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Thread: PET 3032 garbage screen problem

  1. #51

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    Those all look OK. I'm thinking that something is failing getting from the mux to the RAMs or I got the wrong address line.
    can you check SA4 all the way to each RAM? and make sure it looks the same as from the output of the MUX.
    Dwight

  2. #52

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    I hope you ran a wire from the external trigger to either Pin13 or to pin14. Also since a wire is 1X you should set the trigger level to 2V.
    I see you showed a clock on the other channel but I really wanted the input on one channel and output on the other. Also, the display should be set for what is normally called chopped and not alternate( I can't tell which you were using ). The displays towards the end shows almost nothing of use. These are just smudges on the screen.
    The fixed captures are more useful than the videos.
    Dwight

  3. #53

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    I thought I'd tell you what I was looking for in the signals. There are two times relative to the clock signal that are interesting one is when the clock is high and the other when the clock is low. Because of the fault seen on the screen, I was looking for a bad signal on the output for one of those two times. What I saw in the video was that I saw solid looking ones and zeros, on the bottom trace, for both levels of the clock.
    From a possible failure of the mux, I wasn't expecting the levels to look so clean I saw solid 1's and 0's for both clock high and low. There was one point that one could see a small glitch where there was a small spike, instead of a clean level. This is normal because thing don't exactly line up in time for real world electronics. ICs all have some delay.
    But any away, I didn't see what I was expecting to see at the output of the mux on the SA4 signal, if the mux was at fault.
    I'm now suspecting a possible fault someplace else. As I recall on the PET's, the RAMs were originally soldered in. It is possible that the trace bringing the SA4 signal to each socket may be broken someplace. Which is why I wish you to check the SA4 signal at each RAM location.
    As for the wire used for the external trigger. A 1X probe is basically a piece of wire with a nice handle to hold it and a ground connection in a handy place. For the purpose we are using, a piece of wire works fine.
    For a 10X probe, it is a little more complicated. There are two things in the probe. One is a resistor to make a resistor divider between the probe and the input impedance of the scope. The probe drops 9 parts of the voltage while the scope drops 1 part of the voltage. For a typical scope with a 1 meg input resistor, the probe will have a 9 meg ohm resistor. You will notice that notice that I said impedance. Most scopes have a small amount of effective capacitance in parallel with the input connector. It is kept to a minimum but still there from the connector, the wire and the amplifier input on the scope. This is what the input compensation capacitor deals with. One wants a capacitor in parallel with the 9 meg ohm resistor that matches the 9:1 ratio of the resistor, except this is to match the capacitor of the scope input. Since this is never an exact size, there is a small adjustment on the probe to compensate this. It effects the high frequency action of the scope. This is why you look at the edges of a square wave to adjust it. Too much high frequency, bypassing the 9 meg resistor, and it makes a spike. Too little bypassing and it makes a rounded ramp. When just right it makes a square edge.
    You'll notice that in the actual circuit the digital signal is not perfectly square. This is because in TTL they pull down hard but the pull up isn't as strong. This is because that the circuit inside the TTL part uses NPN transistors for every thing. With this, it is easy to make a strong pull down but not a strong pull up.
    OK, I guess that is enough lessons for today, back to the failed PET.
    Dwight

  4. #54
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    Dwight,
    Good post!
    I was hoping for a bad RAM address MUX too, but Fran's clear scope videos shows these signals looked at are good. If the problem is a broken trace, Fran will need patience and a lot of probing to find it. A tough problem.

  5. #55

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    Hi Fran
    I mentioned Chopped/Alternate modes for the inputs. I realize that a digital scope is always in what we'd call chopped mode when both displays are seen. If not, it would not make any sense. I looked at the manual online and saw no reference to Chopped/Alternate. You'll have to excuse me, I rarely use a digital scope and mainly use an analog scope.
    Both have their issues. In another post the OP was using a digital scope to do head alignment. It is generally an easy task for a analog scope. The scope showed the disadvantage of a sampling scope in the form of serious aliasing, making the scope useless for that purpose.
    Sill, a digital scope can easily hold both early ( before trigger ) and after data. It can be done in an analog scope but is a little more complicated and expensive to do. The be able to do this is useful when tracking down cause and effect, in computers. Say you have a subroutine that is causing an issue. You can trigger on that issue then look back in time and find what part of the program called the specific event that cause the problem. This is difficult to do with an analog scope and easy to do with a digital scope.
    In any case, don't spend any time trying to figure out what Chopped/Alternate means. It doesn't have much meaning for a digital scope.
    Dwight

  6. #56

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    Quote Originally Posted by dave_m View Post
    Dwight,
    Good post!
    I was hoping for a bad RAM address MUX too, but Fran's clear scope videos shows these signals looked at are good. If the problem is a broken trace, Fran will need patience and a lot of probing to find it. A tough problem.
    Yes but once located, a simple jumper wire can correct it.
    I'm suspecting that the trace was damaged while unsoldering RAM and installing sockets. If the wire is good then we may have a more serious timing issue where there is a critical timing delay. Things would be so much easier if I were physically there but that is the challenge of debugging over the net, remotely. and across time zones. I'm not sure but I think a shorted across two addresses could also do this but I need to think a little more first. That is why I wanted the reference channel on the mux input rather than the clock.
    Dwight

    Dwight

  7. #57

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    Quote Originally Posted by Dwight Elvey View Post
    Those all look OK. I'm thinking that something is failing getting from the mux to the RAMs or I got the wrong address line.
    can you check SA4 all the way to each RAM? and make sure it looks the same as from the output of the MUX.
    Dwight
    Hi Dwight
    Yes, the values are the same

    CH1 F5 pin-12 and CH2 F7 pin-3, positive and negative

    20190916_210345.jpg 20190916_210535.jpg 20190916_210751.jpg 20190916_211756.jpg 20190916_211918.jpg 20190916_212250.jpg

    CH1 F5 pin-12 and CH2 F8 pin-3, positive and negative

    20190916_213303.jpg 20190916_213422.jpg 20190916_221049.jpg 20190916_221733.jpg 20190916_221923.jpg

  8. #58

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    Thank you very much for the explanations, I am amazed at your knowledge and light years away from you.

  9. #59

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    Quote Originally Posted by Dwight Elvey View Post
    Hi Fran
    I mentioned Chopped/Alternate modes for the inputs. I realize that a digital scope is always in what we'd call chopped mode when both displays are seen. If not, it would not make any sense. I looked at the manual online and saw no reference to Chopped/Alternate
    Dwight
    It is correct, my scope does not have chopped / alternate modes for inputs.

  10. #60

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    Quote Originally Posted by Dwight Elvey View Post
    Yes but once located, a simple jumper wire can correct it.
    I'm suspecting that the trace was damaged while unsoldering RAM and installing sockets. If the wire is good then we may have a more serious timing issue where there is a critical timing delay. Things would be so much easier if I were physically there but that is the challenge of debugging over the net, remotely. and across time zones. I'm not sure but I think a shorted across two addresses could also do this but I need to think a little more first. That is why I wanted the reference channel on the mux input rather than the clock.
    Dwight

    Dwight
    I have not desoldered the video ram and I have not installed sockets.

    20190916_230122.jpg

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