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TRS-80 Model I horizontal video instability

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    TRS-80 Model I horizontal video instability

    Instead of doing a "jumbo" everything thread maybe it makes more sense to do problem specific ones so they might be easier to find later. So:

    I have a TRS-80 Model I that has an interesting issue with the video output. Upon being initially powered up from cold it displays clear text output, but within a few seconds it starts looking a little "jumpy". This lasts for about 45 seconds with subtle worsening of the stability until suddenly it all goes to heck and the monitor comes close to losing horizontal sync. Here is a video captured through a composite cable, the symptoms apply regardless of monitor, including an original Radio Shack one.



    The waving subtly changes amplitude as the machine warms up, so after 10 minutes or so it becomes "almost possible" to read the screen again (with the letters rapidly alternating between forward and backwards italics) but sync never really locks in again. The characters seem properly formed so it doesn't appear to be a major issue with the divider chain. In the tech manual "Z6" is described as the source of the horizontal sync pulse, would a partial failure of that IC cause these symptoms?
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

    #2
    You could drag out your O'Scope and a can of Freeze Mist, then chase the Vertical and Horizontal Sync pulses to the combined SYNC Signal
    on Pin 8 of Z5. Shouldn't take long to see what is going on. You can also use a small heat source to raise the temperature on the suspect
    part. A small heat shrink gun is a good heat source.

    Larry

    Comment


      #3
      After spending some time stepping through the hsync circuit description in the manual I think I have ideas where I need to clamp on the oscilloscope probes, but if someone has a second I could use a quick confirmation about the parts I should have on hand when I sit down to work on it. Just double-checking: this circuit appears to rely on both the switching speed and (possibly) the threshold voltage of the 74C04 (and maybe the 74c00?) components? I’m guessing that subbing HC or HCT isn’t going to fly here.
      My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

      Comment


        #4
        You could purchase a few parts such as:
        R43 or R44 10K
        R20 or R21 100K Pot
        C26 C27 Vertical circuit
        C20 C21 Horizontal circuit
        MM74C04N and MM74C00N

        and have them laying in your parts box for the next xx years, or find the culprit and just order exactly what is needed.
        I typically just order what I need after chasing the signals. Don't overlook cold solder joints, as well as R43 & R44.

        I don't remember what I changed on the last Model 1 I repaired.

        Larry

        Comment


          #5
          Well, I've dug into the machine and no luck so far narrowing it down without shotgunning. The power supply voltages for both +12v and +5v were on the low side (4.8v for 5v) so I adjusted that, and I also replaced the big 10,000 and 2200 input capacitors just because they looked a little sketchy, but no improvement. (If anything it made things "worse" in the sense that the hsync is always wavy even from a cold power-up. Or maybe that's better because it's consistent?) The delay circuit of R20 and C20 seems to be working correctly so far as I can tell.

          R43 reads around 3.5k instead of its marked 10K. If I'm understanding the manual correctly it and C21 are responsible for controlling the length of the Hsync pulse so that's certainly fishy. Tomorrow I'll drag out the oscilloscope and try using the HDRIVE pulse as a trigger to time if the HSYNC pulse is coming out wildly different lengths.

          (I'm also thinking I should check HSYNC relative to VSYNC and make sure the counter chain is consistent about spacing the HDRIVE signal.)

          My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

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