Interesting Mac Classic Upgrade to 4 MB without using RAM expansion card

[Forrest]

Yesterday on Adrian’s Digital Basement YouTube channel, he demonstrated how to upgrade the memory on a Mac Classic without using the RAM expansion board and memory modules. Adrian said this upgrade has never been done before, and involved removing two chips from the logic board, installing some wires and replacing the 8 RAM 1 MB chips on the logic board with 4 MB chips. Instructions on how he performed the upgrade and a video Link

 

[SuperSVGA]

I'm surprised no one has cloned that CAS PAL chip yet, it's simple and much easier than most of the other reverse engineering efforts I've seen people do.

 

[360alaska]

I saw that, here are the instructions also:

How to modify the Mac Classic for 4mb onboard as seen on Adrian's Digital Basement

** Remove two chips:

Remove UL1 CAS PAL (no longer needed!)
Remove UH6 74LS174 (no longer needed!)

** Modify system to think it has 4MB on the motherboard:

Pin 13 of UI4 BBU:
Remove R42 470ohm (under motherboard - this pulls pin to ground)
Pull up PIN 13 via 10K to 5V
(I did this by running a jumper between the left side of R42 and R41)

Pin 17 of UI4 BBU:
Should be pulled up to 5v already (verify but mine was already at 5v)

** Remove old 44256 RAM chips (8 of them, UK1-UK8)
** Install 4400 RAM chips (4x1mbit - I took these off a 4mb 72pin SIMM)

** Make connections to connect CAS lines from UI4 BBE to RAM:
Connect UL1 PIN 7 CAS0H to UL1 PIN 24 (CAS0H to UK1/3 DB8-14)
Connect UL1 PIN 6 CAS0L to UL1 PIN 25 (CAS0L to UK5/7 DB0-7)

Connect UI4 PIN 16 CAS1L to UL1 PIN 27 via 22ohm (CAS1L to UK6/7 DB0-7)
Connect UI4 PIN 19 CAS1H to UL1 PIN 26 via 22ohm (CAS1H to UK2/4 DB8-14)
(22ohm is just to make sure shorted DRAM doesn't kill the UI4 BBU)

Remove (optionally) and jumper over R43 and R56 (22ohm resistors) as these are in series with R3 and R19. This gives the CAS lines just 22 ohms (versus 44 ohms)

This replicates the Mac SE design exactly, in fact the Mac Classic is really a Mac SE... The UI4 BBU chip is the same one on the Mac SE, even though the part# is different, it seems to work in an identical way.

 

[croissantking]

I watched this today. I have no interest in the Mac Classic personally but it’s a fun and interesting mod, which opens up options for new projects.

 

[LaPorta]

While this is interesting...

...why not just use the card?

 

[croissantking]

While this is interesting...

...why not just use the card?

Fair point. But … it’s just for fun: I can see the tinkering appeal.

As Adrian Black said in the video, it opens up possibilities to do a pizza box mod with this logic board if someone wanted.

 

[Fizzbinn]

Seems kinda like the IIsi mod to put 4MB on the logic board (instead of the stock 1MB). In Adrian's specific case I believe he had stolen a logic board IC that is needed to support the expansion card to fix another board.

 

[Forrest]

He borrowed the 74LS174 from another board, because this chip had failed on the board he was working on.

The pizza box mod sounds interesting.