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Thread: IBM 5150 - Beep, disk activity - but blank screen

  1. #11
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    Cant quite make out if there is an AC wave form there !
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  2. #12

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    Yeah, neither could I! Doesn't look any better at different speeds either...

  3. #13
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    Need to adjust the trigger so it stabilises the image but I imagine the faint line is the repeated overlayed image of the peak of the wave forms so it looks as if you have a video signal.

    Using the computerfacts diagram, follow that waveform through the signal path (but play with your scope to get a stable, triggered image of a real waveform) especially check if you have the 20V peak to peak waveform after R212.

    Essentially, as you get an image of the raster with the brightness turned up we know that most of its working so first lets prove that you have a video signal at the cathode (ie the heated bit that produces the electrons as the voltage from the video signal is applied to it, useful description here http://lateblt.tripod.com/bit71.txt)
    Last edited by Gary C; August 10th, 2020 at 01:41 PM.
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  4. #14

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    I did try adjusting the trigger - was one of the first things I tried, didn't seem to make a difference. I'll mess with it some more and try tracing the that signal.

  5. #15
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    Quote Originally Posted by nullvalue View Post
    I did try adjusting the trigger - was one of the first things I tried, didn't seem to make a difference. I'll mess with it some more and try tracing the that signal.
    Sorry, its difficult to know how much experience people have

    So check

    Need clear triggered waveform from pin 7 & 6 of the video card, then

    1 the signal from pin 7 should be present in amplified form at pin 9 of the sn7406 and inverted at pin 8
    2 the signal from pin 6 should be present at pin 1 and at pin 4 of the sn7406 imposed on 5v DC
    Last edited by Gary C; August 11th, 2020 at 05:53 AM.
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  6. #16

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    So I can't seem to get a good reading coming out of the video card either. Here's what pin 9 looks like. But the funny thing is, 6,7,8 & 9 all look exactly the same - this. Where should I be getting GND, pin 1 right?

    20200811_143445.jpg
    Last edited by nullvalue; August 11th, 2020 at 12:28 PM.

  7. #17

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    Yeah I don't think I had a good connection on GND for reading the video card signals. BTW I am a bit of a scope newb - I know the basics but I may be doing something wrong.

    Here's what I have been able to get from the card.. I just got GND from the ISA pin 1

    Pin 9
    20200811_153615_pin9.jpg

    Pin 8
    20200811_153654_pin8.jpg

    Pin 7
    20200811_153742_pin7.jpg

    Pin 6 (looks like nothing/no activity?)
    20200811_153825_pin6.jpg

  8. #18
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    Ground on a scope is something to be careful of. Its not like using a multimeter where you measure a voltage across a component. On most scopes, the ground is connected to earth, so all you need to do is stick your probe on the point to be measured and you get a reading. Problem comes if you try to attach the ground of your probe to a point that isn't at ground potential then you short that point to ground and it goes bang !
    If you needed to ground your probe, to get a reading that is odd. Check the mains earth on the scope and the PC are wired up.

    I will need to check the pin 6 output from my MDA to see what that should look like but pin 9 & 8 are showing synchronising signals and pin 7 seems to show picture information which to me means the card is putting out a display.

    So we need to trace the signal from pin 7 through the circuit shown in the Sams computerfacts sheet - BUT the earth thing confuses/worries me, you shouldn't need to connect the scope ground normally, so be careful only to ground it to ground or you might create a short circuit (anyone else have a comment ?)

    Some info on the problem here https://www.testandmeasurementtips.c...-oscilloscope/
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  9. #19
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    I am wondering about that intensity signal, it should be doing something

    from here https://www.seasip.info/VintagePC/mda.html

    The 'video' and 'high intensity' pins have four possible combinations:

    Video------Intensity--------------Resulting colour
    0-----------0----------------------Black
    0-----------1----------------------Dark green
    1-----------0----------------------Green
    1-----------1----------------------Bright green

    So it should display something, but Im not sure, need to test my PC
    Last edited by Gary C; August 11th, 2020 at 05:46 PM.
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  10. #20

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    Quote Originally Posted by Gary C View Post
    So we need to trace the signal from pin 7 through the circuit shown in the Sams computerfacts sheet - BUT the earth thing confuses/worries me, you shouldn't need to connect the scope ground normally, so be careful only to ground it to ground or you might create a short circuit (anyone else have a comment ?)
    Interesting, what else is the alligator clip on the probe for then, if not for ground? The person who gave it to me (an EE) showed me how to use it and connected the clip to ground. As I'm sure you can tell, it is an older analog scope.. I'll start tracing that signal.

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