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Reverse Engineering the Macintosh LC III Logicboard

marciot

Well-known member
I probably ought to start a new thread for my LC III board from hell, but here is my survey of all the potentially corroded traces on my board. The more I dig into this, the more daunting it looks! The number of broken traces now numbers about fifty 😯

All the traces which pass through the damaged area near the battery are painted in white. I came up with this by using GIMP's magic wand tool to trace the paths from the PNG files. Now have a GIMP file with 50 layers, one for each trace and I can use this to figure out where to place wire bridges to hopefully avoid the damaged traces and vias. Anyone want to call the odds of me actually being able to get a bootable computer from this board?

@max1zzz, if I fail in this insane repair endeavor, I might have to buy one of your brand new boards! 😁

View attachment Damaged Traces.png
 

marciot

Well-known member
@max1zzz: How much you are selling or planning to sell your PCBs for? Do you use JLPCB assembly to prepopulate any of the smaller passive components or must these be carried over manually? What type of tools do you need to transfer the components? I have a hot air rework station, but not much experience with it, and I worry about hand soldering small pitch parts like the CPU and transferring the large plastic parts such as the memory slot and the PDS slot connector from one board to another. Is that a relatively easy job?
 

max1zzz

Well-known member
max, did you change your GAL setup? Looks like you condensed three smaller GALs into that one higher capacity GAL?
Gals? There aren't any GAL's in these guys, it's all apple customs

@max1zzz: How much you are selling or planning to sell your PCBs for? Do you use JLPCB assembly to prepopulate any of the smaller passive components or must these be carried over manually? What type of tools do you need to transfer the components? I have a hot air rework station, but not much experience with it, and I worry about hand soldering small pitch parts like the CPU and transferring the large plastic parts such as the memory slot and the PDS slot connector from one board to another. Is that a relatively easy job?
The boards are £35 + shipping

The boards come without anything pre populated (I did look at it but it was going to add ~£40 to the cost of each board which I didn't think was worth it, I can however generate the PNP data if you want to order some boards and go that route) Personally I move everything that is undamaged across and then replace anything that was damaged

A soldering iron and hot air station is all that is technically needed although some form of good magnification will help too (I assemble the boards under a microscope :) ) I can't say it is a easy job though, the memory controller in particular is a pretty fine pitch chip and is very easy to bend pins on and the PDS slot is a right pain to remove (The PDS connector can be replaced though, you can buy 2 96pin Din 41612 connectors (Or one 96pin and one shorter one) one connector fills the main section and then you can cut the last 6 rows off the second and use that to fill the smaller section)

I can assemble the board for you if you decided against assembling it yourself, this service is £50 (with a 1-2 month lead time)

There is a thread in the trading section too :) https://68kmla.org/bb/index.php?threads/fs-reproduction-macintosh-lc-lc-iii-logicboard-pcbs.39812/
 

Trash80toHP_Mini

NIGHT STALKER
Gals? There aren't any GAL's in these guys, it's all apple customs
My bad, I confuzzled the pair of square ICs in that package type and the larger one "above" them. But was looking at the board from opposite directions it seems. They're still there and the larger package is still on the opposite side of the board. :oops:
 

Trash80toHP_Mini

NIGHT STALKER
Facepalm if I haven't mentioned this? Long ago uniserver removed the built in memory of his LCIII and installed 8MB on tits pads. IIRC he said it could address more than that, but such wasn't available in a matching footprint package. I'll have to try to dig up that old thread sometime.

So anyone building one of these new boards should source an 8MB set of chips. 😉
 

max1zzz

Well-known member
Ooh so if I run some more address lines I could maybe get 4mx4 chips working for 16MB onboard. This might call for some experimentation!
 

mmu_man

Well-known member
Btw, having seen this thread about C22, I just confirmed mine has the + pin of C22 at -5V, which is not really a good idea. It still works but it will probably shorten its lifespan. Not sure what it would do to tantalums either…
 

Trash80toHP_Mini

NIGHT STALKER
Ooh so if I run some more address lines I could maybe get 4mx4 chips working for 16MB onboard. This might call for some experimentation!
Glad thread opped up again. Have you looked at the possibilities yet? I'd think replacing the Bank A with a SIMM Slot makes more sense than any soldered memory config.

BTW, where are my IIcx trace layer references? ;)
 

max1zzz

Well-known member
Btw, having seen this thread about C22, I just confirmed mine has the + pin of C22 at -5V, which is not really a good idea. It still works but it will probably shorten its lifespan. Not sure what it would do to tantalums either…
I have seen that, electrolytics and polys seem to be "ok" with the reverse voltage but it probably isn't great for them long term. Tants I would install the correct way round as they tent to short (and sometimes explode!) with reverse voltage

Glad thread opped up again. Have you looked at the possibilities yet? I'd think replacing the Bank A with a SIMM Slot makes more sense than any soldered memory config.
Not had a chance yet, way to many projects I need to finish off...... Though I have just tried out a new method of creating kicad layouts from the sprint layouts that is much faster than totally redrawing the boards form scratch so some playtime for the LCIII board is coming soon :) (Just need to finish the schematics for it first....)

BTW, where are my IIcx trace layer references?
Ohhh yeah I forgot about that, Will try and remember to do that tonight! :)
 
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