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Thread: Trs 80 model 4 dead

  1. Default Trs 80 model 4 dead

    Hello all.

    I recently purchased a model 4. When I turn it on the screen shows the raster and the drives spin (no red light) for a few seconds. I opened it up and checked the power supply voltages. I am getting 4.9V on the 5V rails and 11.4V on the 12V rail. I also get the -11.9V. As far as I can tell there is no activity on the board. I am getting 4.9V on the chip VCC pins. I am not getting any activity on the clock pin #6 on the z80. I have traced all the way back to the crystal and if I am doing it right (definitely a noob) I still am not getting anything on the oscilloscope. I am just getting the 4.9V.

    Are the crystals a known problem with these boards. I could easily be missing something and it might not be the crystal at all.

    Also I did check the voltages at J4 on the mother board and they appear to be correct (from left to right 4.9,gnd,11.4,-11.4)

    Any ideas?
    Sean

  2. #2

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    By a strange coincidence, I have a model 4 (non-gate array) that exhibits the same (at least outwardly) symptoms as of a couple days ago. I don't want to say much more and co-opt your thread, but I will be watching this with interest. May or may not be the same thing going on with mine.

    BTW, does anyone know where I can get a schematic for a non-gate array model 4? Schematics for the gate array version are all over the place, but I haven't found any for the non-gate array version, and it looks like even the power supply boards have a significantly different layout.

  3. #3

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    The crystals in the Model 4 have nothing special about them. You should get a wave form on the crystal using an oscope. They can go bad, so try replacing it.

    Since you are an admitted noob maybe post some pictures or a video of how you are testing it and we can make sure it's correct.

    I'd also look at the Z80 as I've had a few of these go bad on me.

  4. #4

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    My experience is the oscillators on the non gate array aren't easy to find and you would have to make one using a crystal.

    I have seen CRTC failures more frequently than the crystal.

  5. Default

    Quote Originally Posted by pski View Post
    The crystals in the Model 4 have nothing special about them. You should get a wave form on the crystal using an oscope. They can go bad, so try replacing it.

    Since you are an admitted noob maybe post some pictures or a video of how you are testing it and we can make sure it's correct.

    I'd also look at the Z80 as I've had a few of these go bad on me.
    Ok. Here is the back of the board. It looks like there are 4 pins that the crystal is soldered to the board with. Only two of them are connected on the back of the board. Here are the pictures of the pin/voltage.
    Attached Images Attached Images

  6. Default

    Quote Originally Posted by pski View Post
    The crystals in the Model 4 have nothing special about them. You should get a wave form on the crystal using an oscope. They can go bad, so try replacing it.

    Since you are an admitted noob maybe post some pictures or a video of how you are testing it and we can make sure it's correct.

    I'd also look at the Z80 as I've had a few of these go bad on me.
    And here is the second pin.
    Attached Images Attached Images

  7. Default

    Looking at this more closely I see that my motherboard is a 8709296 REV C board. It only has one crystal. The only scematics I can find are of REV A which has two crystals. Anyone know where to get a proper schematic of my board?

  8. #8

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    Doesnt look like its outputting anything. Is that the 20.375? Ground should be on the case on the other side of the board.

  9. #9

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    Sean,
    Just as a quick check of your O'Scope, hang your X10 Probe on the calibrate signal of the O'Scope and make sure you are
    getting a trace. You might have to adjust the trigger point to get a reliable sweep.

    Larry

  10. Default

    Quote Originally Posted by ldkraemer View Post
    Sean,
    Just as a quick check of your O'Scope, hang your X10 Probe on the calibrate signal of the O'Scope and make sure you are
    getting a trace. You might have to adjust the trigger point to get a reliable sweep.

    Larry
    The trigger point is set correctly.

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