https://en.infinityproducts.co.jp/product-page/smc-fat-mac-switcher
Instruction:
Now on sale!
If the current Mac geeks who have recently acquired a 128K were to upgrade it to 512K, they might consider reverting it back to its original state. Currently, the 128K is extremely expensive and has become a rare Mac, even on platforms like eBay.
While it's unclear how many people would want to upgrade their 128K to 512K at the moment, I believe there are some geeks who would be interested in trying it out once they learn that it's possible. Moreover, a significant portion of the original 128K units available in the second-hand market have been modified by amateurs.
There are many aspects that I find impressive about the Macintosh. Around 1984, RAM chips were very expensive, so many Japanese manufacturers, even in the high-end market, only equipped their computers with 64KB of RAM. Of course, that was an era when GUI was not prevalent, and even hard drives were not popular. Looking back to 1984 and recalling my own computing environment, the existence of the Macintosh 128K during that time is truly remarkable.
In 1985, the 512K was released, which added one A8 address line and replaced the 4164 chips with 41256 chips. This is the origin of the "Fat Mac."
Consider a Fat Mac
Examining the wiring and circuit diagrams of the 128K and 512K, the only significant difference is the RAM capacity. (*The 512K has additional circuitry near the keyboard telephone jack for RAM control, but it can also be reverted back to the 128K state.) People who purchased the 128K just before the release of the 512K were probably frustrated when they realized that newly released software didn't work on their machines. Shortly after the release of the 512K, magazine articles on upgrading the 128K's RAM to 512K began to appear, and people started taking on the challenge. However, merely replacing all 18 DRAM chips is not sufficient for it to operate as a 512KB machine due to address limitations.
I often saw a 74F253 piggybacked on the F3 location, which is the 16-pin IC at that spot. This IC is stacked with another one, and it's commonly seen with three wires jumpered to jumper pins.
Later, I learned about a Supermac kit that could be inserted into the footprint of E3 without requiring jumpers. By chance, my friend Stephen Arsenault had one. When I saw the picture, I was amazed! He kindly showed me by removing the IC from the PCB, as he also had an interest in it. I realized that it was doing the same thing as the circuit diagram I already had, except it had a bypass capacitor for noise mitigation.
Since I was able to quickly investigate the 7-pin pinout, I was able to experiment with this modification easily. The next day, I finished designing the PCB.
Kero's Mac Mods New Design
It features:
- Switching between 512K and 128K, with an LED indicating 512KB usage
- Supports ROM-inator Resurrections
- Supports BMOW FloppyEMU
- Supports SYSTEM 6.0.8 (in 512KB mode)
- Supports HD20 (compatible with FloppyEMU)
- (*Wiring can be cumbersome, but it is also possible to install this in the 512K.)
I had no expectations of it selling well, but I decided to create this Fat Mac Kit. By simply switching the wiring of the +5V and DRAM pin 1, I found that I could install a switch on the motherboard to toggle between the onboard 16 RAM chips (totaling 512KB) without replacing them, effectively reverting to 128K.
Initially, I considered using jumper pins, but for safety reasons, it required using at least a DPDT switch. This allows for switching between 512K and 128K. I read in old articles that there is software that only runs on 128K, but I'm not sure which ones specifically. In any case, I have succeeded in adding a mode switch to instantly change the memory capacity.
I used chip parts to keep it as thin as possible and not obstruct the storage of the logic board. (*Normally, ICs like 74F253 and 74AS253 are used for piggybacking, but I considered the current 74HC253, which has similar specifications. However, this CMOS chip worked fine in terms of signal level, but leakage became an issue.) The basic design was completed in less than an hour.
Prototype
I made a universal PCB for the prototype. Universal PCB is called "Janome_kiban" in Japanese. In the past, I used it to make various things. The prototype was done with DIP parts, using a 74F253.
Note: Cut and separate the router (like a solder jumper) bridging between the 1st and 2nd pins in the empty footprint at E3 on the back of the Mac128K logic board using a cutter. This is the only point that requires modification. The connection here serves as a bridge between +5V and DRAM pin 1. When installing a 41256 chip, pin 1 becomes the A8 address line and does not receive 5V power.
In my 128K, all 16 DRAM chips have been replaced, making it 512KB. However, if you want to install this Fat Mac Switcher on a new unit, replacing the DRAM chips is a prerequisite, of course.
*As mentioned earlier, during the prototype phase, I realized that the CMOS 74HC253 has good noise tolerance but significant internal leakage. Although it does not cause any issues in actual use, I did not want to add extra circuitry just to address this leakage. So, I abandoned the use of HC253 and returned to the NOS F253 without changing the specifications. The cost increases considerably, but I want everyone to use it with peace of mind.
Slimming down the Mac512K to 128KB
Someone on Twitter asked if it could be installed on the Mac512K, and I asked him why he wanted to do such a thing. He said he had never owned a Mac128K and wanted to experience that atmosphere... I immediately thought this device might be useful for people like him.
When referring to old literature, there was a person who posted that a certain software only runs on 128KB. This was back in 1985, so without actually using that software, there is no way to know if it's true. However, there might be software that only works with 128KB.
The Mac512K is almost the same as the 128K, but there is one significant difference. It allows the choice of implementing or not implementing the parts near the keyboard jack and bridging them with jumpers. Again, the Fat Mac Switcher and circuit configuration differ here, but they are essentially doing the same thing. This suggests that we can also apply it to the Mac512K.
I tried this and it worked great. I was able to downgrade Mac512K. Moreover, you can return to the original 512K in an instant. It's also useful for the unfortunate 128K with just the logic board upgrade. Unless you open the case, you won't find out that it's a Mac512K.
In the photo, the jumper pin is placed at a right angle and placed in a position where it is easy to operate with the jumper wire, but it is not recommended.
Mac512K is perfectly completed to 128K specifications!