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Looking LC475 Video related, schematics/info
Posted by: Big Ben on 2020-04-24 04:40:26
Hello LCs enthusiasts!

I recently get one of my LC475 unit out of storage in order to fix it and sell it. (Need to make some space 🙁 )

Unfortunately, despite everything seems fine, the video output is missing HSync.

  • PRAM battery is good
  • I used a VGA adapter with DIP switches to force the video mode (so I can expect particulars readings)
  • There is RGB signals and the VSync signal is a strong, stable and clean 66Hz (totally what I expected), checked with an oscilloscope.
  • Motherboard and PSU fully cleaned and recapped. No broken trace found.



I'll poke around a working 475 and the defective unit to try to find the problem, but if anyone as some info about the video generation or schematics of this motherboard, it'll be very helpful.

I didn't found anything useful on this matter so far. :/

Posted by: Realitystorm on 2020-04-24 18:46:08
I have a 475, did you try a different adapter?  The one I have that works best is tiny, basically just the two ports and seems to only be doing pin remapping.
Posted by: Big Ben on 2020-04-25 14:46:55
Unfortunately it’s not an adapter problem, I used one to force the video mode at boot, and the one I use works fine with others LCs.

I can’t get my Apple Display to show you a picture of what is displayed.

But you can definitely see the happy mac then the desktop, but the video is totally messed up since the screen doesn’t get the hsync.

I only have a lcd but it won’t display a thing in this case, it’s totally binary, either the signal is good, either it’s bad and nothing is displayed.

So I need to find out which part on the motherboard failed to deliver the Hsync signals. Which is harder without documentation or schematics.
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