking for the "Insert diskette" message from the first stage bootloader.

    Other things to check: The character generator and VDP chips. Try swapping them out with the other board or some of your spares.

  2. #72
    Join Date
    Oct 2016
    Location
    Burnley, UK
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    163

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    That's what I get on the screen regardless of the boot disk. Power on, press red buttons (with drive door open) and that screen comes up. I've swapped the various socketed chips.

    Believe me, I'm waiting on that Insert Boot Disk message!

    It would be pretty amazing to get this board working. I've had it most of my life!

  3. #73

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    Beat me to it.

    Also try probing around Z55, the shift register that feeds the character generator. We should have activity on its input and outputs. Check Z56 pins 11, 13 and 8 for pulsing, and Z57 pin 6 for pulsing. This is the video signal, if it's stuck you would get a white screen (or black, depends on which way its stuck).

  4. #74
    Join Date
    Oct 2016
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    Burnley, UK
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    Ok Jon, thanks for that. Have to pause for a bit now as family matters press. I'll report back later ��

  5. #75
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    Oct 2016
    Location
    Burnley, UK
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    I've got pulsing on all inputs and outputs on Z55

    Z56 pin 8 and 11 both pulsing. Pin 13 is high.

    Z57 pin 6 is pulsing.

  6. #76

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    That is the video (luminance) output from the CRT8002. Put a 'scope on it. I'll check mine, but I don't think it should be high. (Edit: Yes, should be pulsing. This is why you have a full on white screen I think!)

    Anyway, speaking of "having a bad day..."

    https://youtu.be/JCmyvacGTCQ

    (wait for the upload to complete)

    It was all working fine until I started filming. What's actually happened is the directory tracks aren't getting read properly for some reason. If I format the disk and start writing stuff to it, all is well; but the moment I power cycle the machine it all goes to pot, because the in-memory buffers are lost.
    Last edited by JonB; February 9th, 2019 at 08:51 AM.

  7. #77
    Join Date
    Oct 2016
    Location
    Burnley, UK
    Posts
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    So near yet so nearly there? Superbrain plus hard drive - the ultimate! All it needs then is my Pixelplotter unit on one side, an S100 adapter on the other, a psu upgrade...

    Let's get to a working SB first...

    On pin 15 of the main ribbon cable between the logic board and psu, I have a signal of some sort according to my scope. That's the video line according to the diagram.

    I also socketed the keyboard chip. Only thing I discovered is that I have one working chip out of the three. With the other two, I loose the drive activity.

    One thing I did discover - I tried an AMD PPI chip. If I press on its lower end, I can make a faint dark.block cursor appear on the screen. It moves back and forth along the top left on a hard reset - as if the insert disk' message is cycling with the reset keys being pressed. Pressing on the PPI chip also makes that cursor jump around the screen from top to bottom under the boot disk message location.

    Wonder if it's worth replacing the PPI socket or looking for bad joints under there?

    Also discovered only one of my VDU driver boards works. Need to recap it. The picture may improve?

  8. #78
    Join Date
    Oct 2016
    Location
    Burnley, UK
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    Definitely need to recap. Picture began to shrink in width as smoke poured out of an electrolytic! Pause for ebay again!

  9. #79

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    Ah, you let out the magic smoke!

    Occupational hazard, I'm afraid.

    Definitely replace the PPI socket. It is clear there's a bad connection there. I had the same thing with my CPU socket; it's worn out and only works with the uIDE Z80 shim installed. Check the other sockets too.

    For the PSU upgrade, buy a Meanwell 12v power supply. Meanwell = "good quality brand", if it is genuine. Don't use generic cheapo PSUs from China because they have a habit of blowing up. Connect the Meanwell PSU to the disk drives' 12v line, disconnecting it from the Intertec PSU, and connect the Meanwell -ve line to the ground on the Intertec PSU.

    This takes a lot of load off the marginal Intertec PSU and since fitting one to my machine I have had zero problems. I actually did mine to resolve screen picture glitches each time a floppy was accessed, as I'd implemented the driver motor control mod so each time a motor started there was a bit too much current drawn, which upset the monitor.

    Anyway, speaking of uIDE..

    Take 2: https://youtu.be/y-gY0S2aVvs

    ..and, because things never go as planned..

    Take 3: https://youtu.be/hBrnUB8LXkE


  10. #80
    Join Date
    Oct 2016
    Location
    Burnley, UK
    Posts
    163

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    Getting closer Jon - just another bug!

    Update on the repair. I've ordered 10 worth of electrolytic caps for the VDU driver board. The picture I have is not filling the width of the screen because of the cap that released the smoke. As it let off, the screen shrank further. Hopefully the picture will improve in stability and quality if i get rid of all the crusty 40 year old electrolytic's off that board.

    I've also had a shift change at work so i no longer have mornings free - further work may be confined to the weekend, so Friday/Saturday.

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