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Thread: Commodore 1702 Brightness Problem

  1. #1

    Default Commodore 1702 Brightness Problem

    Hello guys, need some pointers...I have a 1702 that got from ebay...Restore the unit and change all the Capacitors...The unit work, front and back connections but when it get warm the screen brightness go dark. I have to move the brightness to 100%. If I turn the unit off and let it cool down... I have to put the brightness back to 50%...

    Any Idea what can it be...any pointers...sound light a Potentiometer or something in the brightness control board...any clue or pointers?

  2. #2
    Join Date
    Dec 2012
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    São Paulo country, Brazil
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    Quote Originally Posted by megaimg View Post
    Hello guys, need some pointers...I have a 1702 that got from ebay...Restore the unit and change all the Capacitors...The unit work, front and back connections but when it get warm the screen brightness go dark. I have to move the brightness to 100%. If I turn the unit off and let it cool down... I have to put the brightness back to 50%...

    Any Idea what can it be...any pointers...sound light a Potentiometer or something in the brightness control board...any clue or pointers?
    I have no clues to offer, but I can share a similar experience I had with a PET: the screen became to dim slowly before some time. It was a capacitor in the high voltage section of the CRT board.

    You told you changed all the capacitors... why did you do that? Were they all bad? In my experience, many old caps are better than new, cheap chinese ones. Maybe you replaced a good cap with a bad one, even if it's new...

  3. #3
    Join Date
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    The Service Manual for the 1701/1702 can be found here: https://gona.mactar.hu/Commodore/mon...(1984_Oct).pdf

    The manual contains the schematic diagrams. For example, see page 13 where the brightness and sub-brightness controls are depicted (not sure whether this is for the 1701 or 1702 or they are common).

    It is likely that a component is 'failing' - or changing in value - as things heat up. This may be because the component itself is faulty or (more likely) something else is drawing too much current and whatever is 'shutting down' is doing it to protect the monitor.

    What we have to do first - is to identify the voltage that is changing as things warm up...

    Questions first: can you follow schematics? What test equipment do you have access to (e.g. a multimeter) and do you know how to use it?

    As you are a new member - your posts will be moderated for a while. Hang in there!

    Dave

  4. #4
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    Saint Petersburg, FL
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    Quote Originally Posted by daver2 View Post
    The Service Manual for the 1701/1702 can be found here: https://gona.mactar.hu/Commodore/mon...(1984_Oct).pdf

    The manual contains the schematic diagrams. For example, see page 13 where the brightness and sub-brightness controls are depicted (not sure whether this is for the 1701 or 1702 or they are common).

    It is likely that a component is 'failing' - or changing in value - as things heat up. This may be because the component itself is faulty or (more likely) something else is drawing too much current and whatever is 'shutting down' is doing it to protect the monitor.

    What we have to do first - is to identify the voltage that is changing as things warm up...

    Questions first: can you follow schematics? What test equipment do you have access to (e.g. a multimeter) and do you know how to use it?

    As you are a new member - your posts will be moderated for a while. Hang in there!

    Dave
    I would suspect a bad capacitor. Get some keyboard cleaner (held upside down) or freeze spray. Turn off monitor and spray one small area around a capacitor. turn back on and compare. repeat until you see a change. Avoid going near the suction cup and wire on the back of the crt. deadly voltage there.

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