Mac Classic Repair

[roto]

Hi All. I've been a lurker for awhile but wanted to start a thread documenting my attempt to repair a Classic I recently picked up, hopefully getting some tips along the way. Maybe my experience will be helpful for others as well.

The Classic was picked up at an estate auction, described as powering up, but with a blank screen. I've not tried powering it up myself, but will be taking it apart soon to assess. The last Classic I worked on had significant battery damage and I've yet to revive it, so I'm hoping I'll have more luck with this one.

 

[AndiS]

Open it up and get the battery out as soon as possible!

Good Luck!

 

[dochilli]

And recap the LB and the AB.
Search for cracked solder joint on the LB/AB connection and the neck board (CRT).

 

[roto]

Thanks.

So I got it apart this weekend. Unfortunately the LB had decent Maxell battery bomb damage, but not the worst I've seen. I forgot to get any before pictures, but here are a couple after a clean with vinegar, then IPA. I also decapped the board. The biggest casualty so far was the crystal falling off while cleaning; the legs were badly corroded. Luckily I don't see any fully corroded pins on any chips and things really don't look too terrible:





I connected the 40MB hard drive to my Linux box. Still powers up and spins, and used ddrescue to image it, but with 50K of bad sectors.

Then I decided to plug the board into a good working chassis. As expected, I get a blank grey screen, same as in the auction photo. No garbage or lines though, so that seems positive. Another good sign, when I press the NMI button, I get a clean looking Sad Mac:



I think this means the ROM, RAM, CPU, and video circuitry are working in some capacity. Any thoughts on what I should look at next given these symptoms? I plan to just start probing lines for any damaged traces but if there's somewhere I should focus first, I'll start there. I'll probably also clean it again, it still has some haziness and gunk I can probably get off with more work.

 

[roto]

Success!!

After a bunch of probing, I found just one damaged trace: the R/W line between the CPU and VIA was broken, right under where the battery holder was. Repaired that and the system booted up into the built in ROM:


Also replaced the broken crystal and the removed caps. I now have working time and sound. Booted right up from the hard drive too. Still need to clean the floppy drive and test that.

 

[JustG]

Awesome job! Where did you source a replacement crystal? I have a board that suffered very similar damage to yours.

 

[roto]

I ordered one from Digikey: https://www.digikey.com/short/8qdr8nj8