Reverse Engineering the Macintosh LC Logicboard

[max1zzz]

Oooh, pretty:

Somehow I didn't notice I still had soldermask over the fan / speaker connectors until just now.....

A clear LC case would be very cool for sure. In your OP, you mentioned eventually working your way through the whole LC series. Curious if you plan to actually remake each model, or make some sort of "new hybrid LC" with features you like/want? Looking forward to your tests!

Currently my plan are just to make near 1:1 repro's, but like the plus I may end up modifying the boards once this has been done

 

[cheesestraws]

That looks classy!

 

[joshc]

MMM RED

 

[ajacocks]

Looks fantastic!

 

[mattsoft]

Awesome! Love these projects!

 

[max1zzz]

And today's build progress:

Board lined up with the top side stencil, Trying to align the board with such a big stencil was not fun!


Paste applied to the bits that are being reflow soldered, annoyingly the board is just a little bigger than my hotplate so had to be moved around a bit during soldering

Chips positioned, I wasn't initially going to reflow solder the CPU or VLSI chip as my experience is trying to reflow solder hand placed QFP chips just results in bridges everywhere, but figured I might as well give it a go

and after soldering, the CPU went about as well as I expected, I think there are more bridged pins than non bridged ones The VLSI chip actually went ok though! There are a couple or bridges but most of the pins are clear. All the PLCC / SOJ chips soldered fine which was my main concern ( really hate hand soldering those! )

Tomorrow I'll start on the sea of passives on the bottom of the board

 

[paws]

I've made a couple of boards with QFPs and I'm not sure I will again unless forced. I had access to a fairly good setup with an oven and used stencils, and every time I populated one it was 5 minutes in the oven, then hours of sweating, shouting and self-loathing as I tried to clean up all the bridges.

 

[bdurbrow]

How do you like the hot plate? I'm considering getting one for my FPGA project (BGAs... ugh...).

FWIW, I don't even consider reflowing 1-off (or low volume) QFPs; I just line them up under the microscope, tack a couple of corners, and drag-solder them (with a big "hoof" tip). I wind up with a fair bit of flux left on the board, but I would have been cleaning the whole thing with 99% IPA anyway.

Also, I'm going to play with an ultrasonic cleaner that my dad bought off of eBay... we'll see how well it works. I can say that the tiny one from Harbor Freight didn't work at all - not enough power, I think. I've heard of (well, watched YouTube videos of) people running PCBs and keyboards in the dishwasher, so I might try some dishwasher detergent in the tank (both my dishwasher and the tank of the ultrasonic cleaner are stainless steel; so it should be chemically compatible).

 

[uliwitness]

LC was a great machine. Though my favorite "LC family" computer was probably the LC 475. It was just so amazingly fast. And I suppose you could overclock it to 33MHz as described here and make it even cooler to make a sort of "ultimate LC".

Though I always thought the overclocking was just changing that jumper -- the article makes it seem like you need to solder a few resistors differently as well, and the System 7.5 installer won't recognize it anymore. So if you made such a board, I suppose you'd want to add a "Turbo" button to switch it back to 25MHz to get back to a regular LC in case a reinstall is needed.

 

[joshc]

the article makes it seem like you need to solder a few resistors differently as well,

The jumper on the 475 (J18) is for something else, that just changes the gestalt ID between 605 and 475. A Quadra 605 is gestalt 94 while a LC475 is gestalt 89.

Overclocking to 33mhz just requires moving a few SMD resistors, pretty easy...



(source: https://www.geektechnique.org/projects/68040.html)

As for System 7.5 with this mod, if it's already installed it will work. It's the installer which is picky, not the OS itself.

Anyway... back to the original LC. I like the original LC, so I'm definitely looking forward to seeing Max's board up and running.

 

[uliwitness]

Yeah, the article makes that clear enough, but when I first read about the trick in a Mac magazine back then, it sounded easier. I'm glad I didn't attempt it at the time, because SMDs were beyond my skill at the time. And a Turbo button to switch between the two resistor positions would mean if your hard disk fails, you can still go back to 25MHz and just run the installer to restore. You don't need to keep a copy of a pre-installed 7.5 hard disk around to swap in.

Anyway... back to the original LC. I like the original LC, so I'm definitely looking forward to seeing Max's board up and running.

I'm curious, what things do you like about it?

The LC was a nice machine, but the main thing I personally remember is that it was a huge compromise. The slow speed throttled by its bus, the graphics that couldn't drive the 13" color display (640x480) at 8 bits but only the 12" (512x384), the bad performance with base RAM. I love it because it was my first Mac, but everything I had after it was a better machine to actually use.

Pretty much every Mac I've had since then was wonderful and fast and ran everything I threw at it decently, and only got too slow with successive system updates after a few years. The LC was pretty much our only choice back then because it was the only Mac in our price range. We got the 12" monochrome display (640x480 at 16 greys looked good, 16 colors would have looked horrific) pretty quickly upgraded from 2MB RAM to 4MB RAM and that made it run decently with System 7 (not just System 6 with MultiFinder off).

Not trying to bash the LC, I think this is a cool project and am just curious which positive bits I have missed. Certainly the built-in sound card and included microphone were great for me.

 

[max1zzz]

I've made a couple of boards with QFPs and I'm not sure I will again unless forced. I had access to a fairly good setup with an oven and used stencils, and every time I populated one it was 5 minutes in the oven, then hours of sweating, shouting and self-loathing as I tried to clean up all the bridges.

Yep, that's pretty much my experience with stencil soldering QFP's! it's usually easyier to just drag solder them by hand

How do you like the hot plate? I'm considering getting one for my FPGA project (BGAs... ugh...).

FWIW, I don't even consider reflowing 1-off (or low volume) QFPs; I just line them up under the microscope, tack a couple of corners, and drag-solder them (with a big "hoof" tip). I wind up with a fair bit of flux left on the board, but I would have been cleaning the whole thing with 99% IPA anyway.

Also, I'm going to play with an ultrasonic cleaner that my dad bought off of eBay... we'll see how well it works. I can say that the tiny one from Harbor Freight didn't work at all - not enough power, I think. I've heard of (well, watched YouTube videos of) people running PCBs and keyboards in the dishwasher, so I might try some dishwasher detergent in the tank (both my dishwasher and the tank of the ultrasonic cleaner are stainless steel; so it should be chemically compatible).

I love the hot plate, it's so much easier than using hot air (I get far fewer components moving out of place while soldering)
The only issue I have had with it is that it can slightly warp boards if your not careful so requires some experimenting to get the technique right

I haven't tried it with BGA's though, personally for large BGA's I would still go down the route of using my IR preheater and hot air.

I have access to a couple of cheap ebay ultrasonics and both work well however (particularly with heavily flux coated boards) they are not a magic bullet, often after cleaning they require some scrubbing with IPA to remove whatflux the ultrasonic can't
Also a caution with older boards and ultrasonics - I have personally seen two cases of SE/30 boards which died after a trip through a ultrasonic, on one of the it killed one VRAM IC (Lucklie I had already ordered some VRAM IC's for another SE/30 so had them in stock! ) the other I am yet to diagnose. I haven't had any issue with modern boards though

 

[max1zzz]

Last night I managed to get the board assembled enough to try powering it on, I'm currently just getting a black screen

A little probing around revealed that /RESET is acting very strangley, sometimes it just stays at near ground (Maybe rising to 0.3-0.5V) other times it will pulse however these pulses where very odd, they where not clean transitions (they start at ~2.5v then ramp up to the full 5v) Further investigation revealed that when /RESET is pulsing like this tapping on the EGRET chip would make /RESET go high normally.... The system still doesn't boot after this but something is telling me that EGRET chip is bad!

I'm going to pull one of another board today (After checking it works!) and swap it out and see if that makes a difference

 

[bdurbrow]

Time to show that EGRET the great egress…

 

[chelseayr]

@mattsoft funny enough I was wondering how this project would look like on a LC III+ board before and after some small modifications likewie but I'll wait to see how this first one comes out tho

 

[max1zzz]

We have boot! (OK I might have shouted something a little stronger than that when I got this screen )


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....

Next step is soldering in all the connectors and seeing if I can boot Mac OS on the board!

 

[cheesestraws]

We have boot! (OK I might have shouted something a little stronger than that when I got this screen )

you are on a roll. Very well done, very impressive.

 

[max1zzz]

So, did some testing on the board and it sucesfully booted 7.1 from SCSI and 6.0.8 from floppy, both floppy ports work and ADB works. external SCSI, serial and PDS are yet to be tested

I have encountered one issue, every so often I'll get glitching at the top of the screen:

The board still works fine otherwise, power cycling the board will often clear this up.
If this shows up on boot it will be there for as long as the board is running, similarly if the screen is normal at power on the glitching will not appear while the board is running

I'm not sure what to make of this, it almost looks like interference but since it appears / disappears only on power up I don't really see how it could be

Maybe the video DAC is bad?

 

[bdurbrow]

I’d need to look at the schematics and datasheets, and then do a bit of probing with an oscilloscope and possibly logic analyzer; but… it looks more like a pixel clock timing issue?

 

[bigmessowires]

It looks the the pixels are correct, but some video lines are shifted over by a few pixels? I don't think that's the DAC. It's hard to see in the photo, but are the lines always shifted by a multiple of 8 pixels?

Somewhere there's a counter circuit that will be generating consecutive addresses for scanning through video RAM each frame. Maybe its clock input is glitchy and sometimes it misses a clock edge, with the result that the whole line gets delayed by one byte/word. There's also a shift register somewhere that is shifting out the bits in each byte/word, maybe its clock or enable inputs are glitchy.