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Thread: Original Tandy 1000 no boot

  1. #41
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    Quote Originally Posted by blackepyon View Post
    I'm not sure how it is on other machines, but the Tandy 1000's are able to operate the keyboard lights independently, at least for a while. When I had a blown '245 on my HX, I was able to toggle the numlock and caps a few times before the keyboard controller locked up. That '245 buffer was on the data bus by the expansion header - half the gates were bad, and was causing the entire bus to lock up while the address lines kept going.

    I think the IBM XT had a 4.77MHz processor, same as the 1000. Are you able to swap the CPU from your XT?
    Interestingly the 245s are among the few chips that are socketed. But I have swapped those with others I have and no effect. The only thing that changes things at all seems to be pulling either the CPU or PIC.. if those go the snow changes into lines.

    I have checked the voltages after 5 minutes of run time.. they are all well within spec.

  2. #42
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    Are you pulling the CPU/PIC outright, or swapping them with a known working unit? Which chip number is the PIC you're pulling? I want to follow along on the schematics.

    Can you post pictures of the display during these two states, and tell me which video output you're using? Snow is rather unusual on an RGB display, unless you're referring to random ASCII characters.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  3. #43
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    Pulling them outright, for that particular 'test'.

    This is the screen I usually get, on composite, on power up, with all chips installed. These dots all move around continuously, there is no reaction to Reset or anything, and nothing onscreen whatsoever off the RGB regardless.

    20191124_193101.jpg

    If I pull the 8088 or PIC out completely and power up, I get this screen:

    20191124_193231.jpg

    And every once a while, randomly, I get a screen like this:

    20191124_193138.jpg

    This screen reacts to Reset.. changes intensity for a second before coming back to bright white.

    All of this could be meaningless - but this random video out the composite is the only sign of life there is. That and the continuously shrieking speaker.

    When I first got it it was fully working except for a continuous 'busy' light on the floppy drive, with no disk reading. I pulled the FDC first and cleaned the pins, no change, then pulled the drives and tested in another machine. They were fine. So I tried another cable I had lying around.. nothing. And then on one power up, suddenly that speaker shriek and nothing onscreen again.

  4. #44
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    Oh, and I should mention I have tested the 8284 and CPU in my XT.. they both work fine there. Also have swapped RAM around (haven't tested it in another device or board yet though) without any change. I've tested the BIOS ROM in a 1000EX I have, so that all seems good.

  5. #45
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    The first picture looks like a cursor on an HSYNC signal that's gone way off kilter (to me anyways)

    You've already tried swapping the CPU and the 8284 from the 1000 into a known working system, and they're working, so those are out of the way.

    This all happened after you took everything apart and cleaned it? Just to make sure, is the E5-E6 jumper from the oscillator installed?

    U39 is a binary counter which takes the 28.6 MHz signal from the main oscillator, and feeds the 8284 it's 14.3 Mhz clock signal, as well as the 3.6 MHz signal to the tone generator. Maybe use the oscilloscope to verify the frequencies off of U39 pins 11,12,13,14, and coming in at pin 2 from the oscillator, according to the following timing diagram from the technical manual.
    1000 clocks.jpg

    If these are proper, then we can move on down the list.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  6. #46
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    Hmmm... the signals do seem to be a fair bit off... at least, if I'm using this thing right. Seem to be at 12MHZ where there should be 14.1 and on down. Incoming looks right.

    74F161... can that be substituted with a 74LS161? Not sure what the F means. I have LS161s not but F.
    Last edited by falter; November 25th, 2019 at 10:10 AM.

  7. #47
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    Quote Originally Posted by falter View Post
    Hmmm... the signals do seem to be a fair bit off... at least, if I'm using this thing right. Seem to be at 12MHZ where there should be 14.1 and on down. Incoming looks right.

    74F161... can that be substituted with a 74LS161? Not sure what the F means. I have LS161s not but F.
    Yeah, I think the 74LS161 should work as a drop in replacement. The F series looks to be discontinued, but that's not surprising. The LS and HCT series seem to have taken over most applications.

    The video array is timed by the 14MHz clock signal, so if that's running closer to 12, that would explain why the horizontal sync is wonky. Somehow, it's also supposed to reduce that 14MHz into 15.7 KHz, which would also render the V-Sync on the RGBi output as out of spec.

    Well, fingers crossed, that's all it is.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  8. #48
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    Any luck?
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

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