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Scott Baret

Mac Plus: Flaky Analog Board?

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Yesterday I noticed my screen was jiggling the slightest bit. Could my analog board finally be getting flaky after 22 and a half years?

 

After I had it on for a while it was fine--it was just in the first ten minutes of using it that I had a problem.

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Yes as equill is saying (I think), the power drain will show up weaknesses in the system. So disk access during startup and use will put a strain on the Power Board. Don't add to the draw with any externals as that will only add to the problem and contribute to a catastrophic failure. I got a 512Ke on eBay that worked fine when I turned it on, but I noticed the screen jiggling. Then I made the mistake of plugging the HD20 into it and my J1 exploded in shower of sparks with a nice little burn wound on the vinyl liner.

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You credit me with too fine an analytical insight, Mac128. I sought simply to turn a fellow-soldier towards the source and fount of all troubleshooting expertise about our little furless friends. The double entendre about FDDs was entirely fortuitous.

 

de

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Maybe I should ditch the external floopy for now...just to see if that improves anything.

 

That will help with the diagnosis, but unless you have some external influences, your Mac is on a downhill slope to failure. The bet is to download Dr. Lee's excellent tutorial http://68kmla.org/files/classicmac2.pdf and test and adjust the voltage outputs as indicated. Chances are it is about time you did a complete overhaul.

 

equill, "our little furless friends" I LOVE it!

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I got the drive unplugged and also ditched the numeric keypad, the screen isn't jittering anymore but I've noticed the video isn't the best anymore...one side seems higher than the other (though that may just be a yoke that needs adjusted).

 

I have an extra Plus that I've never booted up, would a transplant of the analog board solve my video problem of distortion if it's not the centering of the screen provided the Plus is good (It's a platinum Plus so I'm thinking the supply should be better in there).

 

Alternately, is there any way to modify this Plus inside so that I can put an SE/30 and its guts inside this case?

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All Pluses are better boards than any 128/512K. Platinum Plus may have the latest greatest which is even better because the 512K has nowhere near the same power demands of the Plus.

 

You can put the SE/30 guts into a Plus, as well as just the analogue board to use with the Plus logicboard (I've done it), see your other post about this issue.

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If your raster exhibits only modest trapezoidal distortion, you can probably fix it by adjusting one or more of the permanent magnets arrayed around the yoke. Rotate only the ones that correspond to the region of the raster that needs correction. I recommend noting the initial positions before tweakage, just in case you discover that you only make things worse. [There's a picture of these magnets in the classicmac2.pdf article.]

 

Before doing any of that, though, make sure that you don't have any magnetic sources nearby. Also try moving the Mac around and see if there are noticeable orientation-dependent effects.

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If your raster exhibits only modest trapezoidal distortion...

I have minor distortion on my SE/30 screen, as shown here:

 

SE30_Screen_Problem.gif

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It is quite likely that you can correct that trapezoidal distortion by adjusting the permanent magnets arrayed around the yoke. I know how careful you are, so I'm sure that you've already made sure that the yoke itself isn't loose. Try rotating one or two of those small square magnets and I'm willing to bet that you can make that raster properly rectangular.

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Tom, thank you again for reminding me about those magnets. You once told me about that many months ago, and at that time I tried adjusting them with the SE/30 turned off. Just doesn't work that way, so I called it quits at that time. But tonight I was determined to get it right. I slipped on a rubber glove (which my wife uses to wash dishes). I then kept the machine turned on without worry of shocking myself even when my fingers slipped all over that yoke and flyback!

 

Here is a Flickr photo I just posted. I added some comments under the photo, so be sure to read those.

 

Simply put, adjusting the top, center magnet shifts the left-top side up, which solved my problem. But I found that many of the menus toward the right side of the screen were leaning a millimeter or so to the left (rather than straight up and down, they were ever so slightly diagonally shifted). I then proceeded to mess around with other magnets, but I couldn't get it quite right. Correcting the top-left side of the screen (the problem area for me, which is shifted down) would cause my menus to shift. So I ended up restoring the top-left part of the screen to the way it originally was. It's not so bad anyway so I can live with it. But it was a bit frustrating that I couldn't fix the problem after much playing with those magnets.

 

The other interesting part is that I do not recall see that shift when I had my old analog board in the machine. I don't think that says anything bad about my existing analog board though, as it is well know that swapping analog boards in classic Macs often requires adjustments to the screen. Even so, I couldn't get the adjustments to work in my favor this time. The top left part of the screen still is a tad lower than the top right.

 

Thanks again for your advice.

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Most curious (and frustrating)! I'm sorry that adjustment of the magnets proved only to be an elaborate shell game. I wish that I had some other suggestions for you, but the only other common cause of trapezoidal distortion in a small monochrome display -- yoke tolerances -- is effectively precluded by your not having noticed the problem earlier.

 

About the only other thing I could suggest is to experiment a bit with moving the yoke connecting cables around. Those carry substantial current, and it's barely possible that the associated magnetic fields could be influencing raster geometry by this tiny bit. Dressing the wires away from the base of the crt would minimize this influence, so if they're presently near the crt gun assembly, perhaps this could help.

 

But it's a long shot at best.

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