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Thread: Looking for help with a TRS-80 Model 1

  1. #1

    Default Looking for help with a TRS-80 Model 1

    I picked up my first TRS-80 model 1. I plugged it in and got a screen full of characters.

    Opening it up I discovered there were (6) wires soldered to the system board and run to a small separate PCB. The problem is 4 of them had separated from the PCB and when I dared to touch it, the other 2 disconnected as well.

    My research seems to indicate this may be level II ROM upgrade but all the info and pics I have found about the upgrade show only (4) connections as seen in the pic below -
    trs80-1.jpg


    Here is what mine looks like - it's hard to see the black wire but there are (6) total.
    20181010_144917.jpg

    I have been unable to find a pic or schematic showing which wires go to which PCB solder points.

    Any help would be greatly appreciated.

    Rick

  2. #2
    Join Date
    Jun 2015
    Location
    Sioux Falls SD
    Posts
    179

    Default

    I have a Model I CPU open, and it only has 4 wires from the satellite board to the solder side of the CPU board. But I have a ROM set I bought recently as NOS, and it has six wires as you described. The Technical Reference manual shows only four wires.

    Do you need info to connect to the ROM board or the CPU board? Or both?
    Last edited by lafos; October 10th, 2018 at 07:50 PM. Reason: clarification

  3. #3

    Default

    Quote Originally Posted by lafos View Post
    I have a Model I CPU open, and it only has 4 wires from the satellite board to the solder side of the CPU board. But I have a ROM set I bought recently as NOS, and it has six wires as you described. The Technical Reference manual shows only four wires.

    Do you need info to connect to the ROM board or the CPU board? Or both?
    So maybe that's what the small board is.

    The connections on the main system board seem ok. I just need to know where to connect the other ends to the small satellite board.

    Thank you.

    Rick

  4. #4

    Default

    The second edition of the technical reference handbook has schematics for three different daughterboard versions: 4-conductor 3-ROM, 6-conductor 3-ROM, and the final 6-conductor 2-ROM board that you have. See page 109.
    -Alan

  5. #5

    Default

    Quote Originally Posted by Petrofsky View Post
    The second edition of the technical reference handbook has schematics for three different daughterboard versions: 4-conductor 3-ROM, 6-conductor 3-ROM, and the final 6-conductor 2-ROM board that you have. See page 109.
    Thanks Alan. With your info, now I know what the small PCB is. Now I just need to know how to determine where each of the (6) wires need to connect.

  6. #6

    Default

    You'll need to follow the circuit traces from the 6 pads to the other components on the daughter board to figure out which pad is which. They're probably in the same order as on the schematic (GND, A11, A13, ROM*, +5V, A12), so all you really need to determine is whether the pad closest to the corner is GND or A12 (i.e., does it have a trace connecting to pin 12 of each ROM and a couple other places (in which case it's GND) or does it connect to pin 21 of Z1 (in which case it's A12)), but you'd be wise to check the other 5 as well.

    Then you need to figure out which color wire is carrying which signal from the main board. Do that by looking at where the wire connects to on the board (e.g., pin 5 of Z38) and then find that on the main schematic (pages 111-115) and see what signal is there (e.g., A12).
    -Alan

  7. #7
    Join Date
    Jun 2015
    Location
    Sioux Falls SD
    Posts
    179

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    I'm tied up through Sunday, but if it will help, I can check the wiring on the one I have then.

  8. #8

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    Quote Originally Posted by lafos View Post
    I'm tied up through Sunday, but if it will help, I can check the wiring on the one I have then.
    That would be quite helpful. Thank you.

  9. #9
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
    Posts
    962

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    When these satellite boards give me problems I normally replace them with a single 27128 Eprom with level II basic on it and get rid of all the clutter related to the original setup.

    Having said that, I have many of these satellite boards lying around... So the wires should go:

    Red Orange Green Yellow (L-R on the 4 pads under the 74LS42)

    Black and White (which are GND and power, respectively) L-R on the 2 pads above the 74LS42.

    Hope this helps,

    Ian.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  10. #10

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    Ian, I think you’re thinking of the 6-wire 3-ROM board, but Rick needs someone to tell him the wire color order for the 6-wire 2-ROM board, which, as his photo shows, has the six pads all in an equispaced row.

    From what you report, and looking at the two schematics, I’m guessing the answer is Black, Red, Green, Yellow, White, Orange, but I wouldn’t bet on it.
    -Alan

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