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Fixing TRS-80 Model III Computer

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  • Al Hartman
    I bought a TRS-80 Model III from the M.A.R.C.H. Swapmeet earlier this month. It started up to a garbage screen. While there, we popped it open, and I pushed
    Message 1 of 5 , Aug 16, 2013
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      I bought a TRS-80 Model III from the M.A.R.C.H. Swapmeet earlier this month. It started up to a garbage screen.
       
      While there, we popped it open, and I pushed down on the the socketed chips. No Change.
       
      I decided to order some replacement RAM so I could try replacing the RAM. It arrived yesterday.
       
      Today, I lifted the Model III up onto my kitchen table (nearly dropping it), and got the top cover off without incident. I disconnected the display and put it to the side. I removed the rear RF shield to gain access to the logic board.
       
      I removed the first row of 4116 RAM chips (16k x 1 DRAM). As I removed them, I noticed the pins of each chip had gone black with oxidation. I may not have bad ram, simply bad connections between the RAM and the socket.
       
      I’m going to clean the pins with a pencil eraser and reinsert them.
       
      But... In testing at the swapmeet, we unplugged the floppy and serial port interconnect cables. They are so old, the backing has dried out, and they cannot be reused easily. I either have to source a pair of replacements (anyone know of any where to get these?), or solder ribbon cable in place of them.
       
      Until that happens, I have no reliable way of testing the RAM since all my test software is on floppy. Anyone know or have a Memory test program that could be recorded to tape and loaded in that way?
       
      Al
    • Al Hartman
      After removing all the RAM, cleaning the oxidation off the pins, and replacing one chip with a broken pin. The Model III now boots into BASIC. The flat
      Message 2 of 5 , Aug 16, 2013
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        After removing all the RAM, cleaning the oxidation off the pins, and replacing one chip with a broken pin. The Model III now boots into BASIC.
         
        The flat interconnect cables from the serial card and floppy controller (such a poor design) are beyond salvaging.
         
        So, the task for another day is to either hard wire the boards together with wire, or install headers that use a connector and cable that can substitute.
         
        Until then, I can’t test the RAM.
         
        But, this was a good outcome for the day.
         
        I think next is to reassemble my C-64 and drill the hole for the JiffyDOS switch, and then add JiffyDOS to my 1541 Drives.
         
        Al
      • Dave McGuire
        ... Excellent! Congrats! -Dave -- Dave McGuire, AK4HZ New Kensington, PA
        Message 3 of 5 , Aug 18, 2013
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          On 08/16/2013 10:00 PM, Al Hartman wrote:
          > After removing all the RAM, cleaning the oxidation off the pins, and
          > replacing one chip with a broken pin. The Model III now boots into BASIC.

          Excellent! Congrats!

          -Dave

          --
          Dave McGuire, AK4HZ
          New Kensington, PA
        • Al Hartman
          After consulting with a few people on the Vintage Computer list, I decided I m going to remove the connectors for the flat cable, and install 4 single row pin
          Message 4 of 5 , Aug 18, 2013
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            After consulting with a few people on the Vintage Computer list, I decided
            I'm going to remove the connectors for the flat cable, and install 4 single
            row pin headers and use two cut down IDE cables (using only one row of the
            female on each end to replace them.

            After beating my head against the wall at Mouser and Digi-Key, and finding
            another place that had them had a $14.00 minimum order (I just placed an
            order with them last week.) I thought to check eBay, and got 10 pcs of a 40
            pin breakaway header for $3.89 with shipping. So, I'm in business.

            Someone sent me a RAM test program, so I'm set there (though I have Super
            Utility which also has one). I need to repair the door on the upper drive,
            and assuming the RAM and Serial board both work, the Model III will be on
            it's feet.

            In parallel, I'll get my C64c reassembled, and tackle the JiffyDOS ROMs in
            my 1541 drives.

            After that, back to getting a working E/I for my "new" Model I. And then,
            getting my original Model I working.

            Al
          • DougCrawford
            Yes! Nice going.
            Message 5 of 5 , Aug 18, 2013
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              Yes! Nice going.

              --- In midatlanticretro@yahoogroups.com, Dave McGuire <Mcguire@...> wrote:
              >
              > On 08/16/2013 10:00 PM, Al Hartman wrote:
              > > After removing all the RAM, cleaning the oxidation off the pins, and
              > > replacing one chip with a broken pin. The Model III now boots into BASIC.
              >
              > Excellent! Congrats!
              >
              > -Dave
              >
              > --
              > Dave McGuire, AK4HZ
              > New Kensington, PA
              >
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