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Reverse Engineering the Macintosh LC Logicboard

mmu_man

Well-known member
My LCIII seems to be okay on the plastic side of things… Maybe some batches had different sources for molding? Or maybe it's just the storage conditions (my bedroom) that suits it better?
The only thing I recall is one of the angled tabs retaining the logic board that was a bit lose, and if I remember I used the hot air station at the lowest setting to soften it and push it back in place.
 

max1zzz

Well-known member
So yeah, no more than 2MB on the motherboard with stock roms at least.
I have a feeling it's more than just the ROM's, if it was just a ROM difference I would have expected it to work with the LCII ROM's installed which it doesn't
Could there been some "interesting" code in those pre-production ROMs?...
Well I have requested a ROM dump on that thread so we can have a look :)
 

max1zzz

Well-known member
So the LC board currently has no schematics available, I thought it's about time that changes!
LC_CPU.jpg
This is going to take ages.... I have got used to how easy it is to work in sprint, going back to using KiCad is kind of painful. That single simple page took a few hours to draw (And I just noticed I missed the label on BERR) and I didn't even make the 68020 symbol form scratch (it's a modified version of the PGA symbol from Sakura-IT available here) I hate to think how long the symbol for the V8 will take to make!

If this turns out well I'll look at making new schematics for the Plus, LC II and LC III too :)
 

max1zzz

Well-known member
So I want peoples opinions here, some sextons of some pages end up being a bit of a spaghetti junction for example the serial section of the 85C80's circuit;
SCSI.JPG
Should I leave these pages as is ans just try and tidy up the spaghetti or should I change to just naming the signals and not actually drawing them? This will make it a bit tidier but in not sure if it will really make the schematic much easier to read....
 

bdurbrow

Well-known member
I find the "spaghetti" reasonably readable; and find just labeling the connections without any lines more of a pain to figure out (having to hunt thru the while schematic to find what went where).


...mmmm... spaghetti... 🍝 great, now I'm hungry. See what you did? ;)
 

Melkhior

Well-known member
@max1zzz That looks so much nicer than my own attempt for my IIci that I'm jealous :) (I only really did the power circuit for a future attempt at finding out the issue with it).

I think it looks very nice as-is, but you could label both ends of the existing lines, at least those in the middle that are somewhat harder to follow. If you see a name at one end and the same at the other, you know you've followed the line correctly (I saw that done by on the Diimo 030 schematics here, and I liked it).
 

cheesestraws

Well-known member
That looks OK to me, too. Agree with @Melkhior that perhaps for lines that cross over each other, labelling each end with the signal name would be nice, but that'd purely be polishing: it's fine as-is, I think.
 

max1zzz

Well-known member
Thanks for the feedback! I'll leave it as is then :)
If I can squeeze in labels at both ends of the lines I'll look to do that

The schematics are nearly ready for a initial release, I just have the PDS connector and a few other small bits to finish off :)
 

max1zzz

Well-known member
Ok, Here is V1.0A!
I have literally just finished drawing it and it has not been extensively checked over so will likely contain errors
If you find any issues let me know and I'll fix them in the next release :)
Aw, cool! So that means I'll have to write Sprint import in pcb_new?
That would be awesome! (I assume it would be possible to marry the board layout to the netlist from the schematic?)
 

Attachments

  • LC_Schematics-V1.0A.pdf
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mmu_man

Well-known member
That would be awesome! (I assume it would be possible to marry the board layout to the netlist from the schematic?)
Probably not entirely but at least import the traces, and compare the netlist yes, footprints might need to be replaced…
Anyway, still haven't finished the netlist part for the OpenBoardView import though, and it needs much less info than for full PCB import.
 

ironborn65

Well-known member
Ok, Here is V1.0A!
I have literally just finished drawing it and it has not been extensively checked over so will likely contain errors
If you find any issues let me know and I'll fix them in the next release :)

That would be awesome! (I assume it would be possible to marry the board layout to the netlist from the schematic?)
thanks, great accomplishment!
best
PF
 

max1zzz

Well-known member
Board layout files, as well the the KiCad files for the schematic are now available :)

These are provided under a non-commercial license however if you provide repair services and want to provide these boards (Or want to use them for any other commercial purpose) please contact me
 

MacFanZA

New member
As one of the early buyers of the LC reproduction logic board, I have some feedback to report.

I sent my dead LC logic board to max1zzz in December 2021, and he used the components to build a new working logic board for me. The computer has been working flawlessly ever since.

There are a couple of welcome touches on this logic board: 1) it can be fitted with a receptacle for a 2032 battery instead of the original half-AA-size battery (I took this option), and 2) it has pin headers for properly connecting the fan and speaker, in addition to the normal contact pads.

I can wholeheartedly recommend this upgrade to anyone with an LC.
 
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