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Recent content by stepleton

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    IBM 5110/5100 video Display card repair (possible diode fix)

    We're pretty sure that those are parallel resistor arrays. The mystery output we're seeing (assuming it isn't caused by odd aliasing) is a strange (relatively) low-frequency 5V p/p sawtooth wave. Those are DIPs but they are not chips: they're resistor packs. Of course, resistor pacts often...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    Thanks for identifying your 'scope. I found the manual, and like manuals for many lower-end oscilloscopes (mine included!), it's a bit thin. The only thing that stands out to me right now is that the acquisition rate is qualified as "single channel". I think that if multiple channels are active...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    A few more thoughts now. Absent a 'scope that can resolve things a little better than your current setup, it's a bit harder to tell what's going on. Can you share information on what you're using? The clock recordings you've posted recently do not look very good, to be honest. What we want to...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    Unusual time again, more hours from now :) A SOIC is probably not what you need: that's a surface-mount package and you could have a hard time replacing a DIP with it.
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    IBM 5110/5100 video Display card repair (possible diode fix)

    Sorry, correction: trace back from pin 3, which is the input to the second driver. And I was wrong: the pin you're probing in the image is pin 4, not pin 11. My bad. In any case, thanks for the clarification: this makes more sense. The hunt continues from the input to the second driver. What's...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    I note the following ICs from your pictures: DIP 2392137 x2: a four-bit binary counter DIP 2396261 x1: a counter of some kind Other DIPs are buffers, nand gates, shift registers. The fact that D02 is flat is a mystery to me, but let's think for a second... C5 is a clock output that could be...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    I'm actually pretty curious about that, because unless there's some detail that the 5110 logic diagram isn't showing, then that line should not be constant on a good board. (Hopefully what I say below makes it clear why this is so.) As above, I'd encourage zooming in on the time scale and seeing...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    Jumping in outside of my usual schedule to say that I wouldn't do this: it seems likely to me that this signal is being driven by the "Character Counter" and probably can't be forced low without straining that IC. D02 looks very weird to me and I think you are correct to be suspicious of it...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    This seems like a good strategy to me. It's possible that you'll need delicate triggering to see the fault; before you know it, you might also find yourself borrowing or buying a logic analyser :) But you can always start simple. If you're lucky, maybe just probing individual output lines will...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    Whew, it's a relief to see logic lines back at logic voltage levels. +8.3V is pretty hot for TTL! Let's talk about cycle steal. I mentioned earlier how the Display Card suspends the processor when it wants to collect video data to display on the screen. This suspension is called "cycle steal"...
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    Tektronix 4050 GPIB Flash Drive - now available

    My apologies, this should all be corrected now. On that note, I think @WaveyDipole may have referenced me in a readme somewhere as Tom Stephenson, although perhaps there really is such a person and I am just presuming too much :)
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    Tektronix 4050 GPIB Flash Drive - now available

    Are you ready for some heart-pounding Option 31 action?? #6 - SEA DOG #7 - ICE RACES #8 - NIGHT RACE ("beginner" mode is boring; be sure to skip ahead to "advanced") #9 - STRAFE (this one is a bit hard to control) Games #6 and #8 take advantage of the Dynamic Graphics capability to...
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    IBM 5110/5100 video Display card repair (possible diode fix)

    I didn't know, but I had a guess. With several things that have the same name that starts with C and has no other hint, it must be something fundamental, something so common that they'd expect a technician to guess right away. I traced it back in the 5100 logic diagrams to its source: Sure...
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    IBM 5100 ROS arrangement

    It's possible, with a mix of "having the features they wanted to put in all along but didn't have time for with the 5100." Yeah, I admit that it's farfetched. I just don't understand what the mechanism could be for! I can't think of a single case where you'd want to wired-or "blend" two ROSs...
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    IBM 5100 ROS arrangement

    Having spent time looking at the logic diagrams in the 5100 MIM for a while, here's some observations. On the 5110, the Common and Language ROS card includes the Common and BASIC ROS but not APL, that's on a separate card. You can see this on the System Logic Manual PDF page 4. There, you can...
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