Portable RAM Card 1MB -> 3MB by adding 658128ALFP chips. Is the B revision acceptable?

[David Cook]

As we all know, the Macintosh Portable is most limited by the lack of RAM upgrades available today. If you only have a 1 MB card (or nothing at all), the computer can't effectively run System 7. While I wait for some genius's modern solution, I would like to try upgrading Apple's 1 MB card for the 5126 backlit model.

As you can see, this card was designed for a 1 MB or 3 MB configuration.

Theoretically, I can just add 16 more 658128ALFP chips (8 on each side). However, I am having difficulty locating these chips at a reasonable price. It seems like the B variety (658128BLFP [Edit: 628128BLFP - '2' not '5']) are readily available. These are fully static, rather than pseudo-static. Fortunately, the refresh pin is NC on the fully static B chip. Otherwise, the pinout and operation seems identical.

Has anyone had any experience upgrading these cards? Has anyone tried the B chips?

Thank you,

David

 

[Garrett]

When upgrading PowerBook 1xx RAM expansion cards, I've found that you can intermix PSRAM and SRAM on the same board. Everything just works. I don't know if that bit of info helps in your case, but it may be worth a shot just to try!

 

[David Cook]

When upgrading PowerBook 1xx RAM expansion cards, I've found that you can intermix PSRAM and SRAM on the same board. Everything just works. I don't know if that bit of info helps in your case, but it may be worth a shot just to try!

Thank you. That's good to know. I've ordered some of the static RAM chips and I'll update this thread after trying them.

 

[Froggy814]

Well, I would give it a try. Only because logical reasoning says: The non backlit can only take SRAM but the backlit can take SRAM and P-SRAM, so I would think it would be okay...

 

[trag]

Too bad you can't use HM658512ALFP chips, or could you? I have about 300 of those sitting here doing nothing.

Got me wondering. Looks like the pinout isn't quite compatible. As might be expected, A17 and A18 are the main problem. I was hoping they were NC on the 658128, which would leave open the possibility that the address lines were present in the upgrade card, but unused. Unfortunately, what's A17, A18 on the 658512 is CE_ and Refresh on the 658128. Booooo.

 

[David Cook]

Booooo.

I concur. Boooo!

But, I'll bet someone will design a replacement RAM board that can use those chips.

 

[David Cook]

It works!!! 1 MB onboard + 3MB on the full Apple RAM board.

Here is one side of the RAM board with 628128BLFP (to be clear 628128BLFP as opposed to the original 658128ALFP) chips added. Eight chips added to each side for a total of 16 chips. It increased the memory on the board from 1 MB to 3 MB.

When I first installed the updated board, the Portable went into a reboot loop. That is, it made the happy chime, initialized the screen to the 'initial test pattern', then about a second later it rebooted and repeated those steps infinitely. The 'initial test pattern' looks like the image below. It has not yet reached the 50% gray checkerboard pattern.

The root cause was some loose pins on the RAM chips that I added. I made the mistake of using the existing solder on the pads (with fresh flux), rather than cleaning it off to a flat surface and then applying new solder. This resulted in chips raised slightly off the board surface and some loose pins. I had visually inspected the board and was proud of my soldering job. Turns out, I needed to make another entire pass with extra solder and flux to catch the offending pins.

It surprises me that the Portable would get stuck in a reboot loop, rather than showing a sad mac. But, I guess it depends on which pins were floating.

- David

 

[Froggy814]

Yay! Good job. Glad you were able to figure the problem out.