Hardware hacking: Let's make a 4mb Mac Classic without using the RAM expansion card

mars 11, 2024 Mourad ELGORMA

After the recent Mac repair-a-thon, I had a Mac Classic motherboard with a missing 74LS174. I thought about just replacing that IC but realized I could actually do a simple mod to the motherboard to give it 4mb of onboard RAM instead of just 1! (And I wouldn’t need to replace the 74LS174, so it would actually save me money.

— Info

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.

— Links

Mini Mac Repair-a-thon:
Part 1: https://youtu.be/lHvGc4LWR0g
Part 2: https://youtu.be/nSoSX6XV7t4
Part 3: https://youtu.be/xx08YXkUOnQ

Recapping the Mac Classic II Motherboard

Previous Mac Repair-a-thon from 2020:
Part 1: https://youtu.be/lKD65I86XGQ
Part 2: https://youtu.be/kNhVQWH318M
Part 3: https://youtu.be/uatqcZRzmt0
Part 4: https://youtu.be/zC0qi5dQsWs
Part 4.5: https://youtu.be/YikOY8WTnLU
Part 5: https://youtu.be/PHTnauC020o
Part 6: https://youtu.be/mqWCPkfcr1A

Adrian’s Digital Basement Merch store:
https://my-store-c82bd2-2.creator-spring.com/

Adrian’s Digital Basement ][ (Second Channel)
https://www.youtube.com/channel/UCbtwi4wK1YXd9AyV_4UcE6g

Support the channel on Patreon:
https://www.patreon.com/adriansdigitalbasement

— Tools

Deoxit D5:
https://amzn.to/2VvOKy1
http://store.caig.com/s.nl/it.A/id.1602/.f

O-Ring Pick Set: (I use these to lift chips off boards)
https://amzn.to/3a9x54J

Elenco Electronics LP-560 Logic Probe:
https://amzn.to/2VrT5lW

Hakko FR301 Desoldering Iron:
https://amzn.to/2ye6xC0

Rigol DS1054Z Four Channel Oscilloscope:
https://www.rigolna.com/products/digital-oscilloscopes/1000z/

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:
https://amzn.to/3adRbuy

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)
https://amzn.to/2wG4tlP
https://www.aliexpress.com/item/33000308958.html

TS100 Soldering Iron:
https://amzn.to/2K36dJ5
https://www.ebay.com/itm/TS100-65W-MINI-Digital-OLED-Programmable-Soldering-Iron-Anti-static-Structure/113382669853

EEVBlog 121GW Multimeter:
121GW Multimeter

DSLogic Basic Logic Analyzer:
https://amzn.to/2RDSDQw
https://www.ebay.com/itm/USB-Logic-DSLogic-Basic-Analyzer-16Ch-100MHz-4Ch-400MHz-Xilinx-Spartan-6-FPGA/202543965672

Magnetic Screw Holder:
https://amzn.to/3b8LOhG
https://www.harborfreight.com/4-inch-magnetic-parts-tray-90566.html

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)
https://www.ebay.com/itm/14-16-18-20-24-28-32-40-pin-IC-Test-Universal-ZIF-Socket-Fs/303206721971

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)
http://www.retrotink.com/

Plato (Clone) Side Cutters: (order five)
https://www.ebay.com/itm/1-2-5-10PCS-Micro-Scissor-125mm-PLATO-170-Electronics-Cutter-Side-Cutting-Pliers/163458746184

Heat Sinks:
https://www.aliexpress.com/item/32537183709.html

Little squeezy bottles: (available elsewhere too)
https://amzn.to/3b8LOOI

— Links

My GitHub repository:
https://github.com/misterblack1?tab=repositories

Commodore Computer Club / Vancouver, WA – Portland, OR – PDX Commodore Users Group
https://www.commodorecomputerclub.com/

— Instructional videos

My video on damage-free chip removal: