NTSC Video Issue After C64R MK2 is Powered On For Extended Time

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  • Jens, is there any update to this issue? I purchased MK2 a while back, but had the same issue and ended up selling it to someone that used it for PAL exclusively. I would like to order the board again, but need NTSC working properly...

    So far, nobody in the US has tried this. What I can do is to mod your board if you order a new one - would be great if you report back if that fixes it with the chips and PSU that you've used before. Please send me an eMail with the order ID, so I can add a note to your order. While this mod is not confirmed, I won't just ship out all boards with it.

  • So I've carefully moved the center frequency of the VCO circuit by reducing a resistor value. The resistor on question is R32, near the PLL chip U23. If you want to try this modification, you will NOT need to remove this resistor, but merely add a 100k resistor in parallel. Putting that high value in parallel will reduce the overall resistance from 3.3k to about 3.2k, and this will result in the VCO control voltage to drop to 3.2V, which gives more headroom for regulation and component tolerances.

    Would you be able to provide the resistor values of R31 and R32? I don't have very precise soldering tools and would rather fully replace the resistors. I'm including R31 because I already accidentally got some solder on it, so replacing it would be safer than risking a sloppy job.

  • As mentioned in the text you've quoted, R32 is about 3.2k after modification. However, this value is hardly available without buying a whole spool of these, as it's only available in 0.5% precision. You might want to play with 3.16k or 3.24k.

    The value of R31 is 27k.

  • Thanks, I'm currently running a stress test and will let you know how it goes. R32 is 3.2k. R31 is 27.17k, since that was the closest I could conveniently get with the basic tools I have on hand. Would you predict that to have any side effects that would invalidate this test?

  • Okay, it's been up for about 24 hours now and the display is still solid. I think it would previously start to wobble around this point. If it's still solid in 24 more hours, I think we can call this a confirmed fix.

  • Ah, I'm about 42 hours in and it looks like the video got kicked slightly out of sync. It must have happened quite suddenly, since I only looked away for about an hour before it happened. The whole screen is shifted about 10 pixels to the right, and all the scanlines are wobbling.

    Very strange, since previously the image would gradually get worse and worse, but this one seems to have happened immediately.

    I guess the best thing to do is to restart the test and see if I get the same result in the same amount of time, just to rule out something like bad soldering (which for me is a strong possibility). I'll report back with my findings.

    For anyone just discovering this thread, my current resistor values are: R32 at 3.2k, R31 at 27.17k. (R31 does not need to be modified from its original value of 27k.)

  • Just finished my second test run - the video degraded again the exact same way about 40 hours into the test. Is there any more information you'd like me to share?

    I would consider this a fix, at least for me. I would never spend 40+ hours straight on any computer platform.

    Even with the original resistor values, I'd only start to see the video signal degrading after around 24 hours of power-on. I think the issue is most pertinent to people who use the C64 for unattended tasks that require it to be always on, such as running a server or interfacing with other equipment.

  • The good news is that we're seeing an improvement. So please continue to test, this time with a slightly smaller value for R32, maybe 3.16k for this run. If you can, please try to locate the 27k/1% resistor for R31, so we'll have a result for prople who only want to make a single change.

  • I'd say "just try it" - we did see an improvement, so the worst thing that can happen is that the improvement is not as good as you want it to be - then we'll need to work on it some more.