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Thread: Mac 512 brightness control

  1. #1
    Join Date
    Aug 2013
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    Larose, LA, USA
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    Default Mac 512 brightness control

    Howdy.

    I just finished recapping my 512 and due to years of bad storage the brightness control pot (R57, 1M Ohm) has failed internally.

    It's got really long legs that look like they've been bent from a horizontal mount to vertical and the support bracket on the back added as an afterthought.

    Does anybody know if it was a specific Apple part or has anyone found a suitable replacement? I don't mind a bit of homebrew hacking to assemble something that fits.

    Thanks


    Phil

  2. #2

    Default

    I don't think Apple's analog board parts are even manufactured by Apple. Only some of the chips on the motherboard.

    On my old Apple II monitor, there are 100KOhm control pots for brightness, contrast, and screen height. I was able to replace them with newer versions of the same type, with some light modification. (i.e. I had to bend the legs a certain way to fit them into their places on the board) The dial stems don't look the same, but they work well enough.

    You should be able to replace the pot with a generic version of the same type of part. That is, if you are able to find a way to remove it from the bracket. (I wouldn't know, because I only opened my 512k to relubricate the mechanical parts of the floppy drive.)

  3. #3
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    Default

    Upon working on it, looks to be moderately standard kit all round. I've seen a couple likely candidates on Mouser, I'll get them ordered and see if I can't get this all back together again soon.

    Phil

  4. #4
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    Default

    Quote Originally Posted by T-Squared View Post
    I don't think Apple's analog board parts are even manufactured by Apple.
    Besides the early Apple I, Apple has never manufactured any of their hardware in-house, it's always been done by a third party. Their current go-to is Foxconn because Foxconn makes everything.

    Quote Originally Posted by PhilipA View Post
    Upon working on it, looks to be moderately standard kit all round. I've seen a couple likely candidates on Mouser, I'll get them ordered and see if I can't get this all back together again soon.

    Phil
    Make sure you get the exact same part, or an equivalent replacement. Even if you know what the ohms range of the pot is, there are logarithmic and linear types that behave very differently from each other.

  5. #5
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    Default

    Looks to be logarithmic, looking at the innards.

    20171024_204010.jpg

    No taper to the track to make it linear.


    Phil

  6. #6
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    Apr 2015
    Location
    Austin, Texas
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    Default

    You got it backwards.

    A logarithmic pot has a taper, either the width of the track, or the thickness of the material.

    A linear pot has a uniform track all around, which seems to be what yours it. Though with the significant wear pattern on that pot, it probably acts more like a log pot now.

  7. #7
    Join Date
    Aug 2013
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    Larose, LA, USA
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    Default

    You're right, I was thinking wrong just after I woke up. Found a likely candidate on DigiKey so I think I'll get it ordered- 1M linear with long mounting lugs.

    https://www.digikey.com/product-deta...395-ND/2620664

    It's going to require a bit of modification but I can deal with that. It's the board and the wiper I need the most out of it.

    --Phil

  8. #8
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    Default

    Well, this appears to have been successful, reading the numbers on the multimeter.

    Ordered the pot that I linked above. The board, overall is the same size with some minor differences.

    20171030_121018.jpg

    Pulled apart carefully with a set of cutters to get the right leverage:

    20171030_121054.jpg

    The build style is slightly different. Cut away the crimped section in the center to allow it to be split into its component pieces:

    20171030_121242.jpg

    My fingers were covered in grease so no photos here but the two differences were the mounting ears on this had 2, the original 1; the gap they sat in was identical so I used a nail file to remove the new ears. The center hole is a larger diameter and the center sweeper on the old shaft was slightly misaligned, so that was bent inward slightly to make contact with the center ring (on this one metal so better than the older carbon style one).

    Crimped it all back together again, tested good with my multimeter throughout the range.

    20171030_123450.jpg

    I'll solder that back in tonight and see how it works.

    Phil

  9. #9
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    Default

    20171030_163601.jpg

    Well worth doing. Brightness minimum set, 5 volts tweaked to 5.00, left 12 at 12.2 so that's good.

    Brightness now adjusts smoothly from very dim to very bright.

    Phil

  10. #10
    Join Date
    Aug 2013
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    Larose, LA, USA
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    Default

    Well, that was short-lived. Periodically, randomly, I'll lose image on the screen (it was too light to see if there was raster still) but giving the case a gentle slap brings the image back. Wiggling the brightness control has no effect.

    I'm guessing I've made a dry joint somewhere on the board. Flyback connections perhaps? I'm going to take it to bits again and reflow all the usual culprits.

    Phil

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