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Last week I fixed an LC III that had this same symptom. Chimed and then nothing, no death chime and no video. The culprit ended up being one of the data lines on the onboard RAM. The ROM was fully connected to the CPU, but the RAM had one broken trace. I think it was somehow hanging up during...

My edit window expired, but I meant to say Atari Falcon here. Completely irrelevant to this thread, but had to correct myself!

I just thought I'd bump this thread since @Dandu didn't receive any responses originally, and add some new info. Here is his blog post about the partition. Does anyone out there have a Performa 550 that has never been reformatted, and would be willing to dump the hard drive to see if the...

Also I just want to preemptively point out that around 16 ohms between ground and the 5V rail on the motherboard is normal. It's often enough to cause your multimeter to beep in continuity mode, but 16 ohms doesn't count as a dead short. That has confused me and many others in the past :)

I also wanted to say thank you @Tarantulas for coming up with a list of capacitors for this power supply. I finally got around to recapping a dead one yesterday, and the new caps worked perfectly and revived it. Here's the list of parts I used, along with Digi-Key links. I had to find a new part...

I know I'm late here, but I figured I'd post this for completeness. I'm in the process of attempting to fix one of these Dyna Comps. Several of the parts have icky green corrosion so it might not be a fun repair, but I can confirm that C2 is X2 and 0.22uF +/- 10%. If I peel that piece of paper...

I'm also very interested in someday being able to play with Volume 1 of the Market Software CDs. Sure hope it's out there somewhere! One thing I should add is when you go to actually do a restore, the installer provides a bunch of info about how the image was created and its history. For...

Hehe, I think it's great that the article linked to the original thread about the issue over here! I didn't expect this to get as much attention as it did, but I'm glad it did -- I'm hoping this helps nudge all the cap replacement guides out there toward being updated to mention this little...

I just wanted to say thanks for this awesome thread and the very useful pictures! I recently bought a Mac LC with a TDK 699-0153 PSU. The logic board caps had leaked, but that repair was straightforward. The PSU gave me fits though. The capacitors on the low voltage side of the board had leaked...

Very interesting! Now I'm confused -- I was assuming that since the first 3 MB of the ROM had a good checksum that the chip dumps were probably good, but I guess something is wrong in the last 1 MB? The first difference at 0x38F21C is 0xBB in @stynx's dump but a 0x4B in your dump, and 0x4B is...

There is data near the start of the ROM that identifies various info about it. For example, the first four bytes are the checksum. At 0x08 you can see 0x077D which is the family version, and then at 0x12 there is 0x45F3 which is the minor version (4.5f3). @eharmon's ROM Fiend template for Hex...

Very interesting! I wrote a quick C program that can combine the interleaved chip dumps back together. Ahh, I think you just beat me to it, but I'll post this anyway. This results in a ROM with checksum 78E842A8. There aren't any Google results for this checksum so it's possibly a previously...

I wonder if the Value 040 shares the exact same ROM? Daystar refers to the Turbo 040 and Value 040 together in a lot of places. This listing on Worthpoint implies the Value 040 is for the SE/30, but it sure looks a lot like an LC PDS card. Also, this 1994 catalog from MacConnection shows the...

Nice work! I can confirm this is correct. Here's some code I quickly threw together to decrypt the ROM dumps based on your provided pin mappings. Comparing against the in-system dumps here, it comes out correct. #include <stdio.h> #include <stdint.h> #include <stdlib.h> int main(int argc, char...

That definitely makes sense. I wonder if they only have to do it that way in a few places, or if there is enough smarts to apply all the patches that same way. Going way back to my discussions in PM with olePigeon in 2011, it looks like it’s an AM27C010-150JC.

I never figured out how to decode the raw chip dumps, but I helped @olePigeon back in the day in my IIci mega-thread (starting around here) with some hacking on the normal Mac ROM with a Turbo 040 installed. At least in the IIci, it ends up being mapped into ROM space at $40880000. If it helps...

Glad to help! Also you probably already know this, but I'll say it anyway: don't forget to order them as 1.2 mm thickness. 1.27 mm would actually be ideal for compatibility, but I'm not sure you'll be able to find a board house that can do it. I know the 1.2 vs 1.27 discrepancy has been a big...

The Wayback Machine is actually back up and running in a read-only state right now, so you can find it there, but I'll repost them here so that they have another home that doesn't depend on the Internet Archive. BTW, I just want to point out that my design was based on @trag's original research...

It's an interesting idea, but the main problem is that A23 isn't available in the ROM SIMM socket. The address lines only go up to A22. So you would need to grab it from somewhere else such as the PDS connector. It's kind of unfortunate that Apple picked A22 for write protection. It really...

Multimeters will have some kind of a threshold for what they consider to be "continuity" for the beep when testing resistance. For example, the manual for my UNI-T UT60A-CN says it will beep if the resistance is less than 100Ω. This originally surprised me when I learned this, because once...