Mac 512k memory expansion question

JC8080

Well-known member
I have a 512ke with a Sophisticated Circuits Mac's-a-Million RAM expansion. When I got it the board was populated with 512k additional RAM, giving 1mb total. I just recently bought 40 used RAM chips, hoping to get 32 good ones, to expand the system to 2mb total. After some shuffling around I got 16 of the new (used) chips in the first bank (row A) on the expansion board and it fired up to the floppy, so it seems it was working fine. I previously had a sad mac, but after swapping some chips out it worked fine. At this point I wish I would have booted with my FloppyEMU and checked how much RAM was showing, but I assumed 1mb since I didn't have any more errors. Then I put the 16 known-good chips that came on the board into bank 2. I turned it on, seemed to work fine, so I booted into the finder and checked the RAM, still showed 1,024k. I had expected to see 1.5mb, and I ran out of time to do any more work on it, so I put it away for the day. I realized afterwards though that maybe 1.5mb isn't an option, and maybe if I had populated the 3rd bank as well I would have seen 2,048k. Then I also wondered if there could be something on the board that needs to be jumpered or reconfigured to allow it to see additional RAM. Does anyone have any thoughts as to whether an expansion like this can see 1.5mb, or if it needs to be fully populated to 2mb? Thanks.
 

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Juror22

Well-known member
I didn't see any documentation yet, but here is a picture of a fully populated board - I cannot see any jumpers that are different from yours...
 

JC8080

Well-known member
Thanks for that pic, I dug around a bit and found one or two more pics online, all populated with 2mb. If you look in the very lower left corner of mine you can see a solder blob inside that little box labeled JB1, and on the pic you posted there isn't any solder there. I'll have to take a look at my board in person, that photo I posted isn't very good quality.
 

Juror22

Well-known member
Good eye. That could be how they manage it.

I suppose JB is short for 'Jumper Solder Blob'?
 

nottomhanks

Well-known member
Circling back to this: we recently picked up a bunch of Mac stuff including a 128k with the Macs-A-Million ram expansion. I bought 50 41256 - 10 chips and fully populated the card. Booted up MacDraw to see how much RAM showed up. It only registered 1024 KB. Then I found this thread and realized that JP1 dictates how much ram is recognized. I pulled out the Hakko desoldering gun, removed the solder, then rebooted to MacDraw- 2048 KB!!! Yay! I love figuring things out!!
 

JC8080

Well-known member
Circling back to this: we recently picked up a bunch of Mac stuff including a 128k with the Macs-A-Million ram expansion. I bought 50 41256 - 10 chips and fully populated the card. Booted up MacDraw to see how much RAM showed up. It only registered 1024 KB. Then I found this thread and realized that JP1 dictates how much ram is recognized. I pulled out the Hakko desoldering gun, removed the solder, then rebooted to MacDraw- 2048 KB!!! Yay! I love figuring things out!!
Awesome, thanks for the update. A friend of mine just asked if I ever made progress with mine (I did not), this gives me some motivation to work on mine.

Are your RAM chips socketed? I'm wondering if part of the problems I am having is due to poor connections in the sockets. Specifically related to some bad RAM errors, not the problem recognizing more than 1,024k. I recently bought a Hakko desoldering gun, so removing all the sockets is now realistic, I was not excited about doing it with just solder wick and a hot air stations.

Edit: @nottomhanks could you post a pic of your board, and one specifically showing the JP1 configuration you found that works?
 

nottomhanks

Well-known member
Here’s some photos. That JP1 needs to be clean (remove the solder bridge and then it sees all chips, not just the first bank.
IMG_0574.jpeg
 

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nottomhanks

Well-known member
When I first installed it, it said 1024, which I think is the motherboard ram as well as the first row of chips. After I removed the solder bridge, it doubled to 2048, which was super cool. I wonder if that means the 128 had been upgraded to 512? It’s hard to tell because you’d have to remove everything to find out.. can anyone do some math on that? That means that I still don’t have an actual 128k Mac then..
 

JC8080

Well-known member
Thanks for the pic. So your photo is before you removed the blob on side 1 of JB1?

It definitely sounds like your 128k has been upgraded to a 512k. You would probably need to pull the expansion board off to tell if you have an actual 512k logic board, or if you have a 128k board that was upgraded to 512k.
 

nottomhanks

Well-known member
Yes, that photo showed what it looked like before I removed it. I think you’re right about the 512k on the logic board. Hard to find an original 128 I guess as most users seemed to upgrade theirs.
 

nottomhanks

Well-known member
The photo I sent with all the chips, you can see the JP1, my cleanup job wasn’t perfect, but it says 2048 now so I’m happy about it. When I was inserting the ram chips, I got a sad mac code. After removing the board I noticed that some of the chips legs were outside their sockets!! I fixed them and then it booted correctly. Glad none of the legs broke. I took the existing 16 chips out of there as I wanted to have the same MS rating for all of them. So I have 18 extras now.
 

nottomhanks

Well-known member
Apparently, if you hold down the mouse button while booting, it will bypass the ram expansion and just show you what’s on the logic board directly. I’ll try that and see what happens.
 
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