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Reverse Engineering the Macintosh SE PCB & Custom Chips for 1:1 reproduction


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So... looks like we're seeing video wraparound that would be characteristic of a low-quality but generally functioning *HSYNC line.  From what I see all of the RAM address and data lines going to the BBU that are used for video are well-behaved.

 

The stretched, mirrored, and dimmed mouse pointer on the right side of the screen seems to indicate pixels being sent to the CRT on the horizontal blanking return path, which should not happen, because the BBU has logic to keep the screen dark on horizontal blanking.

 

So, my general diagnosis: *HSYNC is laggy either from too much parasitic capacitance or too much resistance.

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Solved it! Turns out that C67, while it's on the schematics, was trying to clamp H-sync to ground. I checked the original boards i have here, and they show the silkscreen but C67 itself was never included at all.

 

So, i removed C67 and, well, see for yourselves...

 

@quorten you were correct, but it took so long to film it (first recording failed, didn't notice til i tried to upload)!

 

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Yeah, congrats!  Now, let's think about stretch goals, shall we?

 

The issue with adapter circuit boards for things like ROM, ADB, RTC, BBU/GLUE, and ASC, unless the mounted component is surface-mount and the board area is the same as the original component, is that the extra space consumed can interfere with expansion cards.

 

So, a stretch goal is to design a board, including all adapters, that can handle the mechanical requirements of an expansion card like this MegaScreen SE.  The MegaScreen SE card actually has a plastic peg on its underside that sits flush on top of the BBU, if I recall correctly.

 

pds_card.thumb.jpg.27525659e035a5f4610b2bc6aecb9fb7.jpg

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