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This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

This forum has been around in this format for over 15 years. These rules and guidelines help us maintain a healthy and active community, and we moderate the forum to keep things on track. Please familiarize yourself with these rules and guidelines.


Rule 1: Remain civil and respectful

There are several hundred people who actively participate here. People come from all different backgrounds and will have different ways of seeing things. You will not agree with everything you read here. Back-and-forth discussions are fine but do not cross the line into rude or disrespectful behavior.

Conduct yourself as you would at any other place where people come together in person to discuss their hobby. If you wouldn't say something to somebody in person, then you probably should not be writing it here.

This should be obvious but, just in case: profanity, threats, slurs against any group (sexual, racial, gender, etc.) will not be tolerated.


Rule 2: Stay close to the original topic being discussed
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To put things in engineering terms, we value a high signal to noise ratio. Coming here should not be a waste of time.
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Rule 4: "PM Sent!" messages (or, how to use the Private Message system)

This forum has a private message feature that we want people to use for messages that are not of general interest to other members.

In short, if you are going to reply to a thread and that reply is targeted to a specific individual and not of interest to anybody else (either now or in the future) then send a private message instead.

Here are some obvious examples of when you should not reply to a thread and use the PM system instead:
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Why do we have this policy? Sending a "PM Sent!" type message basically wastes everybody else's time by making them having to scroll past a post in a thread that looks to be updated, when the update is not meaningful. And the person you are sending the PM to will be notified by the forum software that they have a message waiting for them. Look up at the top near the right edge where it says 'Notifications' ... if you have a PM waiting, it will tell you there.

Rule 5: Copyright and other legal issues

We are here to discuss vintage computing, so discussing software, books, and other intellectual property that is on-topic is fine. We don't want people using these forums to discuss or enable copyright violations or other things that are against the law; whether you agree with the law or not is irrelevant. Do not use our resources for something that is legally or morally questionable.

Our discussions here generally fall under "fair use." Telling people how to pirate a software title is an example of something that is not allowable here.


Reporting problematic posts

If you see spam, a wildly off-topic post, or something abusive or illegal please report the thread by clicking on the "Report Post" icon. (It looks like an exclamation point in a triangle and it is available under every post.) This send a notification to all of the moderators, so somebody will see it and deal with it.

If you are unsure you may consider sending a private message to a moderator instead.


New user moderation

New users are directly moderated so that we can weed spammers out early. This means that for your first 10 posts you will have some delay before they are seen. We understand this can be disruptive to the flow of conversation and we try to keep up with our new user moderation duties to avoid undue inconvenience. Please do not make duplicate posts, extra posts to bump your post count, or ask the moderators to expedite this process; 10 moderated posts will go by quickly.

New users also have a smaller personal message inbox limit and are rate limited when sending PMs to other users.


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IBM 5150 Monitor Issue

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    IBM 5150 Monitor Issue

    Recently I stumbled across a IBM 5150. With it came a Magnavox Professional PC Monitor 80 (monochrome). The display is super sharp and great...except for the horizontal line going through the middle that looks like the paint on a road (dashed horizontal line). The PC itself I think works perfectly. I am able to execute basic scripts, boot msdos 3 off either floppy drive (it has a hdd but I think that might be dead as it doesn't spin up). Because of this, I'm assuming this is an issue with the monitor instead of the computer.

    The line itself only appears after the initial blinking cursor and the IBM/DOS logos appear and it always looks exactly the same. I have opened the back of it and looked at the caps and fuses and both at least visibly look fine. A strange thing I noticed is if I touch the "v. lin" dial (see 2nd pic right most) the text on screen very lightly vibrates.

    Anyways, I'm wondering why this horizontal line is appearing and if there is any hope to saving it?

    https://postimg.cc/yJmY4SMR

    https://postimg.cc/FfWCKPXX

    #2
    Refer to your first photo. Notice that the "Ok", compared to the text above it, is spread out vertically. That dashed line is Cassette BASIC's list of function commands (see bottom line of the screen shown at [here]) that is compressed vertically. So there is a vertical deflection problem. From the top of the screen downwards, vertical deflection is starting off okay, then going crazy. If you turn up the brightness, the raster may appear, and if it does, you may see that the raster only occupies the top half of the screen.

    That behaviour is not what I would expect if the 5150's MDA card was putting out a bad vertical sync signal (bad pulse width and/or pulse frequency), but there are others on these forums with more repair experience than me, and they may possibly disagree.

    Originally posted by gbrownie View Post
    A strange thing I noticed is if I touch the "v. lin" dial (see 2nd pic right most) the text on screen very lightly vibrates.
    V. LIN = Vertical linearity. That 'vibrate on touch' symptom may be relevant (or maybe not). Look for a bad solder joint on that V. LIN pot. Try adjusting the pot slightly.

    As a comparison, the vertical stage of the IBM 5151 is shown at [here]. Just a dedicated 'vertical deflection' chip and some supporting circuitry. Your monitor may be very similar. Sometimes the vertical deflection chip fails (fully or partially). But considering the age of the monitor, bad solder joints and bad electrolytic capacitors are, in my opinion, just as likely.

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      #3
      As noted by Modem7 this is a vertical scan amplitude & linearity problem.

      From first principles. the scanning current should be in increasing in a sawtooth linear manner, from top to the bottom of the screen. Clearly the vertical yoke current is inadequate and the scan waveform driving the V yoke coils is defective.

      The common cause would be the V yoke coil's coupling electrolytic cap has gone low uF and high ESR. The power supply voltage could be sagging down to the V deflection IC (or vert deflection circuit). The V deflection IC could be faulty, or a capacitor/resistor supporting it is defective.

      This sort of fault is dead easy to sort out with a scope, checking the power supply rail first and the voltages in the V deflection system. So there is plenty of hope for your monitor.

      Comment


        #4
        This is why I keep coming back to this forum. The amount of learning you can do here is incredible.

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