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Thread: PET 3008. Father/Son repair project. Display looks like a maze.

  1. #1
    Join Date
    Feb 2013
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    Chesterfield, UK
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    Default PET 3008. Father/Son repair project. Display looks like a maze.

    Hi,

    Many years ago, I got a 3008 from a car boot sale. It worked at the time although I remember having to wedge something under the screen to get it to display . It went up into the loft and was recently noticed by my son, who wanted to see if we could get it to work.

    It shows green horizontal stripes when powered up now.
    IMG_20191014_234615341.jpg
    Reading around suggested it wasn't getting a video signal so we took the back panel off the monitor and found a pen top glued underneath the video board (the blue thing in the picture).
    This reminded me of having to wiggle the monitor when it used to 'work'. Someone must have tried to fix it that way.
    IMG_20191014_220021501.jpg

    When I now try to flex the board gently, the display changes to what I can only describe as a maze.
    IMG-20191014-WA0000.jpg

    Confession time: We started to re-seat the roms before taking the monitor back off. I bent the pins on the first one I took out - a 901447-10 (the character rom?), and one of the corner pins snapped off
    IMG_20191014_220200143_LI (2).jpg

    I wish I had remembered the dodgy screen connection problem before I took the chip out. Could the 'maze screen' be a symptom of the damage I did, or have I got other problems too?

    Thanks in advance for any advice.

  2. #2
    Join Date
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    That is pin 12, the ground connection. It is required for the chip to work. Did you solder the pin back on? Or at least tack a wire from the shoulder of the chip to the socket pin. Try not to mess up the socket so you can re-insert a new chip later. I could send you a replacement chip, but it would take a while.

  3. #3
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    Feb 2013
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    Chesterfield, UK
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    That's a very kind offer, thankyou. I had a go at fixing it however and even though I do say so myself, it's not a bad job.
    IMG_20191015_222944187.jpg

    I put the chip back in, applied the pencil-under-the-video-board fix (tm) and tried again. The screen has changed, and now looks like this:
    IMG_20191015_223523341.jpg

  4. #4

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    Yes, the missing pin is at least one thing that can cause your problem.
    As for the video, you might pull the board out and look carefully at solder joints. Since you have raster it is likely away from the flyback transformer. You may need to discharge the high voltage connection to the aquadag. I used to use wire attached to the pering on the aquadag and a screw driver but I've been told that the current surge can damage things. You should use a 1Meg ohm resister in series with the wire ( assuming you are using a well insulated screw driver and not a wood handled one ).
    Leave it connected while you remove the high voltage connector. The usually have a spring wire hook that can be released by carefully pushing the back to the side until one side of the connector comes free ( requires a significant force ).
    Then remove the connector at the gun end. Be very careful here. There is a tiny tit of glass that it underneath the connector. If you rock the connector too much sideways you can break it.
    I believe the yoke is connected by a connector that you can pull. I'm not sure if it is keyed so make a mark on the connector and the board with a permanent marker.
    Be careful around the hole where the high voltage connector was. Because of induction, some of the charge can grow back. Always discharge as describe to the aquadag before getting near it.
    Dwight

  5. #5
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    I'd say that the 'maze screen' shows that the monitor is OK and the video is working; it might be the 'normal' garbage screen without a character generator chip. Before you poke around inside the monitor let's get that chip repaired or replaced and then see what's what.

  6. #6
    Join Date
    Jun 2012
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    UK - Worcester
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    The brightness is too high of course. This may be just a tweak required to the brightness control. Turn the brightness control down until the 'background' just goes black, and (what looks like the line retrace) goes away - if possible.

    I suspect you have more problems than just the character generator though. It looks like you have a problem with the ODD (or is that EVEN - it is too early in the morning for me!) RAM or latch.

    There may also be a problem with the character generator anyhow.

    How many chips are in sockets?

    As Mike says, the fact that we do have a picture at all is good. This allows a bit of debugging...

    Dave

  7. #7
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    Feb 2013
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    I have a friend who restores old arcade games who tells horror stories about discharging old CRT tubes, so happy to delay that for now - but thanks for the details Dwight.


    Brightness adjusted - looking better.
    IMG_20191023_194851067.jpg

    Here is a full picture of the board - there are 3 empty sockets:
    IMG_20191023_194545635.jpg

    Does it help with the fault finding process to know that I witnessed this working before it's stint in the loft?
    I invested in a decent chip extractor. I'm thinking removing and replacing the socketed chips might be worth a try?

    In the meantime, I'll read up on oscilloscopes. I'm anticipating a PicoScope from the birthday fairy soon.

  8. #8

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    Quote Originally Posted by Eightbysix View Post
    I have a friend who restores old arcade games who tells horror stories about discharging old CRT tubes, so happy to delay that for now - but thanks for the details Dwight.


    Brightness adjusted - looking better.
    IMG_20191023_194851067.jpg

    Here is a full picture of the board - there are 3 empty sockets:
    IMG_20191023_194545635.jpg

    Does it help with the fault finding process to know that I witnessed this working before it's stint in the loft?
    I invested in a decent chip extractor. I'm thinking removing and replacing the socketed chips might be worth a try?

    In the meantime, I'll read up on oscilloscopes. I'm anticipating a PicoScope from the birthday fairy soon.
    It sounds like it has a sampling rate of at least 20MHz. That will be useful for looking at the ~1MHz signals the processor uses. The crystal is 16MHz and will be beyond what the scope can likely display. Still it is much better than many of the other cheap DS scopes out there that are just on the edge of being useful.
    Remember, it is a sampling scope. Understand the funny displays you'll get by undersampling. This is where you've set the sweep rate to slow for the rate of data that is being displayed. As high frequency square wave can look like a lower frequency sine like wave. A data bus signal can look like a bunch of spike of different amplitudes.
    There are many advantages of a digital sampling scope but it does come with some disadvantages as well. The most painful one, for fixing old computers, is that one can't use one for the 'cats eye' pattern used to align tracks on a disk drive. That requires an analog scope.
    Anyway, understand the concepts of both undersampling and oversampling and you should be fine.
    Dwight

  9. #9
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    So I got the oscilloscope, I read the book - I need something to sample now.
    What would you guys recommend I check first?
    Is there a schematic for this model (300 that I can use to make educated guesses with?

  10. #10
    Join Date
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    Here are the schematics for the 3000 series. Start with the clocks if your scope can handle 8 MHz. It is on Sheet 6 on chip G5-pin3 called CLK8.

    http://www.zimmers.net/anonftp/pub/c...01N/index.html

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