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Reverse Engineering the Macintosh LC Logicboard

cheesestraws

Well-known member
Yeah—agreed, that looks like the pixels are being shifted out at the wrong times, rather than the pixel clock itself is drifting. Note that the pixels are all lined up vertically, it's just there's an offset of n pixels being applied for some n. What video mode are you in there - what kind of bit drift are we looking at per pixel?
 

max1zzz

Well-known member
Ok, I think it's actually my screen not being as compatible as I thought it was
I tried pulling the VGA cable out the adapter with the system running, on reconnecting it the display was correct with no shifted pixles. If this where a issue with the pixels being shifted out the VRAM at the wrong times I wouldn't expect this to have a effect as the mac has no way of knowing if there is a cable connected to the adapter or not

Also pulled out a couple of LC II boards, annoyingly although both where previously tested and working both jsut show a black screen now, however I found pulling teh VRAM simm form one produced a solid white screen, compleat with shifted pixels in the exact place as the repro board.
 

cheesestraws

Well-known member
That does seem pretty conclusive, agreed. Good that it's not your problem, bad that you spent time chasing it down. Ho hum...
 

uliwitness

Active member
Turns out the EGRET was fine, swapped in a known good one and got the same symptoms, swapped the suspect one onto a known good board and it booted. Found a couple of via's in the EGRET circuit that where not connected to the appropriate planes that solved that issue (Why tapping on the chip made the slightest difference I have no idea....)

Then I found a a trace going from the CPU that was connected one pin over to where it should be, fixing that made no difference

I was half way through swapping the VLSI chip out to verify that wasn't the issue when I noticed one ROM socket had no trace between /OE and /CE...... adding a jumper to connect these two together produced a working LC :) Turns out a 68020 locks up pretty fast when it's only getting data form 3 of the 4 ROM chips....
It's like software development. Miss one small thing and the symptoms are very surprising.

Great work!
 

max1zzz

Well-known member
Test fitted the board in a case, it's a little snug but everything lines up correctly :)
IMG_1627.jpg
I have tried a PDS card in it too and that lines up with the slot in the case just fine

I just need finish testng the board now.... (Audio in, serial and PDS are yet to be tested)
 

max1zzz

Well-known member
Awesome work. Did you test if the board works with 8 memory chips ?
I did, got chimes of death. Also tried dropping the roms form a LC II onto the board but got the same result, I guess the memmory controller just won't see more than 2MB of ram on the motherboard, Presumably the extra positions are to allow the use of lower density ram (where 512Kx4 IC's ever a thing?)

On a slightly amuseing note, the LCI (with a stock RAM config) will boot just fine with the LCII roms, and even identifys in Mac OS as a LC II :) I'm not sure if this gives any real advantage but it was amusing to see
 

demik

Well-known member
I did, got chimes of death. Also tried dropping the roms form a LC II onto the board but got the same result, I guess the memmory controller just won't see more than 2MB of ram on the motherboard, Presumably the extra positions are to allow the use of lower density ram (where 512Kx4 IC's ever a thing?)

On a slightly amuseing note, the LCI (with a stock RAM config) will boot just fine with the LCII roms, and even identifys in Mac OS as a LC II :) I'm not sure if this gives any real advantage but it was amusing to see

Bummer ! Fun stuff about the LC IIs ROM though. It's not that surprising if you consider that the LCII is a LCI with a 68030.
 

max1zzz

Well-known member
It's not that surprising if you consider that the LCII is a LCI with a 68030.
Indeed, in fact sections of the PCB are 100% identical! should make the LCII board fairly easy to reverse engineer once I get started on it. (Currently I'm looking into the LCIII so the II is on hold for now...)
 

demik

Well-known member
Indeed, in fact sections of the PCB are 100% identical! should make the LCII board fairly easy to reverse engineer once I get started on it. (Currently I'm looking into the LCIII so the II is on hold for now...)

Oh you are on a roll! Good luck with the LCIII !
 

cheesestraws

Well-known member
I've got a couple of II boards here earmarked for you, I'll try to get them over to you in the next week... it's been a busy couple of weeks.
 

max1zzz

Well-known member
I've got a couple of II boards here earmarked for you, I'll try to get them over to you in the next week... it's been a busy couple of weeks.
Thats fine, I'm in no immediate rush for them (Since my investigations into the III has occupied for the moment ) so whenever you get around to posting them is good with me :)

Oh you are on a roll! Good luck with the LCIII !
Thanks! I Also have at least 3 more boards on my radar at the moment (LCII, 475 and Classic II). It's going to be a buys few months!
 

olePigeon

Well-known member
I'd love to get one of these boards in black. @max1zzz How much is it for a one-off board if I could send the files off to PCB Way or something? Any idea?
 

max1zzz

Well-known member
@olePigeon Once I have finished testing the boards I'll be getting a batch made (With fixes for the issues found in this version) I can have these made in black (Red was only ever intended to be for the prototype stages :) )
Boards will be approx £35 each

I will also be offering assembly on these for anyone that is interested, since I know not everyone is keen on soldering 150+ pin QFP packages :) (A donor board would need to be supplied for this)
 

max1zzz

Well-known member
Shipping the the USA is not a major problem, It's not the cheapest thing ever but is something I have done plenty of times

I will be releasing the gerbers at some point, but am still undecided on when this will be and under what terms
 

bdurbrow

Well-known member
Shipping the the USA is not a major problem, It's not the cheapest thing ever but is something I have done plenty of times

I will be releasing the gerbers at some point, but am still undecided on when this will be and under what terms
You sure it’s going to be not an issue? I mean shipping something that weighs thirty-five pounds can’t be cheap… perhaps if you got them made with a lighter weight currency?

🤪
 

max1zzz

Well-known member
You sure it’s going to be not an issue? I mean shipping something that weighs thirty-five pounds can’t be cheap… perhaps if you got them made with a lighter weight currency?

🤪
Haha :)

Late breaking news: I have verified the III is a 4 layer board. New board coming up!
 

bdurbrow

Well-known member
4 layer board
So, pepperoni, mushrooms, bell peppers, and onions? 🍕 Um... no, that's a pizza. Nevermind. ;)



It occurs to me, that it might be helpful to people who are restoring original PCBs that got capacitor/battery bombed to have the raw scans of the various layers, so as to see what traces went exactly where... 🤔
 
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