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Thread: PET 2001-32N - Diag works in UD9, but with Kernal it fails to start

  1. #91

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    Quote Originally Posted by dhoelzer View Post
    Interesting. First, after probing pin 3 on UG5 (8Mhz clock coming out of the counter), the system now boots directly into the monitor, which is an unexpected change. Second, the 1mhz output and the 16mhz input are very clean, but the 8mhz output on pin 3 is not regular, and will not measure at 8mhz... and simply probing the pin drops out the video signal. Here's the trace:
    Attachment 56703
    I'll continue around the rest of the clocks.
    I do wish you guys would learn how to use a digital sampling scope. The trash you see on the screen is under sampling a high frequency signal. You can't look at a 8 MHz signal with a 100 Ms/div sweep. If you're going to use one of these digital sampling scopes, at least learn what it does.
    I agree, it is likely right on the edge of failing if it changes, just touching the scope probe. Although, are you using a 1X or a 10X probe? A 1X has a lot of capacitive load.
    Dwight

  2. #92

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    Most 1X probe are can be around 60 pf. Hitting a 8 MHz line is like slapping it with a 330 ohm resistor. Also remember that a square wave is composed of harmonics. The impedance for them is smaller as well.
    Dwight

  3. #93

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    Quote Originally Posted by dhoelzer View Post
    I think you're right, but I also think there's something a bit up since simply probing it is enough to complete disrupt the display.
    how are the actual chips mounted on UG5 and UE11 marked exactly? (I'm interested if any of them is a 74LS or plain 74). Also is your probe a X1 or X10 kind? I agree with Dwight, digital sampling scopes aren't going to be an easy tool for these kind of troubleshooting, but you seem to see correctly a 16 MHz square wave (don't you?) so the 8 MHz one should be easier.
    Measure the actual resistance between UG5 Pin 3 and UE11 pin 2 also.
    Frank

  4. #94

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    When you see this kind of signal on a digital scope, look at the sweep rate. If you are using a auto sweep detect, turn it off and set the rate to an appropriate sweep for the frequency you intend to look at. An 8 MHZ signal should be 0.5 us or 0.1 us ( 100 ns ) or so.
    Dwight

  5. #95

    Default

    Quote Originally Posted by iz8dwf View Post
    how are the actual chips mounted on UG5 and UE11 marked exactly? (I'm interested if any of them is a 74LS or plain 74). Also is your probe a X1 or X10 kind? I agree with Dwight, digital sampling scopes aren't going to be an easy tool for these kind of troubleshooting, but you seem to see correctly a 16 MHz square wave (don't you?) so the 8 MHz one should be easier.
    Measure the actual resistance between UG5 Pin 3 and UE11 pin 2 also.
    Frank
    DM74191N for UG5
    SN74LS165N for UE11

    The resistance between pin 3 and pin 2 is 0.2 ohms, effectively a short.

    Regarding Dwight's response, he clearly did not read the time divisions correctly. That's 100ns, which I zoomed in to make it clear what a single sweep looked like, not 100ms.

    However, I was incorrectly using X1 rather than X10. At X10 I do see an 8MHz signal. That was my error.

  6. #96

    Default

    Quote Originally Posted by Dwight Elvey View Post
    I do wish you guys would learn how to use a digital sampling scope. The trash you see on the screen is under sampling a high frequency signal. You can't look at a 8 MHz signal with a 100 Ms/div sweep. If you're going to use one of these digital sampling scopes, at least learn what it does.
    I agree, it is likely right on the edge of failing if it changes, just touching the scope probe. Although, are you using a 1X or a 10X probe? A 1X has a lot of capacitive load.
    Dwight
    Hey Dwight... I do appreciate your response. I think you have misread the image that I posted. Believe it or not, even I can work out that 100ms is never, ever going to work on an 8MHz signal. That image is, in fact, at 100 ns/div, not ms/div. I think your eyes simply couldn't differentiate the N and the M. I had zoomed it to 100ns to make it clearer what one period was looking like. I didn't realize the impact of being at X1 on a high frequency signal. Switching to X10 properly resolves an 8MHz signal.
    Last edited by dhoelzer; October 16th, 2019 at 08:41 AM.

  7. #97

    Default A small revision

    All along, I hadn't realized I was at X1 rather than X10, so some minor revision is in order.

    Measuring at X10, I no longer see evidence of the craziness feeding back to UG5 and the 8MHz signal, which seems good.

    I do, however, see that same signal coming and going on the 60hz signal on pin 5 of U11. It does not appear to be present on any of the other various 60hz signals coming out of that inverter.

    Awaiting other suggestions, I'm going to continue making my way around to see if I can identify the origins of that signal. At this point, everything has been thoroughly q-tip scrubbed with alcohol.

  8. #98

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    And another weirdness??? Maybe? Maybe this is a clue?

    The system still always drops to TIM now when the BASIC ROMs are switched in... but I have had it running the standard pettest image for the last ten minutes and have *NOT* noticed the spurious signal showing up on pin 17 of the PIA! When I switch to BASIC, it comes back sporadically... So, that seems to imply it has something to do with something that's getting programmed..? Yes? No?

    I'll let it run for a few hours and come back and watch the scope from time to time.

  9. #99

    Default

    Quote Originally Posted by dhoelzer View Post
    DM74191N for UG5
    SN74LS165N for UE11

    The resistance between pin 3 and pin 2 is 0.2 ohms, effectively a short.

    Regarding Dwight's response, he clearly did not read the time divisions correctly. That's 100ns, which I zoomed in to make it clear what a single sweep looked like, not 100ms.

    However, I was incorrectly using X1 rather than X10. At X10 I do see an 8MHz signal. That was my error.
    On my chromebook it looks like a 'm' but on a 21 inch screen it looks like an 'n'.
    It looks like a typical under sampled shot but maybe it was just having troubles syncing. Anytime your looking at such a signal on a digital scope, of a digital signal, it most likely means you are seeing either an under sampled signal or it is not able to synchronize the signal.
    On seeing 60Hz on an output, it is possible it is right on the edge of failing because of load. In such cases, the output can amplify small ripple on the supply lines.
    If you don't find a source for the signal, try helping the output. You can add a reasonable resistor to see if it improves to have less ripple.
    If it helps, suspect one of the loads.
    I'd think a 1K should show something.
    Dwight

  10. #100

    Default

    Quote Originally Posted by dhoelzer View Post
    All along, I hadn't realized I was at X1 rather than X10, so some minor revision is in order.

    Measuring at X10, I no longer see evidence of the craziness feeding back to UG5 and the 8MHz signal, which seems good.

    I do, however, see that same signal coming and going on the 60hz signal on pin 5 of U11. It does not appear to be present on any of the other various 60hz signals coming out of that inverter.

    Awaiting other suggestions, I'm going to continue making my way around to see if I can identify the origins of that signal. At this point, everything has been thoroughly q-tip scrubbed with alcohol.
    Do you mean UI1 (not U11)? Pin 5 is driven by pin 11 of UH10, check page 6 of the 2001N schematic and look for VIDEO ON signal.
    Again if that signal was problematic, you wouldn't see anything on screen probably.
    Frank

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