Reverse Engineering the Macintosh LC Logicboard

[max1zzz]

This is a ongoing project to create a 1:1 replica of the original Macintosh LC Logicboard

This project is in a fairly advanced state now, The board has been drawn up and V1.0 boots with a few patches and one trace cut:

This board has been fully tested and everything on it works, Not bad for a V1.0 board (and one of my first attempts at recreating a board like this!)

I'm currently waiting for the V1.1 PCB's from china which should have been the final revision, except I just realise I didn't remember fixing one of the issues in the V1.0 board...

Yep, the board isn't going to boot with /CE and /OE of one of the ROM chips not tied together....
So it looks like I am waiting for 10 fairly pointless PCB's. At least they are in a fairly fetching matte black the will look good sitting around on my desk for years to come

V1.2 will be sent off for manafacture next month, hopefully with no more issues

 

[Kai Robinson]

So the v1.1's only need /CE and /OE tying together to be enabled?

 

[max1zzz]

So the v1.1's only need /CE and /OE tying together to be enabled?

Should be all that's needed, I need to build one up to see if I made any other mistakes.
I'm just annoyed as that was the fault that was stopping the V1.0's booting and I somehow forgot to fix it...

 

[max1zzz]

And V1.1 PCB's are here!

 

[Kai Robinson]

Mmmm.... Matte black!

 

[max1zzz]

Annnd it begins again! V1.1 is being assembled:

(The camera on my phone wasn't wanting to focus...)

Hopefully this one should work with just the one jumper between /CE and /OE

Still can't decide if the Matte black or purple boards look better.... (I did want to get some made with clear solder resist, but PCBWay quoted about $330 for 5!)

 

[Kai Robinson]

I gotta say, although purple looks good - matte black with ENIG Gold finish is.... :chefskiss:

 

[Stephen]

Annnd it begins again! V1.1 is being assembled:
View attachment 148
(The camera on my phone wasn't wanting to focus...)

Hopefully this one should work with just the one jumper between /CE and /OE

Still can't decide if the Matte black or purple boards look better.... (I did want to get some made with clear solder resist, but PCBWay quoted about $330 for 5!)

Uhm… how the heck can I get one of these black ones? They look awesome!

 

[PotatoFi]

You guys are insane, and I love it. Amazing work.

 

[Branchus]

So glad you copied the later revision of the board with jumpers for the speaker and fan.

 

[Mr. Fahrenheit]

This is an awesome project!! Keep it up!

 

[max1zzz]

I gotta say, although purple looks good - matte black with ENIG Gold finish is.... :chefskiss:

It is a very nice finish, I'll probably go matte black / ENIG on the 1.2's

Uhm… how the heck can I get one of these black ones? They look awesome!

I'll make a thread in the trades section once they have been tested

So glad you copied the later revision of the board with jumpers for the speaker and fan.

Funnily I didn't even know a revision without them existed untill I saw your last livestream! Every LC I have ever seen or worked on has had the headers (even the older unit's that have the weird bar thing for the speaker / fan)

You guys are insane, and I love it. Amazing work.

This is an awesome project!! Keep it up!

Thanks guys!

 

[max1zzz]

Got the board assembled enough for testing last night, Since V1.0 worked I was assuming this was going to be more of a formality than anything however (and much to my surprise....) I flipped the switch and got..... nothing. No chime, no video.

About 4 hours of troubleshooting later (And a few minor issues fixed) I looked at the SWIM and thought something looked wrong... A quick glance at the working board revealed the issue, turns out I soldered the SWIM rotated 90 degrees to where it should have been!

Removing the SWIM produced a chime (But no video)
I was sure the SWIM would be dead, but it was 1AM and I didn't feel like finding a donor board so with nothing to loose I soldered it back on in the correct orientation

annnd...

It boots!

I quickly soldered a floppy header on and much to my surprise it booted form my FloppyEMU just fine, so it appears the SWIM was survived just fine

A couple of quick notes, the video signal looks like junk because the monitor i'm using there doesn't like the mode the adapter is set to, changing that adapter to VGA mode clears this up. And that massive cap is there because I was having some issues with PRAM data not surviving power cycles on the V1.0 board (the best fix being to increase the size of that cap) so didn't want to solder a SMT cap onto there until I have done some experimenting, I figured I should have something there when testing it last night and that massive 1000uf 25V cap was the first radial lead cap I found on my desk

 

[Mr. Fahrenheit]

That’s interesting that you needed that large cap there to preserve PRAM. Apple’s early production boards have a 2200uf 16v cap there. I have found replacing it with a 47uf 16v still works fine.

I have often asked why the early boards (including all of my prototype boards) used such a massive cap there. Did Apple have the same issues as you have?

 

[max1zzz]

I have often asked why the early boards (including all of my prototype boards) used such a massive cap there. Did Apple have the same issues as you have?

I suspect that was the case. Interestingly the prototype boards (at least the ones I have seen) use a EGRET with a different part number to the production boards (343S0100 VS 343S0851) Maybe apple used a version of the microcontroller with a higher drop out voltage on the prototypes?

I can't for the life of me figure out why this happens on my boards though, as far as I can see the circuit for the battery / EGRET is exactly the same as the original boards....

 

[Mac84]

Wow, an exciting project! I realized in my collection I have an LC II and LC III, but not an LC (which I swore I had at one point). Getting one of these logic boards would certainly help scratch that itch. Well done!

 

[max1zzz]

Thanks

And as a little update, the V1.1's have passed testing and the single fault in the PCB has been corrected. V1.2's will be going off to JLCPCB just as soon as I have finished cheking over the V1.1 LCIII design

 

[max1zzz]

Scratch that, I just found a second fault. I *think* this is the cause of the NVRAM weirdness but it's gone 2AM and I have work in the morning so I'm not going to be able to fix the issue and verify my theory until tomorrow (OK later today if we are being pedantic...)

 

[max1zzz]

And after much confused probing of the board the mystery NVRAM issue has been solved!

So what was the issue? something bizarre and abstract? Not really....

One single via which should be connecting pin 4 of the MC34064 at UD9 to GND was instead connected to nothing

One small piece of bodge wire later (Connecting pin 4 of UD9 to a very conveniently placed ground pin on the Egret)

Testing the board with this fix in place reveals that the NVRAM data now survives power cycles just fine with no component substitutions

So what's going on here? UD9 is a MC34064 undervoltage sensing IC, it's output is connected to pin 12 of the Egret. What exactly pin 12 of the egret dose is not completely clear, it's not the reset pin of the MCU and the BOMARC schematic of the LCII mark it as "reset in" so perhaps once the voltage of the +5V rail starts dropping this instructs the Egret to yank /RST low and stop the CPU etc running and this slows down the rate at which the voltage on +5v drops, giving the power supply circuit for the Egret a little more time to switch over to the battery? Or maybe this simply instructs the Egret to go into a low power standby mode? Whatever is supposed to happen UD9 isn't going to be able to tell the Egret do it with no ground reference!

Either way with pin 4 of UD9 grounded the EGRET's supply voltage drops slowly rather than the sharp drop and bounce back I was seeing before and NVRAM data is not lost

 

[max1zzz]

And V1.2 PCB's have been sent off to JLC!
I wanted to go for matte black + ENIG as a finish but that added about £20 to the cost of 5 boards! In the end I went with purple + ENIG as the purple LC III board looked real nice