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Macintosh SE FDHD - 820-0250-A Logic Board schematics

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Hi to everyone!


I am new to the forum, and need some help with my Macintosh SE FDHD.

I just refurbished and fixed one Macintosh Classic and it works perfectly except SCSI HDD (which sometimes requires side kick to start spinning), for which I already purchased SCSI2SD on eBay and can't wait for it to arrive.


Anyway, I started refurbishment of mentioned Macintosh SE FDHD which was in absolutely terrible condition due to battery leakage (more like battery explosion, as it looked like from inside), everything was oxidized ... totally dead. I washed every part of it, besides HDD and floppy, as I did with Mac Classic and I managed to recap everything (except HDD and floppy drive ... really not deeded) and I fixed analog board (few transistors and diodes were shorted). I had to swap RAM with new RAM sicks (now it has 4MB), but when I plug it in and turn the switch ON, HDD starts to spin, but screen is totally black.


OK, I did not put any battery back to the mother board (I will ... these days), but this should not be the reason why the screen is black, right?

I know that there must be some signal generated by that big VLSI chip, but I cannot probe it with the scope because I do not know where to start since the schematics for Macintosh SE is nowhere to be found on the Internet. Of course I can look for datasheets of the ICs, but it will take forever.


To be more specific the motherboard model is 820-0250-A (1988). I found some schematics for SE/30 but not for SE FDHD.


Does anyone has schematics for motherboard model is 820-0250-A for SE FDHD, or maybe knows where to find it?


If you do, please share.


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Sent you a PM.


The battery has no effect on booting, it only holds some basic variables like the time and for some reason the mouse speed setting among others. In fact I run my FDHD with no battery just in case I forget to remove it when storing it away.


EDIT: I should clarify the battery itself doesn't remember these things LOL

Edited by ignatio
i'm an idiot

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Hi ignatio,


Thank you for the tip about the battery. I thought that this is the case, because that is the only logical thing. I was just wandering if it might not be the case because I found somewhere in the swamp of manuals I managed to download that the battery "should" be inserted on the motherboard. And there was one Youtube video in which someone booted Mac after inserting the battery (could be Adrian Black ... can't remember any more who was it exactly, nor what Mac was it exactly).


Also about the battery, it can't remember these things, then it get jealous and explodes of jealousy which destroys these great machines LOL.


Saw the PM, thanks, already got these ... unfortunately no plain SE schematics there, only SE/30 (quite different from plain SE). But I never give up, I will find the problem wherever it is, having hope that all custom ICs are OK.

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Hi, yes, ignatio sent the same. Thank you both.

I do already have these schematics. I was just hoping that someone has a complete one.

I will try to use this one and check some routes on PCB, hopefully it will be the right one.


Seems to me that P4 connector is missing ("hard to read image"), which is a missing link for detecting where the CRT signals coming from. Although, P4 is on the analog board schematics. Also one of the rails is quite low on voltage, around 4.37V (might be one of non DC signals). Digital 5V rail is on 4.75V, and 12V rail is on 13.0V ... a bit high. I might swap PSU with ATX board from PC PSU, it might be more reliable with voltage levels. I have to inspect everything in more details with the scope first.


As I manage to do some progress, if any, I will post my progress.

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