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| Macintosh with 'sort of double screen' |
Posted by: mactjaap on 2017-11-29 13:25:17
Testing a new motherboard. I see some kind of double screen! Who knows what this is? Is it fixable?
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Posted by: techknight on 2017-11-29 16:03:30
you got a stuck most significant address bit in the video circuit somewhere. so as it runs through the lower bits, its re-scanning the same information.
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Posted by: CharlieFrown on 2017-11-30 04:20:28
I have the same issue with my Plus, but it doubles the screen horizontally. Havent resolved it yet. But someone suggested checking certain traces in my old thread.
Cant find this post after forum "remodelling".
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Posted by: apm on 2017-11-30 17:56:27
Yep, definitely a stuck line in the video address circuitry somewhere. Depending on which line it is, you could get any pattern of doubling from 8-pixel vertical stripes (bit 0) to two 256-pixel vertical stripes (bit 5) to horizontal stripes of any number of lines (bits 7+).
My Plus logic board schematic is hard to read, but it looks like you want to probe signal VA5 (video address bit 5), which goes from U1F pin 10 to U3G pin 13. Also check VA4 (U1F pin 11, U3G pin 3) and VA6 (U1F pin 9, U4G pin 3). I *think* the fault would be on bit 5, but if you compare what's happening on those different lines you might see if anything looks different between them. A scope could help.
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Posted by: CharlieFrown on 2017-12-01 06:24:59
link
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