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Thread: Original Tandy 1000 no boot

  1. #21
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    Quote Originally Posted by falter View Post
    Okay so I have done some checking around. Voltages from the PSU look good. I see clock signals being divided into correct frequencies ok. But there is no activity between the CPU and 128K ROM. I have light 'snow' on the screen, persistent tones from speaker.

    One thing that is odd.. probing the READY line on the CPU it it is high, however if I hit Reset a random number of times I will sometimes see an inconsistent signal there.

    Presently am trying to understand how the 8284 works and why occasional reset causes it to 'leak' signal onto the ready line, or if something should be there.
    IDK maybe bad capacitors?

  2. #22

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    Keep in mind that in their typical weirdness, Tandy used the stripe on the edge of the floppy cable to indicate pin 34, rather than pin 1. And their floppy drive controller and cabling follows the original Shugart standard, rather than IBM's modification of it which later became the industry norm.

  3. #23
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    I was playing around with this some more yesterday while waiting for my 100's motherboard to dry from my cleaning efforts.

    I got a 1000 keyboard finally and plugged it in. Interestingly, on first power up, the num lock and caps lock keys allow themselves to be turned on and off. And then about 5 seconds later they lock up.

    I'm getting 'snow' onscreen and that persistent digital howling noise.

    I know the ROM is good - I've tested that. RAM are all good. Socketed chips like the 74245s check out. There are some PROMs I cannot really test. CPU is good.

    I'm trying really hard to follow the schematic but this is orders of magnitude more complicated than what I'm used to.

  4. #24
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    Borrowed my Tandy's PSU to try some diagnosis on my Mindset, and then got to looking at the motherboard again and discovered this:

    20191110_195601.jpg

    I'm not sure if a cap can still work properly like that.. or how it happened, but I noticed C1 is tied to a 7404 that is involved with the disk controller, which is what I was having a problem with initially. Originally when I got the machine it would post fine, but the disk drove light was always on and it would not read anything or even attempt to.

  5. #25
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    That's just a bypass cap on the +5V line. It should be replaced. I doubt that it would cause a non-booting condition, but it might cause read errors on the floppies or reboot problems or "general flakiness" in operation.

    Tom

  6. #26
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    Yeah.. I'm not really sure what's going on with this thing. What I know so far is that regardless, there is always a loud tone on power up. Sometimes I can see activity coming from the BIOS ROM (which I've tested is good in another machine), sometimes there's nothing. Can be totally different at each power up. But whether anything is happening there, regardless the sound does not stop and there's usually no video. I can hit reset 5 times and get 5 different behaviors.

    On power up, the keyboard 'lock' keys (CAPS lock, etc) briefly light up. You can toggle them for about 30 seconds, then they lock up.

    I know it's not the BIOS ROM, the CPU, etc. There are four PROMs on the board... any one of those could be dead and there'd be nothing I could do there, but hopefully not. PSU voltages look good, it responds to reset... there's just no video (garbled video on composite) and no action.

    One thing I did do, thinking about vwestlife's comment above, is I attempted to change the floppy drive cable. If what he said is correct, maybe having certain wires the wrong way fried something?

  7. #27

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    I don't think hooking up the floppy drive the wrong way would fry your system I believe the pins don't carry any power in cable. The only thing that would happen would be the floppy drive lights always being on and not working at all during the initial bootup, but that was what it was doing when you first got the system. Have you just said screw it and reseed everything that is removable? I am assuming that is what you did when you scrubbed the board.

  8. #28
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    Yeah I've reseated everything, and tested every chip I could in my Tandy 1000EX... so far they all come back just fine. I hate that everything else is soldered - I have some suspects but it would be involved to remove and test them.

    I keep going over in my head what I did when it was still posting and then changed.. but I really can't think of anything. All I did was note that the floppy activity LED was always on, tried the drives in another computer (they worked), tried a different cable and drive in this one.. and then all of a sudden no post screen and this tortured, continuous tone. At one point, I did have the floppy drives fall and sort of hit the motherboard on their way down (I had them propped above it).. but I don't see any physical damage anywhere.

    I'm a bit surprised, googling around, that this isn't something that has been encountered more. I've found one reference to a similar problem but no solution.

  9. #29

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    Maybe the cpu socket on the main board got screwed up. The manual that im looking at says that the cpu is one of the first things that get initialized.

    I'd know more but I only have the sx model and tx model and only the actual book for the sx. The rest of my stuff I have in pdf format I found on the net

  10. #30
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    QD (quad) is poorly named if it's just an 80-track 5.25" DD drive like the TM100-4. It's still "double" density, just more tracks.

    I mean you could just use 40 tracks and store it as HD and get the same thing, but that was just not done?

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