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Screen Adjust on a Mac Classic?
- Thread starter Tempest
- Start date
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No, no, no. Those are the very last things to play with. What Scott, I and others have been talking about is a rotation of the yoke itself, under the assumption that the raster has the correct geometry, and that the only problem is a rotation.
Also, do NOT work on the yoke "hot." Only rotate the yoke with the power off (and power cord pulled out). Work carefully, and you'll be able to find the right rotation with ease.
Oh, okay. What I meant by "touching" was what to hold while rotating the yoke. Because he was asking what was "safe" to handle. If this is not the best thing to do. Thanks for warning. 
(I've only needed to mess with the yoke once when I installed a new power board that was a different revision than the original.)
(I've only needed to mess with the yoke once when I installed a new power board that was a different revision than the original.)
I stand corrected. I was assuming the horizontal circuit was like a Mac Plus, which carries about 16 Volts on the yoke coils. In any case, break the glue or age stiction before turning the Mac on, and adjustment will be easy. The horizontal output transformer on the analog board IS dangerous, as it supplies the high voltage for the CRT.
Well I tried this on my old VERY dead Mac SE last night. Obviously I couldn't power it up to see my handy work, but I was able to get a feel for how the yoke moves and how hard I have to twist. After the initial breaking of the glue it moved fairly easily. Since my Classic has been off now for four days I think I have the guts to try this on the real thing.
Tempest
Tempest
Well I braved the tube today and got it straight. It's not 100% but it's better than it was. One question though, how much of a black bar should there be around the picture? I know all older monitors have a picture smaller than the actual tube (overscan?) but I'm curious if this can be adjusted. Right now there's about an inch at the top and an inch and a quarter or so at the bottom.
Tempest
Tempest
There are adjusters on the analog board for raster height, width, and H & V position. They are identified by icons & arrows which indicate which way they change the picture. If you enlarge the raster a lot, you may have to tweak the focus just a bit. Somewhere around a half inch of border is normal. Computers are underscanned (entire raster fits on screen) whereas TVs are overscanned (picture bleeds off the edges of the tube). Digital TVs and DVI monitors don't need to over/underscan, since the exact number of pixels is known for both the data and display. You can get plastic TV adjusting screwdrivers at Radio Shack and other fine electronic stores, which are a good idea. They won't affect the circuits you are adjusting, and of course they don't carry electricity.
Well done, Tempest -- congrats! As to the question about raster size, you have considerable freedom if you don't care about WYSIWYG. The factory endeavors to adjust dimensions so that there are 72 pixels/inch (the same as printed). If such dimensional accuracy is unimportant to you, feel free to tweak the height and width at will. Just try to maintain the correct aspect ratio to minimize geometric distortion. A simple way to do so is to use a piece of paper, oriented with a corner pointing upward and in the center of the screen. Use the default (gray) desktop pattern. The aspect ratio is correct when diagonal rows of dots lie along the edges of the paper.
I put Larry Pina's Test Pattern Generator up at
http://www.io.com/~trag/Classic%20Mac/TestPatternGenerator.img.bin
The Classic screen has the same geometry as the earlier compact Macs, I believe.
http://www.io.com/~trag/Classic%20Mac/TestPatternGenerator.img.bin
The Classic screen has the same geometry as the earlier compact Macs, I believe.
Do you still have this file? The link is not working.
