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PotatoFi

Brightness Knob on Macintosh SE

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Hey! The brightness knob on my Macintosh SE doesn't seem to work all of the way. It goes from medium-bright up to very-bright... but it does not go below medium-bright. It stops very suddenly at medium-bright and does not get any dimmer. I would like to be able to turn the brightness all the way down to basically shut the CRT off to preserve it.

 

Is that just a dirty potentiometer? I cleaned out a Gameboy contrast pot last night by running isopropyl alcohol over it while rotating it... would that work on my Macintosh SE? I just wanted to check before digging the analog board out of there.

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So does the knob continue to rotate and just not dim the display further, or does the knob stop while the display is still medium bright? If it's the former you've probably got an issue with the pot but if it's the latter you'll need to make some adjustments to your analog board to bring your brightness and/or contrast levels within spec so that the pot has the desired effect.

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25 minutes ago, Franklinstein said:

So does the knob continue to rotate and just not dim the display further, or does the knob stop while the display is still medium bright?

The knob continues to rotate, it just doesn't get any dimmer. Dirty potentiometer? Pull the board, flush with alcohol while turning the knob, and let it dry out for a couple of hours?

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Quote

The knob continues to rotate

The knob should not rotate permanently, only go from a minimum to a maximum from the middle position.

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6 hours ago, bibilit said:

The knob should not rotate permanently, only go from a minimum to a maximum from the middle position.

I should have been more clear, it does not rotate indefinitely. It does stop rotating, but it doesn't go any dimmer than "middle-bright".

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Do you see any scan lines when the pot is turned up for full brightness? They are the diagonal lines that zig-zag across the display, and they shouldn't be visible.

 

If they are, you may have to adjust the cutoff, as it is probably set too high.

 

c

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On 1/26/2019 at 1:01 AM, CC_333 said:

Do you see any scan lines when the pot is turned up for full brightness? They are the diagonal lines that zig-zag across the display, and they shouldn't be visible.

Nope, at maximum brightness everything looks fine, it's just very bright. Maybe a tiny bit blown out but it still looks good.

On 1/26/2019 at 9:24 AM, glay78 said:

This happens to my Macintosh Plus too. I recapped the analog board and it’s fixed.

Hrm, I have replaced every cap except for C9 (I accidentally ordered the wrong cap). I am going to order that last cap and install it. If that doesn't fix it, I'll try flushing the brightness control potentiometer with Isopropyl alcohol. This is the *only* thing wrong with this Mac SE FDHD, so I'd like to get it fixed!

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Ok, I recapped the entire analog board (excluding that 3.9 uF cap and the power supply caps), and it didn't fix it. So I desoldered a working pot from my other SE, cleaned it out thoroughly with alcohol, and soldered it in... problem fixed! Except now, I have a new problem: the knob is very stiff due to it's interaction with the front panel (it turned normally before the analog board was installed). So I gotta figure that out.

 

I am also going to try reinstalling the old potentiometer after cleaning it with alcohol.

Edited by PotatoFi

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I took the analog board back out, and gave the knob a bit of a twist to line it up better... a lot smoother now. I think I'm going to call this one fixed! We'll see if the old potentiometer is still bad after a good cleaning when I repair the next SE in the lineup.

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One last update: remember that I pulled the "bad" potentiometer out of my Macintosh SE FDHD, and swapped in a known-good knob from a regular Mac SE? I took the old, "bad" pot, rinsed it thoroughly with alcohol while repeatedly turning the knob lock-to-lock, and soldered it back into the Mac SE that I stole the "good" potentiometer from... and it works great now! 

 

Moral of the story: always try the easy/obvious stuff first. It was a dirty potentiometer all along.

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