Recent content by dougg3

Ah, nice catch! Looks like the Bomarc schematic has an error: I confirmed it's A22 on one of my LC IIIs as well, thanks for the correction! What you found about A22 makes even more sense and is probably more evidence that it has to do with rewritable ROMs. It makes sense they wouldn't need...

Nicely done! On a slightly unrelated tangent, I'd bet money that unpopulated connector J11 has something to do with in-system programming of the ROM. It has GND, +12V (VPP), and a pin that involves write cycles to the Z85C80 SCSI/serial controller. That third pin could probably also act as the...

Haha, that’s pretty wild that we both happened to have the exact same failure on our LC IIIs. Congrats!

Those tests do use A0 and A1 with the *0 and *1 commands to decide the range of addresses to mess with. It might also be smart to use the *4 command to clear the result register each time before performing the test, to make sure you're getting an actual result from the test you're trying. But...

If it’s doing the chimes of death, you should be able to use the serial diagnostics mode to read out the error info and run tests. The RAM tests should be able to tell you what data line(s) are acting up (if it’s a data line anyway) and then you could focus on the related traces/chips.

Yeah, both my LC IIIs work fine without a battery installed. The thing that makes me think the ROM is okay: the startup chime is playing. The chime should only play if the ROM checksum test passes. It's not 100% proof that the ROM is totally fine, but it's a pretty good sign. I tried messing...

I guess one other thing to add is, if you have a known good boot disk like a hard drive or SCSI hard drive emulator of your choice, see if the machine actually boots from it or if it's just hung up forever with the black screen. In the case of mine that was stuck with a black screen after a...

Not dumb at all! I’ve just spent more time than I care to admit troubleshooting multiple LC IIIs. You make a good point, a trace to the onboard VRAM could also be bad

The LC III doesn't require a VRAM SIMM.

I’d normally agree with you, but I saw it with my own eyes on my LC III. Usually bad RAM results in a chime followed by the chimes of death.

I have an LC III that had this exact same symptom (chime and then nothing) and it ended up being a broken trace (well, a bad via) between the CPU and the RAM on the data bus. It was like the RAM test was failing so badly that the Chimes of Death couldn't play. If the adapter you're testing works...

The ROM socket is 64-pin, not 68-pin, so I wouldn't expect it to be compatible for those. Even if you cleverly hacked it to remove 2 pins on each side, I'm assuming the peg locations would be wrong and it wouldn't hold the ROM SIMM in its latched position properly.

I bought some of these on eBay a while back and can confirm that if you cut them apart into single sockets, they are exactly identical to the original VRAM socket that was mounted on my Performa/LC 550 board.

They are. They go to A20 and A21. I haven't tried it myself, but it should work. I'm not sure if you would need to do any MMU reconfig like what was required for the Quadras, but it should be possible to get 4 MB of ROM in the LC III.

Back to the original problem of my IIsi power supply always turning itself back on when I try to shut it off: The original PSU that I saw this problem on is still dead. I've struggled a bit with motivation to fix it after experiencing a couple of MOSFET explosions while testing out more replica...