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Is this a 5120? If so, try adding a temporary bodge wire as shown in purple. There seems to be an electrical noise issue in many 5120 Portables when the sound circuit turns on. It causes the CPU to scramble. The above bodge just keeps the sound circuit on all the time. It is a workaround...

Let me know if you run into anything I can help with. Definitely double check for incompatible memory and make sure the board isn't shorted somewhere.

REPLACEMENT PARTS If priced reasonably, I will pick up battery bombed boards to repair other machines. In this case, the missing CPU and FPU were replaced with ones from a battery bombed board. The tops are ugly but the chips are solid. Replacement/repair summary: 1. New speaker 2. New ROM...

RUST REMOVAL The metal shield on the inside of the case was in horrible shape. The speaker is broken (open circuit). In the past, I've dunked an entire case in Evapo-Rust. https://amzn.to/4sJGM1V However, because I didn't want to remove or stain the weird IIfx label on the front of the...

ROM TROUBLE The seller did not include a ROM with this machine. Whoever removed it (and the CPU/FPU) is a jerk. I prefer the original ROM as it performs the complete RAM test on these finicky RAM slots. Additionally, only the original ROM will pass the MacTest Pro and Home editions of Apple's...

RAM TROUBLE Whenever I am repairing a board, I use known-good parts in everything that is removable. In this case, that includes swapping out the RAM and installing an original IIfx ROM. This approach reduces the number of variables I need to consider when the board isn't working. Earlier, I...

A seller offered this IIfx parts machine on eBay for $200. The seller was completely honest in the listing -- so all problems I describe in this post were risks I assumed. In the eBay image below, you can spot rust, missing CPU/FPU/ROM, and three RAM simms that differ from their mates. Rust on...

Here you go

Also noteworthy, the IIvx using a different system enabler than the Performa 600. Make sure you have both enabler 001 and enabler 304. https://www.macintoshrepository.org/505-macintosh-system-enablers So, if the cache is being enabled in that code (likely), it is critical that the System...

I notice R146 is different between the two board images. It exists on yours but not on @Fizzbinn's board.

Using TechTool or a similar utility, what machine ID does your computer now think it is? The IIvi, IIvx, and IIvm/Performa 600 all have different IDs and thus might be executing different ROM/OS code to enable the cache.

Thank you. BTW, I based my 5126 analysis largely on the PowerBook 100 schematic that appears on your macdat.net link. I then compared it to my 5126 motherboard to be sure. The 5120 recreated hybrid schematic largely reflects the 5126 analysis. Input voltage (6.3B or 7.5V) comes in through RC3...

Let's talk through this using @techknight's video as an example. What could have caused this particular board to be destroyed? It is a 5126 (backlight) with bluing of the power transistors and holes blown in the AC logic chips. (We will assume this is non-abuse unintentional damage. That is...

Thank you!

It looks like the jumpers follow a predictable binary pattern EO=+1MB E1=+2MB E2=+4MB E3=+8MB E4=unknown So, to get +8MB (9 in total with motherboard 1MB), pull jumper E3. To get +7MB (8 in total), keep E3 but pull E2+E1+E0. This explains why the board with 2 megabytes already soldered to it...

That's wonderful news! Congratulations. Your persistence paid off.

Yes. For 8 MB, I moved E3 to E1. So, all jumpers populated except E3. I will play with it further to figure out how to make it 7 MB for use in the 5126. I believe the RAM slot is limited to 4 MB (recent recreations use an extra wire to provide more). Additionally, the system provides the...

As a workaround, adding a >=1MB RAM disk in the Portable control panel seems to prevent crashing. The error is a memory manager error. That's suggests a bug or limit in System 6 beyond 8 MB. I tried using System 6.0.8L without success. I wonder whether a Macintosh LC with > 8MB will crash. I'm...

I have soldered on all the RAM. It works great in System 7.1 on a 5120 Portable. It shows 9MB total and passes all the memory test applications. However, in System 6.0.8, it crashes when doing something as simple as displaying the About box in the Finder. I recall reading about a solution for...

I just bought a Dynamic Engineering Macintosh Portable RAM card with 2 MB of RAM installed. I also purchased new old-stock Toshiba TC 518512FL-80 PSRAM chips to bring the card up to 8 megabytes. My plan is to use this on a 5120 (non-backlight) Portable. The jumpers on the 2MB card are all...