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Reverse Engineering the Macintosh Classic PCB for 1:1 reproduction


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For now this is mostly going to be a placeholder, until the SE boards are finished, but as the title explains, this is designed to save as many Mac Classics as possible. So many have died thanks to the stupid PRAM battery on the board. These are 4-layer PCB's with a mix of through hole and SMD component. You will need at bare minimum, a soldering iron, hot air soldering station, desoldering station and a decent selection of hand tools.  

 

So - how do you reproduce a PCB? 

First, get a Mac Classic PCB - in my case a 'dead' board from Mr Adrian Black!


Then, you get to work desoldering. Literally EVERYTHING. Not a single component must remain! However, 30 year old solder and my ZD-915 desoldering station caused a handful of through holes to just pull out of the board, or some traces would come up with it. I found that pre-heating the joint, with a little flux (AmTech RMA-223), would make the solder melt a lot cleaner when you use the ZD-915 desoldering gun. You must use a lot of heat on these boards. For hot air - i recommend a temperature of 360*c and a flow rate of 4.5 - you must use the right tools for removing the PLCCS - use the A1137 and A1138 nozzles for the larger parts - and the wide round nozzle for smaller 'birdseed' and SOIC parts. 

 

For through hole joints that have been corroded, you must flood the area with flux, heat from both sides, then try and move the pin using a soldering iron (i find 420 degrees C suitable, with a standard flat tip) before trying to use the desoldering gun. Sometimes it won't always come out of the joint, in which case reflood with flux, and attack it with desolder braid. I use 2.0mm GootWick, which is fantastic stuff (thanks to Louis Rossmann for the recommendation).  Preheating the board with hot air works wonders. Speaking of hot air - for SMD parts - be liberal with flux and don't just keep the hot air on the part - you want to move it around the part, on the legs and keep a little pressue on it with some tweezers - don't force the part to move, it'll loosen when the temperature is jsut right. 

You want to save basically everything bar the SCSI and Floppy connectors, ROM Socket and the 'birdseed' (SMD Resistors, Capacitors, Transistors & inductors).

Once you've saved all those parts, it's time to get scanning! 800 to 1200dpi on your scanner. As the Classic PCB is rather smaller than a sheet of A4 paper - it'll all fit without having to merge mutiple images. 

There will be a more to follow...but any questions so far?

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Kai – glad you're taking this on! If you ever want another badly Maxell bombed Classic LB, I'd be happy to send you one via first class parcel, on my tab.

Edited by rplacd
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@rplacd actually - the most beneficial thing would be to strip and sand the board so the inner layers are visible, and scan them! I live in a shared flat and have no 'outside area' that's suitable for it, really. 

 

At the end of Sunday - this is how much i've managed to get done of the top layer;

 

 

macclassic-top-1.JPG

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Are you sure this is just a 4 layer hoard?

Pretty sure everything was 6 layer by the time the Classic came out.

Edited by Bolle
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2 hours ago, Kai Robinson said:

@rplacd actually - the most beneficial thing would be to strip and sand the board so the inner layers are visible, and scan them! I live in a shared flat and have no 'outside area' that's suitable for it, really. 

I have no idea how to do this – but if anyone wants to give this a try, contact me.

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Kai, since you're doing a lot of this type of thing, you might consider investing in an IR pre-heater. I have a T8280. Makes this sort of thing a lot easier, and this Chinese one only costs about $100 shipped. (Preheat the entire board to a temp somewhat below your hot air temp, then the hot air gun has a lot less work to do. Should cause less heat stress to the components as well.) https://www.ebay.com/sch/i.html?_nkw=ir+pcb+preheater

Edited by anthon
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My ME brother told me the way to analyze the layers of a board is to set it up perfectly level and encase it in epoxy. In that state each layer is easily milled away and scanned. I'd told him I'd tried sanding and it wasn't a very effective approach if if done by hand. That was 30 years ago, cast resin should make it easy these days  .  .  .  assuming you have access to a milling machine. :/ 

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The milling machine isn't a problem at all -- every self-respecting maker space will have one. I don't understand the cast resin step though. What would be the process for that? I'm definitely tempted to do this.

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2 hours ago, Kai Robinson said:

@Bolle did you manage to image the inner layers?

 

Kind of at least...

Altough we are slowly getting into working on a replacement logicboard.

We‘re going for a more classic PCB design workflow. We‘ve got all the needed Apple custom and non-standard ICs in Eagle and I should start redrawing the schematics sometime soon.

I will cross reference the schematics once more with the scans of what is left of the inner layers after grinding away like crazy at one of my boards. :evil:

I will implement options for using both modern replacements and original ICs where possible... VRAM comes to mind as well as the video ROM, 29F040s right next to the ROM SIMM, possibly RAM as well.

 

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Would you mind sharing the 1200dpi scans of the inner layers with me, so i can try for a 1:1 layout? :)

 

VRAM is easily available on UTSource as NoS, as is DRAM for making 30-pin SIMM's, i have 50x PCB's for 30-pin SIMM construction, and new PE Connectors SIMM Sockets.

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Top and bottom layers provisionally done

Now, the inner layers should be just ground and power - but i fully expect there to be some surprises. I know there's talk of these being 6-layer boards, but honestly, these are cost reduced SE's with no PDS slot and only 1 ADB, the schematic isn't all that complicated and that's the other thing, we have the schematic!

 

Would anyone like to take a look at the Sprint Layout file so far?

 

macclassic-top-2.JPG

macclassic-bottom-1.JPG

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Good job again. I think the NCR ship is repleacable but the Zilog Z53C8003VSG which is still in production. Pins looks compatible !

They might be a compatible SSC still produced too. Can you give us the PN ? Should be a 8530 IIRC. I will take a look

 

Is the BBU compatible with the SE one ?

 

Edit: found it Z0853004AVSC. Z85C3008VSG looks pin compatible

 

- SCSI: https://www.mouser.fr/ProductDetail/ZiLOG/Z53C8003VSG/?qs=FsLZlD%2B%2Bd9cJEZW2nIhyXQ%3D%3D

SCC: https://www.mouser.fr/ProductDetail/ZiLOG/Z85C3008VSG/?qs=z96q%2FFFR6qb6cf17DanQqQ%3D%3D

Edited by demik
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I have a classic LB I failed to revive after recapping, could be a candidate for contributing to this effort.

 

I don't currently have any hot-air gear, might be a good excuse to get some. I've done some pretty epic stuff with just a soldering iron, but removing a 68000 intact might be a bit of a stretch.

 

I have an idea regarding inner layers involving a local commercial third party, but I should probably talk to them first before saying any more on that.

 

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On 1/16/2021 at 6:17 PM, mogs said:

I have an idea regarding inner layers involving a local commercial third party, but I should probably talk to them first before saying any more on that.

 

So I followed up with this, and unfortunately it's not going to work.

 

There is a local PCB fabrication place in town, and I knew they had an X-Ray machine for checking correct registration of multi-layer boards. I emailed them about the possibility of using it for imaging the internal layers, they replied straight away and were very friendly but unfortunately it only images an area approximately 15mm in diameter.

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9 hours ago, anthon said:

@Trash80toHP_Mini's milling idea sounds promising and do-able to me. I wish I understood the resin mold part though.

I think the idea of resin is to stop the irregularly-shaped logic board from wobbling around, or from being anything but perfectly level as the milling machine slowly removes each layer. Honestly, with every component removed from the board (thus making it flat), this may not be necessary. As far as I know, somehow, milling machines should be able to clamp to a thin rectangle pretty well. They should also be precise enough to remove thousandths of an inch from the PCB, too. 

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6 hours ago, davidg5678 said:

I think the idea of resin is to stop the irregularly-shaped logic board from wobbling around, or from being anything but perfectly level as the milling machine slowly removes each layer.

 

That's my take on it as well, I hope someone familiar with this process chimes in at some point. Meanwhile, WAGs about the process would be:

 

- remove components without worrying all that much about irregular solder surface cleanup.

- moosh the board down into the epoxy with a press to keep its surface roughly parallel to the base of the press with the top surface visible.

- clamp rough casting and mill the top surface to rough flatness just shy of the PCB and mill sides of the casting to a precise rectangle

- clamp the bottom surface to a drill press like device (forgot the name)

- take enough measurements to bare copper (maybe solder mask?) to determine the plane of the PCB in relation to casting base

- clamp upside down and mill the base to match to match top plane and remaining rough sides of the casting

 

At some point you'd probably want to measure and mill the casting to square it up to edges of the PCB for precision clamping and registration on the scanner.

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I understood the purpose of the resin, but not being a mechanical engineer person didn't understand how one would accomplish it. Thanks @Trash80toHP_Mini for explaining one possible way to do it. Sounds like it might be more suited to a CNC mill (for the leveling part) than a manual mill like I had in my head.

 

1 oz copper is 0.035 mm thick... that's some mighty precise leveling to pull off.

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