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Thread: Rev 4 Apple II with weird colour issue

  1. #21
    Join Date
    Dec 2011
    Dallas, TX


    Quote Originally Posted by george View Post
    Scope is needed when you lack of brains in case you need to measure near DC electrical contact. Many of us started when scope was luxury and had to deal without it. By thinking. This reminds me of applefritter forum that some guy despite my advice bought new contemporary scope (the old one was more than perfect for his Apple1). Those kind of thinking is when one is stupid enough that thinks that money would replace good thinking. In our country their is a saying "To usa a cannonball to kill an ant".
    Of course, anybody who randomly probes without thinking gets nowhere. But that person still needs to learn when and how to use the tool.

    Definitely, there is no substitute for thinking about the problem first. But then, once you've started reasoning about the problem, you will generate hypotheses. Getting some concrete data is a great way to focus in on the problem. At first it may seem that probing a circuit is a lot of work, but once you are comfortable with the tool, it will be a huge time saver. The more complex the system, the more true that is.

    I would strongly encourage anybody with even a little interest in electronics to get a scope. I think your friend on Applefritter bought the scope because he liked electronics and wanted to dig deeper. The specific problem, which may have been solvable without a scoep, was probably the trigger. But the ability to see what's happening lets you "divide and conquer" in a way that operating blind does not.

    Also, for analog circuits, there is really no substitute for a scope. You may not even know what to reason about before seeing what the circuit is doing. Digital circuits have, to some degree, a built-in error correction and re-normalizing function at each functional block, so you can think about a digital circuit (within limits) as propagating logic levels. But for an analog circuit, anything can be happening. If you have an oscillation you'll want to know it, and you'll want to see the frequency to start to think about how to compensate, for example.

    One more thing: Having a mentor and watching someone debug a circuit is a huge advantage. That's why, when one of us figures out what went wrong and explains it in detail, it's helpful forever after. Reporting that swapping a chip fixed an issue is helpful, but it's more helpful to say that an output line was stuck high, etc. That way everyone can learn from the process as well as the solution.

    Last edited by dfnr2; October 12th, 2019 at 09:09 AM.

  2. #22


    Quote Originally Posted by falter View Post
    I swapped the original chips back in one by one.. no change. Swapped *all* the original chips back... same thing.. colors stayed good.

    I can only put it down to maybe loose chip in socket?
    When resistance develops between the dissimilar metals of the IC pin and the socket pin, it causes everything from a near open circuit, through to non linear effects (like a rectifier junction) through to some value of resistance. This can cause a delay in the rising and falling edges of a pulse, which is visible on a scope.

    In circuits that generate composite color video, the color is determined by the phase of the subcarrier with respect to the phase of the burst, its very critical timing. So with a poor socket connection the phase of one color signal could in theory be shifted and you could end up with an abnormal color.

    It is important to know what type of sockets you have. The TI ones grab the IC pin from side to side, other types across the flat part of the pin. In addition many IC's have different coating on the pins, some are silver plated, other tin plated, some have a thicker solder like coating.

    Exercising an IC in a socket is only a very temporary fix. The IC's pins needs to be examined under magnification and bright light. It will be noted there is oxidation (& corrosion) and a mark where the pin has been in contact with the socket's claw, it usually presents as a grey to black line. This needs to be completely scraped away, oxides are insulators. I use the back blunt rectangular edge of a number 11 scalpel blade (not the sharp edge), then smooth the surface with 2000 grade paper, folded to make a stiff burnishing tool, if required, and clean with contact cleaner. Whatever system you use the corrosion must be removed for a reliable connection.

    Also it helps to have an IC pin (taken from a defunct IC) and soldered to a stiff wire to make a handle, to insert into every pin on the socket with the addition of some contact cleaner, to feel that the spring tension on every socket pin/claw is normal, some may have been stretched and require repair/replacement. Lubricating the IC pins and sockets can help too. Don't insert any other object than a standard geometry IC pin into a socket or it can stretch the contacts.

    In repairing boards with over 50 TTL IC's on them of mid 1970's vintage, all IC's in sockets, I found that reliability cannot be assured unless every IC pin is attended to and cleaned and every IC socket pin checked, even missing one is asking for trouble.

    Successful repairs nearly always require a scope . I use a 400MHz bandwidth analog scope, but most of the time could get by with a 50MHz bandwidth scope easily. (Low bandwidth digital sampling scopes are fairly "useless toys" and the sampling errors throw in a lot of confusion as seen on other threads)

    The scope is the window into the world of analog and digital circuits. It is not just about on off levels in digital, its all about timing, and without the scope, you can still have all "pulses" present and accounted for, but if the timing relationship is off, just one example if a chip is not enabled before it receives a clock pulse.

    It takes some experience to get the best out of a scope especially with synchronization, and it pays to have a scope with a delay timebase so you can select a pulse edge to view downstream of the sync point. Also, its helpful to have a scope with an H and V sync separator in it when looking at composite video waveforms. (This is why I have suggested in the past the Hitachi V509 as an entry level 50MHz scope with these sync separators & the delay timebase on this forum).

    The reality is for most complex faults in digital or analog circuitry you are flying blind without the scope. Sure some ingenuity can help, when I was a boy I could fix most broken transistor radios with nothing more than a meter, a germanium detector diode and a crystal earpiece. While a scope is not a substitute for a brain, it helps the brain work better an accelerates problem solving.

    A famous quote:

    "the best instrument in the laboratory is always the thing between the ears"

    And that instrument, if its working properly, should know the value and utility of a good scope.


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