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Mac 512k memory expansion question

JC8080

Active 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

Active 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'?
 
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