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48MB in a PowerBook 500 Series – an Illustrated Guide
Posted by: croissantking on 2026-01-30 06:50:27 A few months ago, I worked out how to run 48MB in a PowerBook 500 Series. Inspiration was taken from Toyoki's 64MB mod for PPC upgraded 500-Series notebooks. The concept is similar – tap into unused RAS lines on the memory controller to bump maximum RAM above 36MB. Spoiler content hidden. You will need: A 32MB RAM expansion card like the one in the following photos 8x 2MB 28-pin TSOP (thin profile) chips Repair wire Patience, decent eyesight and a steady hand Mods required: RAM Card Convert a 32MB RAM card to 48MB by stacking chips Run wires from the /RAS pin of the new banks to unused pins on the RAM card connector CPU Card Desolder the onboard RAM. Cut 3 traces Run wires from RAS0+1 to unused pins on the RAM card connector Starting with the RAM card, stack all 8 donor chips on top of the chips on the bottom side of the card. Bend all the legs down and solder them to the lower chip –except for pin 7 (RAS) and pin 8 (NC): lift those two up slightly. (Technically pin 8 can be soldered down as well, it shouldn't matter - but I chose to lift it up as in Toyoki's example.) See this diagram for the pinout: Here's my RAM card with 6 of the 8 new chips soldered down, and a 7th in position. You can hardly tell they are stacked. Note the two lifted pins per chip. Here's a side view of the stacked chips soldered down. Use repair wire to connect up the RAS pins across the four chips at the top, and another repair wire to connect up the other four chips at the bottom. Don't connect them together! Run two trailing wires (one from each bank) to the RAM card connector. I left the NC pins floating/disconnected. Pins 28, 29 and 30 are labelled 'reserved' (more on this in a bit) in the Dev Note, page 35. I used pins 28 and 30 to send my RAS signals back to the CPU card, as shown: That's your RAM card done. Now, take your CPU card, remove the onboard RAM, and run two repair wires as shown to pins 190 and 191 of the Pratt MemIC. Make a cut on the trace leading out from pin 190/RAS0. Side view of RAS0 (Pin 190) and RAS1 (Pin 191), showing repair wires soldered on. Definitely a fiddly job. Moving to the RAM card connector on the top side of the CPU card, we need to peel back the serial# sticker and cut the traces leading to Pins 28 and 30 as shown. These go back to unknown pins on the Pratt MemIC, but aren't needed. @demik theorised they could have been used during development for ROM cards. Finish by running your repair wires up to the pins. Assemble your machine, and there you go – 48MB in a PowerBook 500 Series. This was one heck of a difficult mod to do, it took me two attempts! Worth it though. I'm hoping to eventually create a 500 Series 'Reloaded' CPU card with 16MB onboard and programmable oscillator, if there's enough interest.
Posted by: pax on 2026-01-30 10:25:17 Very nice.
Posted by: EmmyOcelot on 2026-01-30 13:41:28 Very sick to see! Those wires sure do look finicky to get connected correctly.
Posted by: croissantking on 2026-01-30 13:53:25 Appreciate the kudos! Tagging @jmacz @Callan @frontein1, as this mod may be of interest. Thanks also go to @terrier, who helped me work out which were the unused RAS lines.
Posted by: Fizzbinn on 2026-01-30 14:23:02 Impressive indeed! Curious, in the last pic, with a finned heatsink like that installed you just don't install the original heat spreader thing?
Posted by: croissantking on 2026-01-30 14:25:00 Impressive indeed! Curious, in the last pic, with a finned heatsink like that installed you just don't install the original heat spreader thing? It was just during testing, it's all now buttoned up in a 540c chassis.
Posted by: jmacz on 2026-01-30 19:40:21 Awesome! 🙂
Posted by: Callan on 2026-01-30 22:55:11 Awesome for sure!!! Thanks for the detailed write up! It's definitely on the to-do list now!
Posted by: gsteemso on 2026-01-31 11:44:46 Super impressive! I'm awed. I'm hoping to eventually create a 500 Series 'Reloaded' CPU card with 16MB onboard and programmable oscillator, if there's enough interest. This would be a great idea! Even if it were less capable than this proposal, there are only so many 550 boards out there and having something more attainable would be a big help.
Posted by: frontein1 on 2026-02-01 19:02:18 Wow this is pretty cool! Nice work!
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